External masonry walls form the structural envelope of your new build home, providing weather protection, thermal insulation, and structural support. The NHBC Standards 6.1 set out comprehensive requirements for the design and construction of these critical elements. This guide explains the key standards that apply to external masonry walls in new build properties.

The NHBC Standards Chapter 6.1 for External Masonry Walls represents decades of construction knowledge and experience condensed into practical guidance. These standards exist because masonry construction, while traditional and well-understood, involves numerous details that must be executed correctly to achieve a durable, weather-resistant, thermally efficient, and structurally sound building envelope.

Common Themes Throughout the Standards

1. Weather protection is paramount: Multiple provisions address keeping water out—DPCs, cavity trays, weepholes, appropriate materials for exposure, proper jointing, and adequate overhangs. Water penetration remains one of the most common defects in new builds.

2. Thermal performance must be achieved without compromise: With increasingly stringent energy efficiency requirements, proper insulation installation is critical. Gaps, compression, or thermal bridging undermine performance and can cause condensation problems.

3. Materials must suit their location: Not all bricks are equal—exposure conditions, frost risk, sulfate levels, and structural requirements all influence appropriate material selection. Using inappropriate materials leads to premature deterioration.

4. Workmanship quality cannot be compromised: Even the best materials and design fail if construction quality is poor. Proper bedding, fully filled joints, clean cavities, correct tie spacing, and careful finishing all matter.

5. Details at junctions are critical: The greatest risks for defects occur at junctions—around openings, where walls meet floors or roofs, at changes in materials, and at penetrations. These areas demand particular care.

Taking Action on Identified Defects

Minor cosmetic issues (poor pointing, mortar smears, minor alignment issues) should be addressed before completion and are typically straightforward to remedy.

Significant defects (missing DPCs, inadequate cavity trays, incorrect materials, poor insulation installation) require more extensive remediation and should be addressed urgently, as they affect the building’s long-term performance and durability.

Structural concerns (inadequate ties, missing restraint straps, structural cracking) require immediate attention from qualified engineers and must be resolved before the property is occupied.

Potential long-term issues (materials inappropriate for exposure, details that may allow moisture penetration) should be documented, discussed with the builder, and resolved according to their significance.

Your Rights and Remedies

New build homes come with:

• Builder’s contract obligations: Rectify defects identified during snagging period
• NHBC warranty protection: Covers major defects affecting structural stability, weather resistance, and essential services for 10 years (with 2-year builder guarantee period first)
• Building Regulations compliance: Homes must meet minimum standards—failure to comply can be challenged

A professional snagging inspection provides:

• Documented evidence of defects in standardised format
• Technical assessment against recognised standards
• Priority categorisation helping you focus on significant issues
• Leverage in discussions with builders about remediation
• Peace of mind that hidden and visible elements have been properly checked

External masonry walls are complex assemblies requiring careful design, appropriate materials, and skilled construction. The NHBC Standards provide a clear benchmark against which quality can be assessed. Understanding these standards empowers you to:

• Ask informed questions during the buying process
• Recognise the value of quality construction
• Identify when standards may not be met
• Take appropriate action to ensure your home meets required standards
• Appreciate the complexity of seemingly simple construction

6.1.1 Compliance

External masonry walls must comply with the Technical Requirements set out by NHBC. When builders follow the guidance provided in Chapter 6.1 of the NHBC Standards, their work will generally be considered acceptable for warranty purposes.

6.1.2 Provision of Information

Designs and specifications for external masonry walls must be produced in a clearly understandable format, include all relevant information, and be distributed to site supervisors, specialist subcontractors, and suppliers.

Key information that should be specified includes:

• Materials: The specific bricks, blocks, mortar, and insulation to be used
• Dimensions: Wall layouts with all dimensions, position and size of openings
• Structural elements: Position and type of lintels, restraint straps, padstones, and wall plates
• Weatherproofing: Location of damp proof courses (DPCs), cavity trays, and weepholes
• Insulation details: Type of insulant, cavity ties spacing and location
• Fire safety: Position of cavity barriers and fire-stopping requirements
• Finishes: Acceptable pointing or mortar joint finishes
• Setting out: Masonry coordinating dimensions for proper coursing

6.1.3 Structural Design

External masonry must be designed to support and transfer loads to foundations safely and without undue movement. This section addresses six critical aspects:

Compliance with Relevant Standards

Structural design should comply with:

• BS EN 1996-1-1 (Eurocode 6): Design of masonry structures
• PD 6697: Recommendations for masonry design
• BS 8103: Structural design of low-rise buildings
• BS EN 845 series: Specification for wall ties, lintels, and bed joint reinforcement

For buildings over three storeys containing homes, walls must be designed by an engineer in accordance with Technical Requirement R5.

Lateral Restraint

Walls need horizontal support to prevent them from bowing or collapsing. This restraint is typically provided by:

Concrete floors: When properly bearing onto walls (minimum 90mm), concrete floors provide adequate restraint. Where floors run parallel to walls, restraint straps are required.

Timber floors: In buildings up to two storeys, timber joisted floors can provide adequate restraint when joists have minimum 90mm bearing (or 75mm onto timber wall plates), or when carried by restraint-type hangers. In taller buildings, restraint straps connected to the wall are necessary.

Buttressing walls: The ends of walls should be bonded or tied to buttressing walls, piers, or frames throughout their full height. Long walls may require intermediate buttressing to divide them into distinct supported sections.

Concentrated Loads

Where heavy loads are concentrated at specific points (such as under lintels, steel beams, or multi-ply trusses), the design must be prepared by a qualified engineer. Padstones and spreaders may be required where bearing stresses exceed the strength of the supporting masonry.

Bonding

Where partition walls meet external walls of similar materials, they should be fully bonded or tied. Tied joints are preferable where:

• Materials have different shrinkage or expansion characteristics (such as dense concrete and aircrete blocks)
• A load-bearing wall on foundations connects to a non-load-bearing wall on a ground-bearing slab

Tied joints should use expanded metal, wire wall ties, or equivalent at maximum 300mm intervals. Crucially, dissimilar materials should not be used in the same wall.

Movement Joints

Movement joints control expansion or contraction of masonry panels and reduce unsightly cracking whilst maintaining stability. These vertical joints should:

• Run the full height of the superstructure masonry wall
• Not coincide with window and door openings
• Continue from substructure to superstructure where appropriate
• Be positioned at corners or hidden features where possible

Spacing requirements vary by material:

The first movement joint from a corner should be no more than half the standard dimension. Clay bricks expand and require easily compressible filler materials, whilst concrete products contract and require semi-rigid materials.

Damp Proof Courses in Structural Context

DPC materials must be selected with consideration for their structural characteristics, particularly their adhesion to mortar and ability to resist sliding and shear stresses. In frame or crosswall structures requiring shear resistance, the deemed-to-satisfy rules may not be appropriate, and manufacturers’ guidance on compression, tension, sliding, and shear resistance should be sought.

6.1.4 Fire Resistance

External cavity walls must adequately resist the passage of fire. The cavity in a masonry wall can provide a route for smoke and flames to spread, so cavity barriers must be installed in accordance with Building Regulations.

Where Cavity Barriers Are Required

Cavity barriers should be provided:

• At cavity edges: Around all openings including windows and doors
• At compartmentation: Where the cavity meets compartment walls and floors
• To break up extensive cavities: Preventing them from acting as routes for fire spread

Vertical cavity barriers should extend below the damp proof course, with careful attention to continuity where cavity trays are installed at DPC level.

Critical Installation Requirements

Cavity barriers must be sized appropriately for the cavity dimensions. They are normally installed under compression, making the maintenance of design cavity width critical. Significant reduction in cavity width means barriers cannot be fitted without problems for the following masonry leaf. Conversely, significant widening means barriers may not achieve appropriate compression, impairing their performance.

Concealed Spaces Behind Cladding

Where cladding is fitted to a masonry substrate wall, cavity barriers should be positioned:

• At cavity edges, including eaves and verges
• Around openings such as windows and doors
• At service entry and exit points
• At junctions between external cavity walls and every compartment floor and wall

6.1.5 Acoustic Resistance

External walls adjacent to separating walls must be designed to resist flanking sound transmission between homes. Inadequate design can allow sound to travel around the edges of party walls, undermining the acoustic separation between properties.

Achieving Acceptable Sound Reduction

Sound reduction between homes may be achieved by:

• Sufficient density in the inner leaf: The inner leaf of external cavity walls needs adequate mass to resist sound transmission
• Sealing air paths: Particularly important at junctions between the masonry cavity and separating elements
• Appropriate spacing of openings: Windows and doors in external walls should be positioned with adequate spacing relative to the party wall
• Avoiding structural bridging: Structural members should not transfer across or breach separating walls

Material Considerations

The density of external walls and the position of openings adjacent to separating walls should comply with Building Regulations and, where relevant, an assessment meeting Technical Requirement R3. The Robust Details Handbook provides specifications for separating wall and floor constructions and their associated flanking walls.

Where different block materials are used (for example, aggregate separating walls with aircrete inner leaf), differential drying can cause cracking. In these situations, it’s important that the separating wall goes through the inner leaf rather than stopping at it to prevent this issue.

Testing and Compliance

For new build homes, acoustic performance is typically demonstrated through:

• Pre-completion testing on selected plots
• Compliance with Robust Details (which may exempt properties from testing)
• Sound insulation calculations by qualified acousticians

What this means for you: Poor acoustic performance in new builds often only becomes apparent after moving in. Common complaints include:

• Hearing conversations, television, or music from neighbouring properties
• Noise from footsteps or impacts in adjoining homes
• Sounds travelling through party walls at specific frequencies

If you have concerns about sound insulation after moving in, NHBC warranty claims for acoustic performance issues must be supported by professional sound testing, which should be arranged through qualified acoustic consultants.

6.1.6 Exposure

External walls must be suitable for their exposure conditions and resist the passage of moisture to the inside of the home. The UK’s varied climate means that exposure to driving rain, freeze-thaw cycles, and other weather conditions differs significantly across regions and even within the same area based on topography and local shelter.

Durability

Masonry can become saturated and remain so for extended periods, particularly in exposed locations. Special precautions should be taken to resist frost damage and sulfate attack in vulnerable areas:

• Parapet walls and copings
• Sills and projections
• Masonry below DPC at ground level
• Freestanding walls

Material requirements: Masonry units and mortar should comply with BS EN 1996-1-1 and be used in accordance with Table 15 of PD 6697 alongside manufacturer’s recommendations. Mortar mixes should be selected from BS EN 998-2 Table NA.1, using only cement types listed in BS EN 998-2 NA.1.2.

Cement with sulfate-resisting properties should be used where S1 clay bricks are specified in:

• Areas below the DPC where sulfates are present in the ground
• Below the DPC where high saturation risk exists
• Retaining walls, parapets, and freestanding walls
• Rendered walls
• Areas of severe or very severe exposure to driving rain

Reclaimed bricks should only be used where they comply with Technical Requirement R3, as their durability history and characteristics may be unknown.

Rain Penetration

In prolonged periods of driving rain, water will penetrate the outer leaf of masonry walls. The UK is divided into exposure zones based on wind-driven rain:

Exposure Categories:

• Sheltered: Less than 33 litres/m² per spell
• Moderate: 33 to less than 56.5 litres/m² per spell
• Severe: 56.5 to less than 100 litres/m² per spell
• Very Severe: 100 litres/m² or more per spell

The NHBC Standards include an exposure map showing these zones across the UK. Scotland, Northern Ireland, western coastal areas, and elevated locations generally experience more severe exposure.

Design considerations for rain penetration:

The following approaches can reduce rain penetration risk:

• Providing cladding to the wall (render, tile hanging, or other protective finishes)
• Increasing cavity width (minimum 50mm clear cavity for partial fill, or wider fully filled insulation)
• Rendering with crack-resistant backing materials
• Designing protective features such as projecting sills and deep overhanging eaves
• Ensuring mortar joints are fully filled (recessed joints should not be used with full cavity fill)
• Following assessment recommendations for insulation systems
• Ensuring cavities are not bridged by mortar or debris

Special requirements for severe exposures: In Scotland, Northern Ireland, the Isle of Man, and areas of very severe exposure to driving rain, masonry should form a rebate at window and door reveals (minimum 12mm overlap) to avoid straight-through joints, or a proprietary cavity closer assessed under Technical Requirement R3 should be used.

Sills, copings, and features should meet BS 5642 requirements and be weathered and throated unless adequate alternative protection is provided.

Freeze/Thaw Cycles

Common factors affecting freeze/thaw risk include:

• Degree of exposure (incidence of frost)
• Saturation level of the masonry
• Frost resistance of the masonry
• Local protection from roof overhangs, trees, or other buildings

Good detailing to limit frost attack:

• Paths should drain away from walls to avoid saturating bricks near ground level
• Sills, copings, and similar features should have weathered upper surfaces
• Copings or cappings should be provided for all parapet walls, chimneys, and freestanding walls (unless using F2, S2 classification clay bricks to BS EN 771-1)

Coping requirements:

• Overhang beyond the wall face
• Throatings minimum 30mm clear of the wall
• Continuous supported DPC projecting beyond the wall line

Brick selection for frost resistance:

The Standards classify areas based on three criteria measured simultaneously:

• More than 60 days of frost per year
• Annual rainfall over 1 metre
• Elevation 90m or more above sea level

In areas meeting these criteria (shown hatched on the frost exposure map), only frost-resistant bricks are acceptable:

• Clay bricks: F2,S2 or F2,S1 to BS EN 771-1
• Clay bricks classified by manufacturer as satisfactory for the exposure
• Concrete bricks: minimum 20N/mm² strength to BS EN 771-3
• Concrete blocks: minimum 1,500kg/m³ density or compressive strength greater than 7.3N/mm²
• Most aircrete blocks with render

Exceptionally severe frost exposure locations face long stretches of open countryside within severe frost areas. In these locations, only F2,S2 or F2,S1 frost-resistant clay bricks are acceptable for the superstructure, with written manufacturer confirmation required.

Additional considerations:

• Manufacturers’ recommendations must be followed, including mortar choice and joint finish type
• Bricks not classified as frost-resistant (F0,S2 or F0,S1) are unacceptable externally unless completely protected by impervious cladding
• Where brickwork may be persistently wet, low soluble salt bricks should be specified
• Painted or decorative finishes can trap moisture, increasing frost damage risk—manufacturer advice should be sought
• In Scotland, all external facing clay bricks must be F2,S2 or F2,S1, and concrete bricks must be 22N/mm²

Longer-term exposure problems that may develop after completion include:

• Frost damage (spalling, flaking, or crumbling bricks)
• Sulfate attack (mortar deterioration, expansion, cracking)
• Persistent damp penetration
• Efflorescence (white salt deposits)

Understanding your property’s exposure category helps you appreciate whether the specified materials and construction details are appropriate for your location.

6.1.7 Thermal Insulation

Thermal insulation must be adequate and installed correctly to meet Building Regulations requirements and prevent cold bridging, which can lead to condensation, mould growth, and heat loss.

Thermal Insulation Materials

Insulation materials placed in the cavity between the outer and inner masonry leaves should be satisfactorily assessed by an appropriate independent technical approvals authority accepted by NHBC and comply with Technical Requirement R3.

Acceptable standards for insulation materials:

• BS EN 13162: Mineral wool (MW) products
• BS EN 13163: Expanded polystyrene (EPS) products
• BS EN 13164: Extruded polystyrene foam (XPS) products
• BS EN 13165: Phenolic foam (PF) products
• BS EN 13166: Extruded polystyrene foam products
• BS EN 13167: Cellular glass (CG) products
• BS EN 14064-1: Loose-fill mineral wool products
• BS EN 16809-2: Loose-fill and bonded expanded polystyrene beads

Types of cavity insulation:

• Built-in during construction: Boards or batts installed as walls are built
• Retrofitted after construction: Injected or blown-fill insulation
• Fully filled cavity: Insulation designed to be in full contact with both masonry leaves (though a narrow residual cavity may remain between outer leaf and insulation)
• Partial fill cavity: Insulation installed against the inner leaf with a minimum 50mm clear cavity to the outer leaf

All injected and blown-fill systems must be installed by certificate holders or approved installers under surveillance schemes, with operatives trained and approved by the assessment holder and assessing organisation.

Installation Requirements

Proper installation is critical to avoid gaps that create routes for moisture ingress and localised cold spots where condensation forms.

Insulation material should be:

• Close-butted with no gaps between boards or batts
• In full contact with the inner leaf
• Taped at joints where formed of rigid boards with non-compressible edges (tape face must have same low-e value if insulation has low-e facing)
• Fully engaged with adjacent board edges where ship-lap or interlocking edges are provided
• Installed according to manufacturer’s recommendations

Where cavity insulation is used:

• Mortar joints, including perpends, must be solidly filled
• Mortar droppings must be removed from wall ties and insulation edges
• Excess mortar should be struck flush from the inside of the outer leaf
• Excess mortar should be struck flush from the cavity side of the inner leaf

Supporting insulation boards:

• The first row should be supported on wall ties (minimum two ties per board)
• Ties should coincide with horizontal joints in the insulation
• Rigid boards should be stored flat without bearers to prevent warping
• Warped boards should be rejected

When installing fully filled insulation:

• Fibres must be laid parallel to the wall (not perpendicular, which can bridge the cavity)
• Slabs should be butt-jointed, not bent at corners
• Exposed insulation should be covered at day’s end or in driving rain to prevent moisture absorption and expansion pressure on fresh masonry
• Vertical joints should be staggered when layering insulation slabs

At lintels and openings:

• Wall insulation should abut the window/door frame head
• Insulation should be cut and fitted to voids below cavity trays
• No uninsulated gaps should remain around openings

Important restriction: Rigid insulation should not be continuous across the end of a separating wall or floor. A flexible (mineral wool) cavity closer should be used in line with these elements, with rigid insulation butted up to it.

Injected and Blown Fill Insulation

These systems must meet relevant standards (BS EN 14064-1 and BS EN 14064-2 for mineral wool; BS EN 16809-1 and BS EN 16809-2 for EPS beads) and hold satisfactory assessment confirming suitability for masonry cavity walls and the site’s exposure rating.

Pre-installation requirements: Before installation, the home should be ready:

• Cavity wall inspected by the installing team
• Roof in place with tops of walls protected from rain
• All cavity edges at openings closed with permanent cavity closers as per design

Areas that cannot be accessed during filling (such as below gas membranes, low-level continuous DPCs, where cavity trays are one course above lintels, or where separate lintels are used) should be insulated with appropriate built-in materials.

On completion, the installer must provide a declaration of compliance with the relevant product standard.

Construction Types and Exposure

Partial Fill Cavity Insulation:

• Only fixed against the cavity face of the inner leaf
• Minimum 50mm clear cavity maintained between insulation and outer leaf
• In very severe exposure areas in England and Wales with fair-faced masonry, 75mm residual cavity required
• Wall ties must be long enough for minimum 50mm embedment in each leaf

Fully Filled Cavity Insulation:

The suitability of wall construction with fully filled insulation depends on exposure category and protective measures:

Important notes:

• Render on external leaf of F2,S1 or F1,S1 clay bricks in severe or very severe exposure is not permitted with full cavity insulation
• In very severe exposure, fair-faced masonry with full cavity insulation requires minimum 150mm cavity width
• Fair-faced masonry includes clay and concrete bricks/blocks and dressed natural stone with struck, weathered, or bucket handle joints
• Random rubble or random natural stone cavity walls should not be fully filled
• Painted finishes on bricks or render are not acceptable where likely to cause damage

Insulated Dry Linings

Where insulated dry lining contains combustible insulant, the plasterboard should be:

• Minimum 12.5mm thick
• Mechanically fixed to the masonry inner leaf

This prevents early collapse of the lining in a fire.

Indicators of potential problems:

• Condensation or mould growth on internal walls (suggesting cold spots)
• Walls that feel notably colder than others
• Higher than expected heating costs
• Evidence of cavity bridging (mortar droppings on wall ties or insulation edges)

For injected or blown-fill insulation:

• Installation certificates should be available
• Declaration of compliance should be provided
• Installer should be approved under appropriate certification scheme

6.1.8 Concrete Blocks

Concrete blocks shall be capable of supporting intended loads, have appropriate thermal resistance, and be resistant to the adverse effects of climate.

Intended Loads

Concrete blocks should:

• Comply with BS EN 771-3 and 4, PD 6697, and be used according to BS EN 1996-2 and PD 6697
• Be of sufficient compressive strength for the application
• Be used in accordance with manufacturer’s recommendations

Typical strength requirements:

• One and two-storey homes: Blocks with minimum 2.9N/mm² compressive strength could be used
• Three-storey homes or storey heights over 2.7m: 7.3N/mm² blocks required for certain structural parts, unless structural design demonstrates lower strengths are suitable

The maximum load-bearing capacity must not be exceeded. Other factors may dictate higher strength requirements, such as sulfate resistance.

Freeze/Thaw and Sulfate Attack

Concrete blocks used in the outer leaf without protective cladding or render should:

• Have compressive strength >7.3N/mm² or density of at least 1,500kg/m³
• Be made with dense aggregate to BS EN 12620, or
• Be aircrete blocks with manufacturer confirmation of suitability

Where sulfate levels in ground are DS-2 or above at the blockwork level, manufacturer confirmation of suitability is required. Where permissible, sulfate-resisting mortar with appropriate mix should be used.

Other Characteristics

Concrete blocks may be specified according to specific characteristics beyond basic strength. Tolerances should be declared according to the relevant product standard.

After completion, signs of inadequate blockwork might include:

• Cracks in internal plasterboard, particularly at corners or junctions
• Doors or windows becoming misaligned
• Cracks above openings
• General settlement issues

6.1.9 Bricks

Bricks shall be capable of supporting intended loads and have appropriate resistance to the adverse effects of freeze/thaw and sulfate attack. The design strength must comply with BS EN 1996-1 and the structural design.

Clay Bricks Classification

Clay bricks are classified by freeze/thaw resistance and soluble salt content:

When Frost-Resistant Bricks Are Required

F2,S2 or F2,S1 bricks should be used where:

• External facing work in Scotland
• Exposed parts including copings, sills, parapets, and chimneys without protective overhang
• Areas subject to exceptionally severe freeze/thaw exposure

In severe freeze/thaw exposure areas outside Scotland: F1,S1 or F1,S2 bricks may be used for general wall areas if classified in manufacturer’s published recommendations as satisfactory for the exposure.

F0,S2 or F0,S1 bricks: Not acceptable externally unless completely protected by impervious cladding that satisfactorily resists water passage.

Where brickwork may become saturated: F1 bricks are not appropriate if vulnerable to frost. In saturated conditions, sulfate-resisting cement mortar is required for S1 designation bricks.

Strength Requirements

One and two-storey homes: Clay bricks with minimum 9N/mm² compressive strength should be adequate.

Three-storey homes: Clay bricks with minimum 13N/mm² compressive strength are acceptable.

Concrete Bricks

Concrete bricks have a direct relationship between strength and durability, including freeze/thaw resistance:

• Most concrete bricks have 22N/mm² strength, are durable in most situations, and equivalent to F2 frost resistance for clay bricks
• For copings and sills, 36N/mm² compressive strength bricks should be used
• For one, two, or three-storey homes, minimum 22N/mm² concrete bricks are acceptable

Reclaimed Bricks

Reclaimed bricks:

• Should be used in accordance with Technical Requirement R3
• May require independent certification of suitability if durability category cannot be determined
• May be unsuitable for external work due to high salt content or lack of freeze/thaw resistance
• May be unsuitable if previously used internally or were fully protected

It is advisable to know the source of reclaimed bricks and whether they were used internally or externally.

Special Shaped Bricks

Special shaped bricks (clay and concrete) should conform to BS 4729. These include purpose-made bricks for forming curves, features (plinths, cappings), or angles other than 90 degrees (bay windows) that cannot be satisfactorily formed with standard bricks.

Cut and stuck specials may be used where standard specials are unavailable, though they may not be appropriate for buildings with storeys 18m or more above ground level where combustible material use is restricted.

Projecting Brickwork

Where decorative patterns include projecting brickwork exposing brick width or length, consideration should be given to:

• Perforations or frogs that may be exposed
• Visual appearance of exposed bed or differing face surfaces
• Suitable weathering of flat surfaces or ‘ledges’

Exposure of projecting bricks, particularly with frogs or perforations, may affect durability and service life. Written manufacturer confirmation should be obtained that the brick can achieve required durability for the proposed use.

Tolerances of Clay Bricks

Guidance on tolerances can be found in the Brick Development Association publication “Designing to Brickwork Dimensions.”

Installation quality:

• Damaged, chipped, or cracked bricks in facework
• Colour variations suggesting bricks from different batches weren’t properly blended
• Poor bonding or irregular coursing
• Incorrect joint profiles for the exposure category (such as recessed joints in severe exposure)

Longer-term indicators of problems:

• Frost damage (spalling, flaking, surface deterioration)
• Efflorescence (white salt deposits, particularly with S1 bricks in saturated conditions)
• Mortar deterioration (suggesting sulfate attack where S1 bricks used without sulfate-resistant mortar)
• Staining or discolouration

For reclaimed bricks, documentation should confirm their source, previous use, and suitability for external application in your property’s exposure category.

6.1.10 Stone Masonry

Stone masonry shall be constructed to an acceptable standard, including the performance standards for brick and block where applicable. Walls shall be capable of supporting intended loads and have appropriate resistance to freeze/thaw effects.

Standards for Stone Masonry

Stone masonry as the outer leaf of a cavity wall should comply with:

• BS EN 771-6: Natural stone masonry units
• BS EN 771-5 or BS 1217: Cast stone masonry units
• BS EN 1996 and PD 6697: Design of masonry structures
• BS EN 12059: Stone for copings and sills

Acceptability Criteria

Stone masonry as an outer leaf will be acceptable where it:

• Provides adequate weather-resisting structure in conjunction with brick or block backing and/or vertical DPMs
• Complies with the guidance in Chapter 6.1 for brickwork/blockwork
• Complies with BS EN 12370 or has evidence it is not susceptible to salt crystallisation when used below DPC level
• Follows good local recognised practice to provide a high standard

Special Considerations for Stone

Squared or random rubble: When used, it’s important that stone masonry is brought to course at regular intervals of not more than 450mm. This maintains structural integrity and allows for proper integration of other elements like lintels and DPCs.

Bed joints: Joints up to 25mm wide may be acceptable for stone masonry (compared to the typical 10mm for brick), reflecting the natural variation in stone dimensions.

Wall ties: Selection and installation of correct wall ties should be carefully considered, particularly with random rubble where irregular masonry thickness creates challenges for achieving proper tie embedment.

Cast Stone

Cast stone (also called reconstituted stone or manufactured stone) is a masonry unit formed from aggregate, cementitious binder, and other materials moulded under pressure and/or vibration. It’s designed to resemble natural stone.

Cast stone must meet the requirements of BS EN 771-5 or BS 1217 and should be used in accordance with manufacturer’s recommendations regarding:

• Appropriate locations and exposure categories
• Installation requirements
• Treatment of joints
• Support requirements
• DPC and flashing interfaces

Stone masonry work is often more expensive than brick, so quality expectations are correspondingly higher. Natural stone, in particular, should showcase the material’s inherent beauty with skilled craftsmanship. Poor stone masonry work not only affects performance but can significantly impact the property’s appearance and value.

6.1.11 Construction of Masonry Walls

Construction shall ensure a satisfactory standard of brickwork and blockwork. The construction of masonry walls should comply with BS EN 1996-2 (Eurocode 6) and BS 8000-3 (Code of practice for masonry workmanship).

Finished Appearance

The appearance of masonry walls depends on materials used, setting out, and workmanship quality. A site reference panel should be constructed to agree a benchmark for workmanship and products—this becomes the standard against which all subsequent work is measured.

When setting out masonry:

• Avoid cutting bricks or blocks except when essential
• Avoid irregular or broken bonds, particularly at openings
• Fair-faced masonry bricks should be set out dry before work commences

All work should be reasonably level and true:

• The bond detailed in the design should be used
• Perpendicular joints kept in line and plumb
• Courses kept level using lines and spirit levels
• Work should meet tolerances defined in Chapter 9.1 (A consistent approach to finishes)

Practical techniques:

• Use a gauge rod marked with heights of windows, doors, and floors to maintain correct course heights
• For multiple openings of similar width, use a rod cut to the required size to check opening widths as work rises
• Avoid subjecting brickwork and blockwork to vibration until mortar has set

Bonding

A regular bonding pattern should be maintained. External walls should be bonded to partitions and party walls as required by design, either by:

• Toothing every alternate course, or
• Tying with wall ties, expanded metal, or equivalent at maximum 300mm vertical centres

Where joist hangers are not used: Joist filling should be brickwork or blockwork without excessive mortar joints. The recessed portion of timber joists should be treated.

Joist filling requirements:

• 12mm below the top of flat roof joists to allow for timber shrinkage
• Checked to ensure cold roof ventilation is not blocked

Critical rule: Clay bricks and concrete blocks should not be mixed. Where different sized masonry units are needed for correct coursing, small units of the same material should be used to reduce cracking and problems from different thermal properties.

Where the inner leaf provides thermal insulation and different sized units are required for coursing, units should have similar thermal insulation properties to the rest of the wall.

Construction Process

The difference in heights between the two leaves of a cavity wall under construction can be up to six block courses, provided ties are sufficiently flexible to ensure coursing without breaking the bond.

Important: To keep walls plumb, do not overreach at changes of scaffold lift—wait for the next scaffolding level.

For thin layer mortar construction with Technical Requirement R3 assessment, it’s normally permissible to build the inner leaf to storey height ahead of the outer leaf. The assessment recommendations and manufacturer’s recommendations must be followed.

Cavities should be constructed so that:

• They are uniform and in accordance with design, including wall tie specification and cavity width
• Mortar is struck from all joints as work proceeds
• Cavity trays and wall ties are clear of droppings and debris
• Mortar droppings are removed
• Where cavity insulation is used, droppings are removed from the top edge
• Where partial cavity insulation is used, it’s against the inner leaf
• External leaf thickness is consistent (stone should not project into cavity)
• Cavity barriers are installed as work progresses

Board technique for keeping cavities clean: A board can be positioned on wall ties and raised as work proceeds, catching mortar droppings for easy removal.

Laying Bricks and Blocks

Bricks and blocks should have solid mortar bedding and fully filled perpends to reduce rain penetration and dampness risk.

Joint thickness: Bed joints and perpend joints made with general purpose and lightweight mortars should be nominal thickness to suit gauge and masonry coordinating size, not less than 6mm nor more than 15mm unless manufacturer specifies otherwise. Joints should be consistent in width, profile, and alignment.

Frog positioning: Unless otherwise advised by manufacturer:

• Single frog bricks: laid with frog facing upwards
• Double frog bricks: laid with deeper frog facing upwards
• All frogs should be fully filled with mortar

Where cutting bricks is required:

• Use standard work sizes (quarter, half, three-quarter cuts)
• Cut cleanly and accurately
• Avoid cutting facing brickwork with a trowel

Special bricks in accordance with BS 4729 should be used to form curves, features (plinths, cappings), or angles other than 90 degrees (bay windows) that cannot be satisfactorily formed with standard bricks. Cut and stuck specials may be used where standard specials are unavailable, though restrictions apply for buildings with storeys 18m or more above ground level.

Protection During Construction

Masonry cavity walls shall be protected whenever work stops (for inclement weather or overnight). The tops of both leaves, as well as the cavity and any insulation, should be covered with sacking or plastic sheet, appropriately secured in place.

Openings

Masonry may be built around either:

• The frame in-situ, or
• A profile or template enabling frame fitting later

Openings should be:

• The correct size and square
• Allow spacing between masonry and frame for movement and tolerance
• Not distort the frame by forcing bricks against the jamb

When frames are built in, they should be fixed with:

• Frame cramps
• Proprietary cavity closers, or
• Plugs and fixings

Proprietary cavity closers should be fitted in accordance with manufacturer’s instructions, in one continuous piece unless jointing is accepted by manufacturer with suitable installation instructions. The closer must be assessed as suitable for the site’s exposure zone.

Where opening sizes don’t match brickwork setting out: Brick bonding pattern should be set out at wall base to ensure cut bricks occur below openings, maintaining proper bonding patterns around windows and doors.

Corbelling

For feature brickwork sections, masonry should only be self-supporting.

Where courses are corbelled outwards:

• In solid walls: corbelling should not exceed T/3 (where T is wall thickness)
• In cavity walls: corbelling should not exceed T/3 on either leaf (where T is leaf thickness)
• The thickness should not be reduced on the inside face

Where reinforcing is used, corbels should be designed by an engineer in accordance with Technical Requirement R5.

Chasing for Services

Chases for pipes and cables should:

• Not be cut with impact power tools (can damage the wall)
• Not cut into hollow blocks unless specifically permitted by manufacturer
• Be cut with care
• Be limited to 1/6 of leaf depth where horizontal
• Be limited to 1/3 of leaf depth where vertical
• Maintain 15mm residual thickness between chase and void for hollow/cellular blocks unless manufacturer recommends otherwise

Protection of Ancillary Components

Wall ties, tension straps, hangers, lintels, bed joint reinforcement, and windposts should be manufactured from appropriate materials with adequate protection against corrosion.

Material/coating specifications are provided in NHBC Standards Table 4, including:

• Austenitic stainless steel (various grades)
• Zinc coated steel (various coating masses)
• Zinc precoated steel with organic coatings

The level of protection required depends on:

• Component location (outer leaf, inner leaf, internal use)
• Exposure conditions
• Building height
• Contact with damp masonry

Components in contact with or embedded in an inner leaf that is damp or exposed to periodic wetting (eg below DPC) should be protected to the same level as components in the outer leaf.

The construction phase is when most defects are created. A pre-completion inspection allows identification of issues whilst scaffolding is still in place and before finishing materials conceal the structural elements.

6.1.12 Lintels

Lintels and supporting beams shall be installed correctly, safely support applied loads, and be of the type and dimensions appropriate to their position within the structure.

Materials and Standards

Acceptable materials:

• Concrete lintels
• Steel lintels
• Reinforced brickwork lintels

Timber lintels should not be used unless:

• Protected from weather, and
• They do not support masonry or other rigid or brittle materials

Lintels should:

• Comply with BS EN 845-2 (steel or concrete)
• Be designed in accordance with Technical Requirement R5 or manufacturer’s recommendations
• Be provided where frames are not designed to support superimposed loads
• Be wide enough to provide adequate support to walling above
• Not have brickwork or masonry overhanging more than 25mm beyond the lintel
• Have cavity trays where specified in design
• Have padstones and spreaders under bearings where necessary
• Not have concentrated loads applied before manufacturer’s requirement for fully bedded brickwork is met (to avoid overstressing)

Lintel End Bearings

Lintels should extend beyond the opening at each end by the minimum lengths specified:

Where steel lintels are used:

• Manufacturer’s recommendations for fire resistance should be followed, particularly for the lower steel flange
• Inner and outer leaves should be built up together to avoid twisting the lintel flange
• Height difference between leaves should not exceed 225mm

Thermal Insulation and Condensation

The risk of condensation at cold bridges (reveals and soffits) increases with higher wall insulation levels.

To avoid cold bridging:

• Wall insulation should abut the window/door frame head
• Insulation should be provided at the lintel underside unless manufacturer provides an alternative

Durability and Weather Protection

Cavity tray/damp proof protection should be provided:

• Over all openings, either combined as part of the lintel or separate
• Where outer leaf is fair-faced masonry or fully filled insulation is used, all cavity trays (separate or combined) should have stop ends

Separate cavity tray protection is required when:

• Corrosion protection to the lintel is inadequate
• Required by manufacturer
• Lintel shape is unsuitable (profile doesn’t form a cavity tray, or steel lintels have material/coating reference L11, L14, or L16.1)

In Scotland, Northern Ireland, Isle of Man, and severe/very severe exposure areas: A separate cavity tray should be provided over all lintels.

Lintels should be:

• Austenitic stainless steel (material reference L1) in aggressive environments (eg coastal locations)
• Located and sized so external edge projects beyond and protects the window head

Separate Lintels for Inner and Outer Leaves

Where separate lintels support inner and outer leaves:

• The cavity at opening head should be closed with an insulated cavity closer
• A cavity tray should be installed to protect the cavity closer from moisture
• The cavity tray should be built into the inner leaf and taken to the outer face of the external wall directly over the outer lintel (not between cavity closer and lintel)

Placing Lintels

The design should be checked before installation.

Lintels should:

• Be appropriate size for the opening and end bearings
• Have padstones where required (eg for long spans)
• Be installed level on a solid bed of mortar (not soft or non-durable packing)
• Be set out to ensure bearing on a full masonry unit
• Not have brickwork or masonry overhanging more than 25mm

Composite lintels and steel beams should be bedded on mortar to both leaves for even load distribution. Bedding thickness should accommodate unevenness whilst maintaining coursing.

Concrete floor units or heavy components bearing on lintels should be positioned carefully to avoid damage or shock loading.

Lintel defects are significant because they affect structural integrity, weather protection, and thermal performance. Many lintel issues only become apparent after several months or years, particularly those related to inadequate weather protection causing moisture penetration or rust staining below windows.

6.1.13 Materials Suitable for Mortar

Materials used for mortar should comply with appropriate requirements and standards.

Relevant Standards for Mortar Materials

Cement:

• BS EN 197-1: Cement—Composition, specifications and conformity criteria

Masonry cement:

• BS EN 413-1: Masonry cement—Composition, specifications and conformity criteria

Building lime:

• BS EN 459-1: Building lime—Definitions, specifications and conformity criteria

Mortar:

• BS EN 998-2: Specification for mortar for masonry—Masonry mortar

Admixtures:

• BS EN 934-3: Admixtures for concrete, mortar and grout—Admixtures for masonry mortar

Pigments:

• BS EN 12878: Pigments for colouring building materials based on cement and/or lime

Aggregates:

• BS EN 13139: Aggregates for mortar

If mortar-related defects become apparent (cracking, poor adhesion, frost damage, sulfate attack), investigation may need to confirm whether appropriate materials meeting these standards were used. Documentation should be available showing material compliance.

6.1.14 Mortar

Mortar shall be of the mix proportions necessary to achieve adequate strength and durability, and be suitable for the type of masonry.

Mortar Mix Proportions

Unless otherwise recommended by brick manufacturer, the following mixes should be used for clay bricks:

Air-entraining plasticiser can be incorporated in 1:1:5½ or 1:½:4½ cement:lime:sand mortars.

For concrete bricks: Particular attention should be paid to manufacturer’s recommendations.

Retarded Mortar

Retarded mortar and most premixed mortars can be used over longer periods than site-mixed cement:lime:sand mortars.

When using retarded mortar:

• Follow manufacturer’s recommendations and timescales
• Do not use beyond the time for which it’s effective
• Protect against freezing prior to use
• Temporary bracing of larger walls may be necessary due to delayed setting times

Sources of Sulfate

Mortar is vulnerable to sulfate deterioration, especially when masonry is saturated for long periods. Clay bricks contain soluble sulfates (S1 designations have no limit on sulfate content).

Cement with sulfate-resisting properties (or CEM II cements based on blast-furnace slag) should be used:

• Below DPC level when sulfates are present in ground
• When clay bricks (F2,S1 and F1,S1) are used
• When high saturation risk exists

High saturation risk situations:

• Below the DPC
• Areas of severe or very severe exposure to driving rain
• Parapets
• Retaining walls
• Freestanding walls
• Rendered walls
• Chimney stacks

Admixtures and Additives

Admixtures should:

• Only be used where agreed with designer
• Not contain calcium chloride
• Be dosed and used according to manufacturer’s recommendations

Mortars containing air-entraining plasticiser are more resistant to freeze/thaw damage when set but do not prevent freezing before curing.

White cement (BS EN 197) and pigments (BS EN 12878) may be used, but:

• Pigments should not exceed 10% of cement weight
• Pigments should not exceed 3% where carbon black is used

Preparing Mortar

When preparing mortar:

• Ensure mix is appropriate for use and location
• Keep plant and banker boards clean
• Keep mixers clean to operate efficiently
• Ensure colour is consistent

When laying bricks and blocks:

• Mortar should be correct mix and used within two hours (unless retarded mortar)
• Mortar that has started to set should not be retempered

Note: Thin layer mortars are supplied in bag form, mixed with water on site, strictly following manufacturer’s recommendations.

Joints

Jointing is preferable to pointing because it leaves mortar undisturbed. Struck (weathered) and bucket handle joints are preferable for external walls. Unless design states otherwise, only bucket handle or weathered joints should be used.

Recessed joints should not be used where:

• Bricks are not frost-resistant (F1,S1 or F1,S2) unless manufacturer confirms suitability in writing
• Home is built on steep sloping ground, facing open countryside, or within 8km of coast/large estuary
• Bricks are perforated closer than 15mm to face
• No reasonable shelter from driving rain exists
• Cavity is to be fully filled with insulation

Mortar defects may not be immediately apparent but can develop over months or years, particularly where inappropriate mixes were used for the exposure conditions or brick type. Early identification allows rectification before long-term damage occurs.

6.1.15 Render

The surface to which render is applied shall be appropriately constructed and satisfactorily resist the passage of moisture.

Walls to be rendered should be constructed in accordance with the relevant parts of Chapter 6.1. For detailed guidance on render systems, application, materials, and performance requirements, see Chapter 6.11 Render.

Key Points for Rendered Masonry Walls

Substrate preparation:

• Masonry should provide suitable substrate for render adhesion
• Surface should be clean, sound, and free from contaminants
• Appropriate key or mechanical bonding should be available

Moisture resistance:

• Render provides additional weather protection to masonry
• Substrate design should still resist moisture penetration
• DPCs and cavity trays remain essential
• Render should not be relied upon as sole moisture barrier

Compatibility:

• Render mix should be compatible with substrate material
• Movement joints in substrate should be reflected in render
• Crack-resistant backing materials recommended in exposed locations

Specific considerations for fully filled cavity walls:

• Render on external leaf of F2,S1 or F1,S1 clay bricks in severe or very severe exposure is not permitted with full cavity insulation
• Render provides additional protection but increases moisture retention risk if substrate becomes saturated

For comprehensive guidance on render specifications, application methods, and quality standards, Chapter 6.11 of the NHBC Standards should be consulted, as this provides extensive detail beyond the scope of the masonry chapter.

6.1.16 Cladding

Cladding shall satisfactorily resist the passage of moisture and be of the quality, type, and dimensions required by the design.

This section addresses low-rise applications where masonry is used directly for weathertightness or in conjunction with other vertical cladding. For curtain walling and other structural cladding systems, see Chapter 6.9 Curtain Walling and Cladding.

Important: Where external claddings such as fibre cement, timber, or tile hanging are installed on masonry walls, cavity barriers should be provided:

• At cavity edges, including eaves and verges
• Around openings such as windows and doors
• At entry/exit points for services
• At junctions between external cavity walls and every compartment floor and wall

Masonry Cladding to Framed Structures

Where masonry forms cladding to framed structures (rather than being load-bearing), allowance must be made for differential movement between cladding and frame.

Precautions to prevent buckling and fracturing:

Movement joints: Flexible horizontal movement joints should be provided at the underside of each horizontal support member to accommodate:

• Deflection of substrate and angle support system
• Limited compressibility of joint fillers and sealant
• At least 1mm movement per continuous metre of vertical clay masonry
• All movements anticipated at the joint

Support: The masonry outer leaf should have minimum two-thirds of its width supported securely by concrete frame or metal angle.

Vertical movement joints: Should be provided at corners where appropriate.

Inner leaf connection: The inner leaf should be adequately tied to the structural frame.

Support system components:

• Shelf angles supporting masonry
• Brackets connecting to frame
• High-density compressible filler at joints
• Flexible shims for adjustment
• Sealant at exposed joints
• Weepholes for drainage

Joints Between Claddings

Joints between claddings and adjacent materials should:

• Be detailed to be watertight under the site’s particular exposure conditions
• Have provision for differential movement where necessary

Materials for Cladding

Tiles and slates:

• BS EN 1304 (clay tiles for tile hanging)
• BS EN 490 (concrete tiles for tile hanging)
• BS EN 12326-1 (slates for vertical slating)

Timber boarding:

• BS EN 942
• Timber should comply with and be at least class J50
• Should be naturally durable species or pre-treated with preservative
• Battens should be pre-treated with preservative and of size specified in design
• See Chapter 3.3 for timber preservation requirements

Proprietary cladding systems:

• Must have satisfactory assessment meeting Technical Requirement R3

Vertical Tile or Slate Cladding

Vertical tile or slate cladding to walls should:

• Conform with Chapter 6.9 Curtain Walling and Cladding
• Be fixed in accordance with manufacturer’s recommendations

Prefabricated Lightweight Brick Clad Arches

These decorative non-load-bearing arches comprise cement particle or fibre cement boards, injected polyurethane core, and clay fired brick slips attached with epoxy adhesive.

Requirements:

• Must hold satisfactory assessment by appropriate technical approvals authority accepted by NHBC
• Should be supported by load-bearing lintel
• Must be installed per manufacturer’s instructions
• Should include cavity tray over all openings
• Where manufacturer requires cavity tray over arch, a lintel with suitable profile and durability plus proprietary stop ends and weepholes should be provided

Stone Veneer Cladding Systems

Stone veneer cladding systems should comply with:

• BS 8298 when mechanically fixed
• Technical Requirement R3 when adhesive fixed
• Chapter 6.9 when used as brick slip/rainscreen system

Cladding defects may not be immediately obvious but can lead to significant moisture problems if water penetrates behind the cladding system and becomes trapped against the masonry substrate.

6.1.17 DPCs and Cavity Trays

DPCs and related components shall be provided to prevent moisture rising or entering the building. This is one of the most critical aspects of masonry construction, as failure of DPC systems leads to dampness, which is a primary cause of defects in new build homes.

Acceptable Materials

Bitumen-based materials: BS 6398, BS EN 14967

Polyethylene: BS 6515, BS EN 14909 (except as cavity trays in walls, below copings, and in parapets)

Thermoplastics and Elastomers: BS EN 14909

Proprietary materials: Must meet Technical Requirement R3

High bond DPC: Can be used for applications including parapet walls and beneath copings/cappings.

High load DPC: Should be used where subject to over three storeys of masonry.

Positions Where DPCs and Cavity Trays Are Required

Provision of DPCs and Cavity Trays

DPCs and flexible cavity trays should:

• Be of correct dimensions to suit detailed design
• Be supported over cavity to prevent sagging (below copings)
• Be fully bedded in mortar
• Be specified to achieve good key with mortar
• Be sealed to prevent water seeping through joints (in parapets)

Horizontal DPCs should:

• Be correct width
• Lap the DPM where appropriate
• Be laid on wet, even bed of mortar free from projections
• Have masonry above bedded on wet mortar (sandwiching the DPC)
• At ground level, be minimum 150mm above finished ground or paving
• Be lapped minimum 100mm at joints, or be sealed/welded (where preventing rising damp)
• Be considered in design of masonry wall panel
• Be used per manufacturer’s recommendations

Concrete fill in cavity walls should stop at least 225mm below base DPC (may be reduced to 150mm for special foundations like rafts).

At stepped sites: Where DPC steps, horizontal sections should be minimum 150mm, with vertical steps also minimum 150mm.

At sills: Where jointed or permeable sill is used:

• DPC should be placed between sill and outer leaf
• DPC should be turned up at back and ends of sill
• Where DPC is provided, it should lap with reveal DPC
• Where no DPC, vertical DPC should continue 150mm below sill level

Cavity Trays

Cavity trays should be provided at all interruptions to the cavity (window and door openings, air bricks) unless otherwise protected (eg by overhanging eaves).

Cavity trays must:

• Meet BS EN 14909 requirements
• Hold certification confirming suitability as cavity tray
• Provide impervious barrier ensuring water drains outwards
• Always be provided with stop ends where discontinuous
• Project sufficiently beyond lintel ends and cavity face of cavity closer/vertical DPC
• Form stop end in nearest naturally occurring perpend joint
• Be laid on wet, even bed of mortar
• Have masonry above bedded on wet mortar
• Provide drip protection to door and window heads
• Have 140mm minimum upstand from inside face of outer leaf to outside of inner leaf
• Not be low-density polyethylene (LDPE) to BS 6515
• Be shaped to provide 100mm minimum vertical protection above mortar dropping collection points
• Be provided where cavity is bridged by air bricks (extending 150mm beyond each side)
• Be provided over meter boxes where not otherwise protected
• Be in one continuous piece or have sealed/welded joints with rigid support, lapped minimum 100mm
• Be preformed where used at complicated junctions
• Be used per manufacturer’s recommendations

Where fair-faced masonry or fully filled insulation is used: All cavity trays (separate or combined) should have stop ends.

Stop ends should be:

• Sufficient height to contain water and discharge safely
• Height of a full brick perpend
• Formed by turning up flexible sheet material, OR
• Proprietary plastic stop ends adhered to lintel surface per manufacturer’s instructions (where lintel has necessary corrosion protection and profile)

Upstand requirements: The upstand part of cavity trays should be returned into inner leaf unless stiff enough to stand against inner leaf without support. In Scotland, Northern Ireland, Isle of Man, and very severe exposure areas, the upstand must be returned into inner leaf (doesn’t apply at sloping abutments).

Weepholes in cavity walls should be equivalent of full brick perpend joint (65mm x 10mm where exposed in cavity). Proprietary weepholes may have smaller discharge openings if designed to discharge collected water safely.

Weephole requirements:

• Cavity trays above openings and penetrations: at least two per opening at not more than 450mm centres
• At least one to bottom tray in series of stepped cavity trays
• Cavity trays in parapet walls or horizontal roof abutments: not more than 1m centres
• End within cavity should be kept clear of mortar droppings

Complicated Junctions

Changes of direction or interfaces with other cavity elements are more complex than simple joints and require preformed cavity trays. Clear drawings should be provided.

Complicated junctions include:

• Pitched roof abutments (stepped trays)
• Steps in horizontal level
• Internal corners
• External corners
• T-junctions (intersection of parapet wall)
• Door thresholds (with gas membranes, flat roofing, etc)
• Penetrations in horizontal cavity tray arrangements
• Interfaces with windposts, balcony supports, balustrading

Site folding of internal/external corners may be permitted where installation quality is highest standard and follows manufacturer’s recommendations.

Where preformed cavity trays are used, joints with flexible DPC cavity trays should be sealed per manufacturer’s recommendations.

Vertical DPCs

A separate vertical DPC should be provided around openings, extending to lintel underside, and be either:

• Proprietary material assessed per Technical Requirement R3, or
• 150mm wide DPC material nailed to full frame height, protruding 25mm into cavity

A fillet joint of sealant should not substitute for good workmanship or DPCs, but a bead of mastic should be used around openings.

Cavity Trays and Insulation

Where fully filled or partial fill insulation is installed, particular care ensures insulation continuity is maintained around cavity trays.

Connections with Flashings

Where flashings link with DPCs (horizontal or preformed stepped cavity trays), 25mm of mortar below DPC should be raked out as work proceeds to allow flashing to be tucked in.

Joints between masonry and flashing should be pointed with cement mortar or suitable exterior grade sealant (polysulfide or neutral-cured silicone) per manufacturer’s recommendations.

Arches

At openings with arched heads or circular windows, it’s not practical to shape flexible DPC and achieve correct protection—preformed cavity trays should be used.

Stepped Cavity Trays

Where roofs abut walls at angles, preformed stepped cavity trays should be provided.

To minimise water ingress risk below abutments:

• Preformed stepped cavity trays should be provided where roof abuts cavity wall above enclosed area
• Should have two stop ends at lowest cavity tray
• Should have weephole to drain water from cavity
• Are not necessary where roof is not over enclosed area (open carport, open porch)

Installation requirements:

• Install per manufacturer’s recommendations
• Position to suit flashing dimension (minimum 65mm width)
• Position so stepped cavity tray cannot discharge behind flashing
• Where cutting bricks/blocks is necessary, maintain bond in following joint

Parapet Details

Parapet walls should have:

• DPC under coping
• DPC tray starting 150mm minimum above roof
• Coping throating 30mm clear of brickwork
• Copings complying with BS 5642 Parts 1 and 2

DPCs in parapets should be:

• Supported over cavity to prevent sagging below copings
• Fully bedded in mortar
• Specified to achieve good key with mortar
• Sealed to prevent water seeping through joints

Copings should have:

• Overhang beyond wall face
• Throatings minimum 30mm clear of wall
• Continuous supported DPC projecting beyond wall line

DPC and cavity tray systems are mostly hidden once construction is complete, making pre-completion inspection particularly valuable. Once walls are built and finished, rectifying DPC defects is extremely difficult and expensive.

6.1.18 Wall Ties, Bed Joint Reinforcements and Windposts

Wall ties, bed joint reinforcements, and windposts of the correct type shall be installed where required, and be suitable for their intended use and location.

Wall Ties

Wall ties should:

• Be in accordance with BS EN 845-1 or Technical Requirement R3
• Be of the type specified in design
• Be long enough to be embedded minimum 50mm into each leaf
• Be stainless steel or non-ferrous materials per Technical Requirement R3
• Be spaced above and below DPC per Table 9 (see below)
• Be of type/classification appropriate for end use (including cavity width) and geographical location
• Be specified to accommodate movement where required by design
• Be used per manufacturer’s recommendations

Bed Joint Reinforcements

Bed joint reinforcements should:

• Be in accordance with BS EN 845-3 or Technical Requirement R3
• Be of type specified in design
• Be sufficiently wide so minimum 20mm cover is provided from external masonry face
• Be stainless steel or non-ferrous materials per Technical Requirement R3
• Have minimum 225mm lap length (laps between lengths always staggered)
• Be used strictly per manufacturer’s recommendations

Windposts

Windposts should:

• Be designed by engineer per Technical Requirement R5
• Comply with Clause 5.12 and Table 2 of PD 6697:2019
• Be austenitic stainless steel (material reference 1 or 3) where connected to or embedded in outer leaf of external cavity wall in buildings exceeding three storeys
• Be austenitic stainless steel (material reference 1) in aggressive environments (eg coastal locations)

Spacing of Wall Ties

Notes:

• Vertical spacing and number of ties may need adjusting to produce equivalent number when using insulation boards
• Vertical spacing and number of ties may need adjusting when using studded or spandrel panels

Preventing Water Crossing the Cavity

Care should be taken to avoid:

• Ties sloping down to inner leaf
• Drips being off-centre
• Ties having mortar droppings on them

Cavity walls should be coursed so wall ties are level or slope outwards.

Wall ties should be:

• Built in (not pushed into joints)
• Sufficient length to achieve minimum 50mm embedment in each leaf, allowing for normal cavity width tolerances
• Positioned so drip is centred in clear cavity and faces downwards

Ties for Partial Fill Insulation

Where partial cavity fill insulation is used, it should be held against inner leaf by retaining devices, which may be clipped to wall ties.

Retaining devices should be:

• Compatible with wall ties
• Used per Technical Requirement R3

Where 1,200mm boards are used with partial fill cavities:

• Wall ties should be spaced closer to provide adequate support and restraint
• Ties should be spaced at 600mm centres in rows (not staggered)

Cavity Widths Over 100mm

Where cavity width is 100-150mm, wall tie spacing in Table 9 may still be used in dwellings up to three storeys high in sheltered and moderate exposure locations, provided ties are correct length with 50mm minimum embedment.

Where dwellings are exposed to severe and very severe winds (including exposed and elevated locations over 150m above sea level and coastal locations), site-specific assessment of wall tie requirements should be undertaken. Table 9 spacing may still be acceptable if used with stiffer wall tie types (Type 1 or 2 per PD 6697).

Wall ties are largely hidden once construction is complete, making inspection during construction crucial. Wall tie failure typically develops over many years but can lead to collapse of the outer leaf if not addressed.

6.1.19 Handling Materials

Materials shall be handled in such a way as to ensure that the construction is neat, clean, and undamaged upon completion.

Storage and Handling

Deliveries:

• Should be undertaken safely to protect operatives and materials
• Should only use pallets provided by manufacturer
• Stacks of bricks and blocks should be protected from rain and mud splashes by covering with waterproof covers
• Suitable level and safe place should be identified on site for masonry deliveries

Cement:

• Should be stored off ground
• Should be protected from weather

Sand:

• Should be prevented from spreading
• Should be protected to remain clean

Insulation materials:

• Should be handled and stored per manufacturer’s instructions
• Normally should be protected from weather

Handling During Construction

Materials should be handled with care during construction to avoid damage and staining.

Chipped or fractured bricks are not acceptable for facework.

Bricks tipped on delivery or moved about site in dumper trucks often have high wastage degree.

Unloading of all bricks and blocks, especially facing bricks, should be:

• By mechanical means
• Directly onto firm level surface

Unless bricks have been blended by manufacturer: Bricks from different batches should be mixed to avoid colour patching.

To reduce efflorescence risk: Newly erected masonry should be covered. This also prevents mortar being washed from joints and stops masonry becoming saturated.

Bricks and blocks that become excessively wet can suffer from:

• Staining and efflorescence
• Increased drying shrinkage with greater cracking risk
• Lack of mortar adhesion to mud-stained surfaces

The workplace should be kept clean to reduce mortar splashes to minimum. Any accidental mortar smears should be lightly brushed off face after mortar has taken initial set.

Good site housekeeping and careful material handling are signs of a quality-conscious builder. Conversely, poor material handling and storage often correlate with poor workmanship in other areas.

6.1.20 Protection of the Works During Construction

Precautions shall be taken to protect walls from damage during construction.

Cold Weather Working

Freshly laid mortar may fail where it freezes. Air-entraining agents give better frost resistance to set mortar but do not aid setting. Accelerating admixtures and other admixtures should not:

• Be relied on as anti-freeze precaution
• Contain calcium chloride

Setting times of additives should be checked and adhered to per manufacturer’s recommendations. Cold weather retarders increase setting times.

In cold weather:

• Brickwork and blockwork should not be built when air temperature is below 3°C and falling
• Work can resume when temperature is 1°C and rising, with expectation it will exceed 3°C
• Walls should be protected from frost until mortar has set sufficiently to resist frost damage
• Covers should be provided to form still air space to insulate wall
• Walling damaged by frost will not regain strength and should be taken down and rebuilt when conditions improve

Note: Thin joint mortars successfully tested for use down to 0°C are acceptable when temperature is 0°C and rising, used strictly per manufacturer’s instructions and Chapter 3.2 Cold Weather Working.

Hot Weather Working

In very hot weather above 30°C, main concern is rate that water is removed from mortar by suction of warm masonry or evaporation. Mortar also tends to lose plasticity faster due to water evaporation.

Mortar mixed at high temperatures may have higher water content, lower air content, and shorter board life. The bond between mortar and brick/block depends on correct water amount, so this bond may be affected.

To reduce impact of higher temperatures:

• Store bricks and blocks in shade to control heat gain
• Spraying with modest amounts of clean water can keep temperature down and stop suction (do not soak)
• Mixing equipment can be shaded from direct sunlight prior to use
• Mortar tubs and boards should be rinsed with cool water before contacting mortar
• Where ready-to-use mortar is stored on site, keep well covered in tub
• Dry silo mortar offers advantage of mixing small batches used quickly

In dry hot weather: Absorbent clay masonry units may be wetted by lightly spraying to reduce suction. Care should be taken not to over-wet. Low absorption units (engineering bricks) should not be wetted.

Newly built masonry should be protected with suitable material (hessian or sheeting) to insulate and prevent drying too quickly. Hessian should be laid dry (not wetted).

Excessive Rain Working

Bricks or blocks should not be laid in excessive wet conditions.

New brickwork and blockwork should be completely covered to protect from elements.

If brickwork and blockwork is exposed to water for prolonged periods, risk of leaching, cement residues, and efflorescence increases.

Protection of Cavity Walls During Construction

Masonry cavity walls should be protected whenever work stops (for inclement weather or overnight). Tops of both leaves, as well as cavity and any insulation, should be covered with sacking or plastic sheet and appropriately secured in place.

Weather-related defects during construction can cause problems that persist throughout the building’s life. Proper protection is not just good practice—it’s essential for achieving the specified durability and performance.

6.1.21 Further Information

For additional guidance on external masonry walls, the following resources provide comprehensive technical information:

BS 8215 — Code of Practice for Design and Installation of Damp-Proof Courses in Masonry Construction This standard provides detailed guidance on DPC materials, positioning, installation methods, and performance requirements. It’s the primary reference for ensuring moisture protection in masonry structures.

PD 6697 — Recommendations for the Design of Masonry Structures to BS EN 1996-1-1 and BS EN 1996-2 This published document provides recommendations for implementing Eurocode 6 (the design standard for masonry structures) in the UK. It includes:

• Structural design calculations and methods
• Material specifications and properties
• Wall tie requirements and spacing
• Lateral restraint provisions
• Movement joint design
• Corrosion protection specifications

These documents are referenced throughout the NHBC Standards and provide the technical foundation for masonry construction requirements.