If you’re purchasing a new build home with flat roofs, terraces, or balconies, it’s essential to understand the NHBC standards that govern their construction. These standards, detailed in Chapter 7.1 of the NHBC Technical Requirements, ensure your home’s flat roof elements are built to last and perform effectively. This guide explains what these standards mean for you as a homeowner.

7.1.1 Compliance

The NHBC standards establish that all flat roofs, terraces and balconies must comply with the Technical Requirements to be acceptable. This isn’t just about ticking boxes – these requirements are designed to ensure your roof will protect your home from water ingress, provide adequate insulation, and meet fire safety regulations.

Your builder must follow established British Standards and codes of practice, including BS 6229 for flat roofs with flexible waterproof coverings and BS 8579 for balcony and terrace design. These standards have been developed over many years and represent best practice in the construction industry.

If your home is above another property, the flat roof must also provide satisfactory acoustic performance to prevent noise transfer between dwellings. Where applicable, flat roofs, balconies and terraces must meet relevant fire protection requirements in accordance with Building Regulations.

7.1.2 Provision of Information

Before construction begins, detailed designs and specifications must be produced and distributed to all relevant parties, including NHBC, site supervisors, and specialist contractors. This comprehensive documentation ensures everyone involved in building your home understands exactly what’s required

The documentation must include:

Structural and drainage details:

• Complete design showing how water will drain effectively with no back falls
• Extent and direction of falls, with outlet positioning
• Analysis of roof deflection for medium to large or complex roofs

Construction specifications:

• Sections showing the construction layers and how slopes are formed
• Size, specification and position of components including insulation and waterproofing
• Details of fixings, their frequency and installation method
• Method of ventilating voids where ventilation is required

Critical junction details:

• How different elements connect at vulnerable points
• Balcony support and safety barrier specifications
• Survey requirements and deck preparation before waterproofing

Proper documentation reduces the risk of errors, omissions, or misunderstandings during construction.

7.1.3 Flat Roof, Terrace and Balcony General Design

Your flat roof, terrace, or balcony should be designed using one of several established construction methods, each suited to different situations and performance requirements.

In warm roof construction, the insulation sits directly below the waterproofing layer, keeping the structural deck at a similar temperature to the building interior. This design eliminates the need for ventilation and is often the preferred option because it reduces the risk of condensation problems.

Cold roof construction places insulation below the deck with a ventilated void between the deck and waterproofing membrane. This design requires careful ventilation to prevent condensation problems and is more complex to detail correctly, making it less commonly used in modern construction.

In an inverted warm roof, the insulation is placed above the waterproofing layer and protected by ballast, paving, or other surface treatments. This protects the waterproofing membrane from temperature extremes, UV damage, and mechanical damage, potentially extending its lifespan.

Uninsulated Construction: Used for some balconies and terraces where thermal insulation isn’t required, though effective waterproofing remains essential for weather protection.

Green Roofs: These sustainable systems can be extensive (lightweight systems with sedum or wildflowers requiring minimal maintenance) or intensive (heavier systems supporting shrubs, trees, and amenity spaces requiring regular garden maintenance). Both require root barriers and specialised drainage systems.

Blue Roofs: Designed for temporary rainwater storage to reduce flood risk, these systems use void formers and flow restrictors to control drainage rates whilst preventing permanent water retention on the roof.

7.1.4 Drainage

Effective drainage is perhaps the most crucial aspect of flat roof performance. Poor drainage leads to ponding water, which can cause premature failure of waterproofing membranes, structural problems, and water ingress into your home.

Adequate outlet provision: All roof areas must have sufficient drainage outlets sized to handle expected rainfall intensity in accordance with British Standards. The system must account for the cumulative effect of water from multiple roof levels draining onto lower areas.

Overflow protection: Systems must include overflows to prevent flooding if primary outlets become blocked. Overflows should be visible when operating, positioned to discharge safely away from the building, and have higher capacity than the primary outlets.

Proper outlet positioning: Outlets should be positioned at the lowest points in the roof, recessed to facilitate free water flow, accessible for maintenance, and insulated if passing through habitable areas.

Your roof must be constructed with appropriate falls (slopes) to direct water to drainage outlets without creating areas where water can collect.

Design falls versus finished falls: Roofs are designed with steeper falls than the minimum required to allow for structural deflection and construction tolerances. The design fall is typically twice the minimum finished fall.

Different requirements for different systems: Membrane waterproofing requires minimum 1:80 finished fall, while metal roofing systems may require steeper gradients depending on the profile and fixing method.

Methods of creating falls: Falls can be formed using sloped structural elements, tapered insulation, firring pieces, or applied screeds. The method chosen affects both performance and cost.

Special Drainage Considerations

Balcony drainage: Must prevent water cascading from upper levels onto lower balconies or building entrances. This is particularly important in multi-storey developments where water runoff can progressively increase as it flows downward.

Free-draining balconies: Where balconies drain over their edges, ground-level drainage must be designed to handle the cumulative water discharge and prevent ponding at entrances or access areas.

Zero fall restrictions: Zero fall roofs (completely flat) are not acceptable for exposed waterproofing systems due to the risk of ponding and premature failure.

Deck Survey Requirements

Before waterproofing installation, a formal survey must verify that the structural deck achieves the required falls without back-falls or ponding areas. Any deficiencies must be corrected, and additional outlets may be required at remaining low points.

7.1.5 Flat Roof, Terrace and Balcony Structural Design

The structural design must safely support and transmit all loads to the main building structure whilst maintaining serviceability throughout the building’s design life.

Dead loads: The permanent weight of the roof construction including waterproofing, insulation, ballast, paving, soil (for green roofs), and any permanent fixtures.

Imposed loads: Including maintenance access loads, anticipated user loads for terraces and balconies, and allowance for construction loads during building.

Environmental loads: Snow loads calculated according to location and altitude, and wind loads including both downward pressure and uplift forces.

Special load considerations: Roof gardens require higher load allowances, and blue roofs must account for temporary water loading.

Deflection limits: Structural deflection must be limited to maintain effective drainage and prevent damage to waterproofing systems. Both initial deflection under load and long-term creep deflection must be considered.

Movement accommodation: Large roofs require movement joints to prevent cracking from thermal expansion and structural settlement. These joints must be continuous through the waterproofing system.

Drainage coordination: Structural analysis should coordinate with drainage design to ensure outlets are positioned at points of maximum deflection where possible.

Wind Resistance

Uplift resistance: The roof system must resist wind uplift either through self-weight or mechanical anchoring to the structure. Holding-down straps may be required at maximum 2m centres around perimeters.

Edge details: Particular attention is needed at roof edges and corners where wind forces are concentrated and uplift forces are highest.

Structural Quality Requirements

Professional design: Structural design must be carried out by qualified engineers in accordance with relevant British Standards and Technical Requirement R5.

Material standards: All structural elements must meet specified standards and be appropriate for their intended use and exposure conditions.

Construction accuracy: Supporting steelwork and joists must be square, true, and free from twists or sagging that could affect the roof build-up or drainage performance.

Lateral restraint: Where structural elements provide lateral restraint to walls, minimum bearing requirements must be met, or alternative restraint systems provided.

Quality Control

Crack control: Adequate measures must be taken to prevent cracking in concrete elements that could damage directly applied waterproofing systems.

Dimensional stability: Materials and details must accommodate normal structural movement without compromising the waterproofing system integrity.

7.1.6 Timber Structure and Deck

Structural Requirements for Timber Construction

Timber flat roofs, balconies and terraces must be adequately strong and durable whilst providing a suitable base for waterproofing systems. However, the NHBC standards place important restrictions on timber use in balconies due to durability and safety concerns.

Timber Limitations in Balconies

For balcony construction, timber is restricted to elements supported by other materials. Timber must not be used for critical structural elements including gallows brackets, support posts or columns, guardrails and their supports, infill joists, or cantilevered structural elements. These restrictions exist because timber in external environments faces increased moisture exposure and potential decay, which could compromise structural safety.

Timber Quality and Treatment

All timber used must be properly treated and stored. Requirements include checking for conformity with design specifications upon delivery, rejecting wet, damaged or poor-quality timber, and storing timber under cover to prevent wetting whilst avoiding condensation problems.

Timber must be preservative treated or naturally durable in accordance with NHBC standards, with cut edges re-treated using coloured preservative. Temporary covering is required to prevent wetting unless waterproofing installation follows immediately.

Structural Standards

Timber structure must comply with relevant British Standards, use regularised timber that is dry graded and marked appropriately, and follow approved load-span tables. Joists must be properly sized, spaced at maximum 600mm centres, installed level, and supported on hard packing where necessary.

Decking Materials

Timber decks must use approved materials including plywood to BS EN 636 (minimum 15-18mm thickness depending on joist spacing), oriented strand board to BS EN 300 type OSB3, or pre-treated tongue and grooved timber planking with maximum 100mm width and appropriate moisture content.

Installation Requirements

Joist hangers and straps: Must be correctly sized for the timber, fixed in accordance with design requirements, and protected against corrosion. Where holding-down straps are required, they must be positioned at maximum 2m centres with specific fixing requirements.

Strutting: Required to prevent excessive movement, using herringbone, solid blocking, or proprietary systems. Spacing depends on joist span, with strutting needed at span centres for joists over 2.5m.

Decking installation: Must be carried out in dry conditions with materials protected from weather, boards laid flush with minimal deviation, and all fixings punched below surface level.

7.1.7 Concrete Decks

Concrete Construction Requirements

Concrete flat roofs and balconies must form a satisfactory base for waterproofing systems whilst achieving required strength, durability and surface quality. The construction process requires careful attention to formwork, curing and surface preparation.

In-Situ Concrete Requirements

In-situ reinforced concrete decks must use low shrinkage concrete mixes, have accurately constructed and adequately supported formwork, and achieve an even surface suitable for the selected waterproofing system. For adhesive-bonded membranes, the surface should be slightly roughened through wooden floating or light brushing.

Concrete must be protected during curing and must not contain additives that could affect waterproofing adhesion. Surface treatments applied to assist curing can adversely affect membrane bonding and should be checked for compatibility.

Precast Concrete Elements

Precast concrete decks require minimum 90mm bearing unless specifically designed otherwise, allowance for anti-crack reinforcement to prevent differential movement, movement joints approximately every 15m and at abutments, installation to provide even surfaces without back falls, and proper grouting as specified.

Screed Applications

Where structural falls haven’t been formed in the concrete, screeds may be used. Sand/cement screeds must be free from surface irregularities, laid on properly prepared surfaces, finished with wooden floats for smooth surfaces, and achieve minimum thicknesses depending on the bonding method (40mm minimum for bonded, 70mm for unbonded).

Lightweight screeds require specialist installation with 15mm sand/cement toppings. The compatibility of any levelling materials with the intended waterproofing system must be confirmed.

Drying Requirements

Critical to waterproofing success is ensuring adequate concrete drying before membrane installation. Permanent waterproofing must not be installed until the concrete has cured and dried sufficiently to prevent moisture damage and ensure proper adhesion.

For adhesively bonded systems, bond testing should verify adequate drying, typically around 28 days or according to manufacturer recommendations. Permanent metal shuttering significantly prolongs drying times, though perforated shuttering and mechanical dehumidification can assist.

7.1.8 Profiled Self-Supporting Metal Roof Decks

Metal Deck Construction

Profiled self-supporting metal decks provide both the structural support and base for flat roof construction. These systems require careful design and installation to ensure structural adequacy and compatibility with insulation and waterproofing systems.

Structural Performance Requirements

Metal decks must achieve required strength and durability, be checked for design conformity upon delivery, comply with manufacturer’s load and span tables with appropriate safety factors, and conform to relevant British Standards for steel or aluminium construction.

The decks must be adequately protected from construction loads, properly stored to prevent damage, and fixed with side laps stitched to ensure performance as a continuous structural plane.

Material and Profile Standards

Profiled metal decking should be galvanised steel (typically 0.7-1.2mm gauge) or aluminium (minimum 0.9mm gauge) used in accordance with relevant structural standards. The profile must have adequate crown width – at least 45% for bonded systems and 40% for mechanically fixed systems – and suitable quality finish before roof build-up installation.

Roof Build-Up Requirements

Metal deck systems must use warm roof or inverted warm roof designs, with drainage falls formed either by sloped installation or tapered insulation. The insulation must have adequate compressive strength to span across profile troughs without crushing, or be supported on boarding fixed across profiles.

For warm roof construction where insulation spans unsupported across troughs, reinforced air and vapour control layers may be required. Inverted warm roof designs require support boarding fixed across profiles to fully support the waterproofing layer.

Installation Considerations

Proper fixing is essential to avoid bimetallic corrosion, following manufacturer recommendations for compatible materials and protective coatings. The installation sequence must ensure adequate temporary weather protection and proper integration with the overall roof system.

7.1.9 Profiled Self-Supporting Metal Roofing

Complete Roofing Systems

Profiled self-supporting metal roofing provides both weather protection and structural function, forming complete roof systems without additional waterproofing membranes. These systems must provide adequate strength, weather resistance, thermal performance and durability.

System Types

Metal roofing can be site-assembled systems with separate inner liner and outer profile with insulation between, supported by structural framework, or factory-made insulated panels (sandwich/composite panels) with metal skins bonded to insulation cores forming self-supporting assemblies.

Essential Requirements

All systems must include air and vapour control layers on the warm side of insulation with vapour resistance of at least 5,000MNs/g, fully sealed around penetrations and at perimeters. Insulation must contact both inner and outer metal layers, with voids formed by outer profiles properly ventilated.

Ventilation Requirements

Voids formed by the outer sheet profile must be ventilated to prevent condensation problems. Ventilation can be achieved by leaving profile ends open above insulation or using profile fillers with minimum 5% free area of void cross-section.

Where insulation might be affected by condensation, breather membranes should be provided above insulation to discharge moisture externally, following manufacturer recommendations.

Panel Installation

Profiled panels must be fixed using suitable fixings that avoid bimetallic corrosion, with panel side and end laps sealed to provide air barriers as part of overall building air leakage control, following manufacturer instructions for proper sealing.

Performance Standards

Systems must be designed and constructed according to relevant British Standards including BS 5427 for profiled sheet practice, BS EN 14782 for self-supporting metal sheets, BS EN 14509 for insulated panels, and appropriate standards for different metal types.

7.1.10 Thermal Insulation and Vapour Control

Critical Performance Requirements

Thermal insulation and vapour control systems must ensure satisfactory thermal performance whilst preventing condensation formation that could damage the construction. Poor vapour control is one of the most common causes of flat roof problems.

Roof Type Characteristics

Uninsulated roofs: The deck temperature follows the more extreme of internal or external conditions. Cold roofs: Keep the deck at external temperature and require effective cross-ventilation to control condensation risk. Warm roofs: Maintain deck temperature close to internal conditions with no ventilation required. Inverted warm roofs: Keep deck warm while protecting insulation from cooling by rainwater infiltration.

Insulation Materials and Applications

Different insulation types suit different roof constructions. Expanded polystyrene (EPS) can be used in warm roofs when suitably protected but requires third-party assessment for inverted applications. Extruded polystyrene (XPS) suits both warm and inverted roofs. Rigid foam boards (PUR, PIR, phenolic) are suitable for warm roofs but not inverted applications due to moisture sensitivity.

Mineral wool rigid boards work in warm roofs but not inverted constructions, while vacuum insulation panels can be used in both applications when properly assessed.

Installation Requirements

Warm roof insulation: Must be either bonded/mechanically fixed to resist wind uplift with adequate penetration into supporting structure, or part of loose-laid ballasted systems. Insulation must be kept dry, installed in manageable areas for quick covering, dimensionally stable at working temperatures, and tightly butted to avoid gaps.

Inverted roof insulation: Must be suitable for external exposure, ballasted against flotation and wind uplift, able to withstand traffic and design loads, and protected by water flow reducing layers (WFRL) to minimise cooling effects from rainwater.

Vapour Control Systems

The movement of water vapour through roof construction must be controlled to prevent interstitial condensation. Air and vapour control layers (AVCL) on the warm side of insulation are essential for warm roofs, with all laps, joints and penetrations fully sealed.

AVCLs can include reinforced bitumen membranes, self-adhesive membranes, high-density polyethylene with metal foil, or mastic asphalt. For traditional hard metal roofs, minimum vapour resistance of 4,000MNs/g is required with full support.

Cold Roof Ventilation

Cold roofs require effective AVCL at ceiling level, unobstructed minimum 50mm ventilation space above insulation, adequate cross-ventilation with minimum 25mm equivalent gaps at both ends of each joist void, and maximum 5m clear distance between opposite ventilators with protective mesh.

7.1.11 Waterproofing Layer and Surface Treatments

The Heart of Flat Roof Performance

The waterproofing layer is the primary barrier preventing water ingress into your home. Its proper installation and protection are critical to long-term roof performance, making this one of the most important aspects of flat roof construction.

Installation Requirements

Before waterproofing installation, surfaces must be even and dry with all fixings properly recessed, penetrations for services and drainage formed, and manufacturer’s preparation requirements followed including priming of upstands and outlets.

Environmental conditions must be suitable – membranes should not be installed when temperatures are 5°C or below, on damp or frosted surfaces, or during precipitation. These conditions are essential for proper adhesion and long-term performance.

Installation Standards

Waterproofing must be secured to resist wind uplift whilst allowing for metal deck expansion, installed by specialist contractors where proprietary systems are used, and applied by the same contractor responsible for vapour barriers and insulation to ensure system compatibility.

Critical installation practices include checking that deck and insulation are weatherproofed each day with temporary joints, ensuring membrane laps don’t impede drainage around outlets, and preventing water entrapment between successive layers.

Waterproofing System Types

Reinforced Bitumen Membranes (RBM): Traditional systems using polyester-reinforced membranes, often with SBS (elastomeric) or APP (plastomeric) modifications for enhanced performance. These systems require multiple layers with specific installation sequences.

Single-Ply Membranes: Including PVC, EPDM, and TPO systems that can be bonded, mechanically fixed, or loose-laid with ballast. These systems use welded joints and require careful sealing of all penetrations.

Mastic Asphalt: Applied hot in multiple layers to specified thicknesses (typically 20mm on flat areas), providing seamless waterproofing but requiring skilled installation.

Liquid Applied Systems: Cold or hot applied systems that cure to form seamless membranes, particularly useful for complex shapes but requiring careful application in suitable conditions.

Inverted Roof Considerations

Inverted roofs require special attention to ballast provision for wind uplift resistance and fire protection, typically using minimum 40mm paving slabs or 50mm of rounded shingle ballast. Separating layers may be required between waterproofing and insulation.

Surface Treatment Requirements

Different roof types require appropriate surface protection. For maintenance access only, mineral-surfaced cap sheets, reflective chippings, or ballast systems provide adequate protection. For trafficked areas, precast paving slabs on supports or proprietary decking systems are required.

Surface treatments must meet fire protection requirements and prevent loose materials from entering drainage systems. The choice affects both performance and maintenance requirements.

Integrity Testing

Waterproofing integrity should be verified after installation. Roofs over 50m² or those difficult to access require visual inspection and appropriate integrity testing by qualified surveyors. Electronic testing methods are available for suitable membrane types, with test reports provided to NHBC.

7.1.12 Green and Biodiverse Roofs – Including Roof Gardens

Sustainable Roofing Systems

Green and biodiverse roofs provide environmental benefits including improved insulation, biodiversity habitat, stormwater management, and urban heat island reduction. However, they require careful design and installation to ensure structural adequacy and long-term performance.

Design Requirements

Green roofs must be designed with minimum 1:60 finished fall at the waterproofing layer, in accordance with industry best practice codes, and using certified waterproofing systems endorsed by manufacturers for green roof applications.

The design must account for wind uplift and flotation forces, particularly when plant establishment is incomplete, and include accessible drainage outlets with visible inspection hatches for maintenance access.

System Types

Extensive Green Roofs: Lightweight systems typically supporting sedum, grasses, and wildflowers with shallow growing medium (typically under 150mm). These require minimal maintenance – essentially annual attention – and are suitable for steeper slopes up to 45°.

Intensive Green Roofs: Heavier systems supporting shrubs, trees, and amenity spaces with deeper growing medium (typically over 150mm). These provide garden environments requiring regular maintenance like conventional gardens and are generally limited to 10° maximum slope.

Construction Requirements

Both system types require root barriers or root-resistant waterproofing membranes to prevent plant root penetration, protection layers above the waterproofing, filter layers above reservoir systems, and moisture control appropriate to the plant requirements.

The structural design must accommodate full wet soil loads, with concrete decks generally required for intensive systems due to weight considerations. Extensive systems may use profiled metal decks depending on loading calculations.

Waterproofing Systems

Approved waterproofing types include reinforced bitumen membranes, mastic asphalt, single-ply membranes, or liquid applied systems. The system must be endorsed by the manufacturer for green roof applications and integrity tested before covering with growing medium.

Installation Requirements

Both the green roof system and waterproofing should be installed by contractors trained and approved by system suppliers to ensure proper integration and performance. The installation sequence must protect the waterproofing during subsequent construction activities.

Fire Protection

Extensive green roof systems require non-combustible perimeter strips at building abutments, roof lights, and regular intervals across the roof. Design must comply with building regulations and industry fire risk guidance to prevent fire spread.

7.1.13 Blue Roofs

Sustainable Water Management

Blue roofs provide temporary rainwater storage to reduce flood risk by controlling discharge rates into urban drainage systems. These systems require careful design to balance water retention with building protection.

Design Principles

Blue roofs must be designed according to relevant standards and technical guidance, using certified waterproofing systems endorsed for blue roof applications, with supporting data demonstrating compliance with material standards and codes of practice.

The design must ensure complete drainage over the specified retention period – permanent water retention is not acceptable on roof waterproofing systems. Specific flow restrictor outlets control discharge rates whilst maintaining accessibility for inspection and maintenance.

System Requirements

Flow Control: Specific flow restrictors meet required discharge rates while remaining accessible for maintenance. Overflow Protection: Independent overflows prevent building water ingress if attenuation levels are exceeded, with visible operation to warn of potential blockages. Minimal Penetrations: Only essential rainwater outlets and overflows penetrate the waterproofing layer.

Construction Types

Blue roofs typically use warm roof or inverted warm roof construction. For inverted systems, additional insulation thickness may be required to compensate for cooling effects of water penetrating the insulation layer.

Adequate ballast, paving, or green roof toppings must prevent insulation flotation, which can occur before the system reaches full attenuation capacity. Water flow reducing layers (WFRL) must perform as the principal drainage layer to reduce water penetration to lower levels.

WFRL Installation

The water flow reducing layer requires careful installation with proper lapping and sealing, connection to attenuation chamber lips, finishing minimum 50mm above maximum attenuation levels at upstands, and sealing around all penetrations and behind parapet chambers.

7.1.14 Raised and Buried Podiums

Complex Waterproofing Systems

Podiums present particular challenges as they must provide both weather protection and structural support whilst accommodating various access and loading requirements. These systems require comprehensive waterproofing strategies coordinated with basement protection where applicable.

Design Requirements

Podiums require fully coordinated waterproofing and drainage systems using products with accredited third-party certification and performance testing. For specific applications, this testing should include root penetration resistance and durability under vehicular loading where relevant.

The waterproofing must provide continuity across all interfaces between podium structures, superstructure, and basement elements, creating an integrated water-resistant envelope throughout the building.

Podium Types

Raised Podiums: Decks or terraces over non-habitable spaces where thermal insulation may not be required but effective waterproofing remains essential. These typically don’t require coordination with basement tanking systems.

Buried Podiums: At or below ground level requiring integration with basement waterproofing systems. The waterproofing strategy must address both roof-level water penetration and ground water exclusion.

Loading Considerations

Podium design must consider emergency vehicular access requirements identified during planning, with waterproofing and structural systems capable of accommodating foreseeable loading. Standard pedestrian loading assumptions may be inadequate for emergency access requirements.

Waterproofing Integration

The waterproofing system must bridge between different structural elements whilst accommodating structural movement. This often requires complex details at interfaces between podium decks, rising walls, and basement structures.

Where podiums connect to basement structures, the waterproofing must integrate with basement tanking systems to provide continuous water exclusion throughout the below-ground envelope.

Construction Coordination

The complexity of podium waterproofing requires careful coordination between structural, waterproofing, and landscape contractors to ensure proper installation sequence and protection of completed work during subsequent construction activities.

7.1.15 Detailing of Flat Roofs

Critical Junction Points

Proper detailing at junctions between different elements is crucial for flat roof performance. These transition areas are where most water ingress problems occur, making careful attention to standard details essential for long-term success.

Flashing Materials

Acceptable flashing materials include rolled lead sheet (minimum Code 4), aluminium alloys (0.6-0.9mm thick, protected from mortar contact), zinc alloys (0.7mm thick), copper (0.6mm thick, fully annealed), stainless steel, galvanised steel, and proprietary materials assessed under Technical Requirement R3.

Material compatibility is essential – different metals must not cause bimetallic corrosion or must be properly isolated from each other.

Standard Details

Upstands: Minimum 150mm height above finished surface level, with waterproofing extending up walls and properly terminated with cover flashings and cavity trays. Similar principles apply to inverted roofs with allowance for ballast thickness.

Penetrations: Pipe penetrations require careful sealing with sleeve systems and apron flashings, with insulation maintained around penetrations and minimum 150mm upstands above roof level.

Roof Outlets: Must be positioned at lowest points with proper recessing to facilitate drainage, accessible for maintenance, and insulated where passing through heated spaces.

Movement Accommodation

Larger roofs require movement joints to accommodate thermal and structural movement. These joints must maintain waterproof integrity whilst allowing movement, typically using twin-kerb systems with flexible waterproofing connections.

Edge Details

Roof edges require careful detailing to direct water away from the building whilst providing adequate protection for roof build-up edges. This includes verge details, eaves connections, and abutments with pitched roofs or walls.

Parapet and Coping Details

Parapet walls must be weatherproofed with copings or by extending roof waterproofing over the wall. Copings require minimum 45mm projection beyond wall faces with drip features at least 30mm from wall surfaces, proper fixing systems, and integration with cavity trays and DPCs.

Quality Control

Detailing quality often determines overall roof performance. Poor details can compromise otherwise well-constructed roof systems, making attention to these junction points critical during construction.

7.1.16 Accessible Thresholds and Upstands

Level Access Challenges

Accessible thresholds present particular challenges for flat roof and balcony construction, as they must provide weather protection whilst accommodating level access requirements. Poor threshold design is a common source of water ingress problems in new builds.

Standard Upstand Requirements

Generally, waterproofing should extend up walls to form minimum 150mm upstands measured from the drainage layer. The waterproofing must link directly under cavity trays to ensure proper cavity drainage, with weepholes provided at maximum 1m centres to assist drainage.

Reduced Upstand Conditions

Where door thresholds or window sills are located less than 150mm above the balcony drainage layer, special accessible threshold requirements apply. This includes doors with level access and windows adjacent to internal floor levels where external paving/decking creates reduced clearances.

Accessible Threshold Standards

Accessible thresholds must comply with specific fire, thermal, waterproofing and acoustic requirements, have maximum 15mm upstand at the door position with optional sloping transitions either side (maximum 15° slope), include minimum 45mm projecting sill to shed water away from the waterproofing interface with drip feature minimum 30mm from upstand face, and provide 75mm minimum balcony upstand below the projecting sill measured from drainage layer.

Prohibited Design Approaches

Designs that continue waterproofing horizontally through cavity walls to form upstands against inner leaves are not acceptable. These create problems because waterproofing materials aren’t suitable as DPCs supporting masonry loads, cavities must drain freely without water retention, waterproofing drainage shouldn’t enter cavity walls, and hidden upstands cannot be inspected or maintained.

Drainage Requirements

Effective drainage is crucial for accessible thresholds. Requirements include minimum 1:80 finished fall to outlets without back-falls or ponding, designs ensuring blockage cannot cause building flooding, full protection from direct foot traffic, ability to withstand point loads from paving supports, and UV resistance or complete protection from daylight.

Drainage System Options

Acceptable drainage arrangements include at least one outlet with overflow having capacity exceeding the outlet, one outlet chute and hopper sized for twice the discharge capacity, two independent outlets where one can handle full discharge if the other blocks, or setting balcony kerbs minimum 25mm below door thresholds for safe discharge.

Gap Requirements and Splash Protection

Proper gaps must be maintained between paving units (6-8mm between individual units, 10-12mm along perimeter upstands) to ensure drainage flow. Spacers and supports must not obstruct water flow to outlets.

Splash barriers minimum 150mm above paving level must protect wall areas that could be adversely affected by moisture, using impervious finishes, cladding, or extended waterproofing with cover flashings.

7.1.17 Metal Balcony Decking Systems

Specialised Systems

Metal balcony framework structures and decking systems require design and construction according to BS 8579 guidance for balconies and terraces. These systems must address both structural and weatherproofing requirements whilst accommodating thermal movement and corrosion protection.

Design Standards

Metal balcony systems must comply with structural design standards for loading, deflection and stability, incorporate adequate corrosion protection appropriate for the exposure environment, accommodate thermal movement without compromising waterproofing integrity, and provide suitable interfaces with building waterproofing systems.

Material Considerations

Metal components must be adequately protected against corrosion, particularly in aggressive environments such as coastal locations. Material selection and protective treatments must consider the full service life requirements and maintenance accessibility.

Integration Requirements

Metal decking systems must integrate properly with building waterproofing systems, provide adequate falls for drainage, accommodate structural movement without compromising weather resistance, and maintain long-term performance under imposed loading conditions.

Installation Quality

Proper installation requires attention to fixing adequacy and corrosion protection, drainage integration and falls, thermal movement accommodation, and long-term durability of protective coatings and sealants.

7.1.18 Parapets and Guarding to Terraces and Balconies

Safety Requirements

Terraces and balconies with regular access require adequate guarding to minimise falling risks. These safety systems must be properly designed, constructed and maintained whilst integrating with weatherproofing systems.

Guarding Standards

Guarding must not be easily climbed by children, achieve adequate height as required by Building Regulations, use appropriate materials (toughened or laminated glass where glazed systems are used), and not inhibit drainage flow or overflow operation in blocked outlet situations.

Structural Stability

Guarding systems must be designed according to BS EN 1991-1-1 for horizontal loading and Building Regulations requirements. Particular care is needed for balustrading fixed to parapet walls to ensure stability and prevent overturning, potentially requiring end fixings or returns for stability.

Cavity walls and DPCs can create slip planes limiting horizontal force resistance, potentially requiring ring beams or additional support to ensure adequate stability.

Masonry Parapet Construction

Masonry balcony walls must be built according to external masonry wall standards, incorporating strengthening as required by design, movement joints as specified, and weatherproofed tops using copings or extended deck waterproofing.

Copings must be firmly fixed, project minimum 45mm beyond wall faces with drip features discharging at least 30mm from wall surfaces, and incorporate DPCs and cavity trays linked to waterproofing upstands. Cavity trays should discharge externally with weepholes at maximum 1000mm centres.

Balustrading Installation

Balustrading should not be fixed through copings or cappings due to waterproofing difficulties, or through waterproofing layers unless suitable waterproof junctions can be provided (such as raised waterproofed kerbs or pitch pocket details).

These problems are avoided by fixing balustrading to wall faces below copings or cappings, maintaining coping integrity and waterproofing continuity.

Durability Requirements

Materials and finishes must resist corrosion and staining in aggressive environments, particularly coastal zones. Ferritic stainless steel can suffer surface problems in coastal conditions and may require alternative specifications.

Maintenance Access

Provision must be made for safe future access to flat roofs for maintenance purposes, including consideration of safety systems for workers accessing roof areas.

7.1.19 Further Information

Additional Standards and Guidance

The NHBC standards reference numerous additional British Standards and industry guidance documents that provide detailed technical information for specific aspects of flat roof construction. These include thermal insulation product standards (BS EN 13162 through 13170), waterproofing material standards, and construction practice codes.

Industry Bodies and Resources

Various industry associations provide additional guidance including the National Federation of Roofing Contractors (NFRC), Single Ply Roofing Association (SPRA), Mastic Asphalt Council (MAC), Lead Contractors Association, and specialist organisations for green roofs and metal roofing systems.

Professional Assessment

Many aspects of flat roof construction require professional assessment under Technical Requirement R3, particularly for innovative materials, non-standard details, or challenging environmental conditions.