Chapter 7.2 of the NHBC Standards focuses on the comprehensive guidelines for the design, construction, and maintenance of pitched roofs. This chapter provides detailed definitions and classifications of various roof types, such as cold pitched roofs, warm pitched roofs, and hybrid roofs, along with the necessary materials and structural components required for each type. It also outlines the technical requirements for ensuring compliance with British Standards, particularly BS 5534 for slating and tiling. Key aspects include the provision of adequate ventilation, weatherproofing, and insulation to prevent issues such as condensation and ensure the roof’s durability and energy efficiency.
The chapter also addresses the importance of clear and detailed design and specification information to be provided to relevant personnel, ensuring that all elements from the layout of roof trusses to the details of ventilation and fire-stopping are well-documented and accessible. It includes specific guidelines for the installation and integration of solar roof panels, emphasizing the need for appropriate fixing methods and compatibility with existing roofing materials to maintain the roof’s integrity and performance. The guidelines aim to ensure that pitched roofs are constructed to the highest standards, offering safety, durability, and efficiency in various environmental conditions .
7.2.1 Compliance
Section 7.2.1 of the NHBC Standards outlines the mandatory compliance requirements for pitched roof structures and coverings. It mandates that all pitched roofs must adhere to the established Technical Requirements to ensure safety, durability, and performance. Specifically, roofs with tile or slate coverings must conform to BS 5534 standards, which detail the specifications for slating and tiling. Compliance with these guidelines ensures that the pitched roofs will generally be acceptable under NHBC regulations, offering reassurance of quality and adherence to best practices in construction.
Furthermore, this section emphasizes the importance of meeting these compliance standards to avoid potential issues such as structural failures, water ingress, and other defects that could compromise the integrity of the building. By adhering to these guidelines, builders and contractors can ensure that their pitched roof constructions are robust, weatherproof, and long-lasting, thereby protecting the interests of homeowners and maintaining the overall quality of the housing stock.
7.2.2 Provision of Information
Section 7.2.2 highlights the critical need for clear, comprehensive, and accessible design and specification information for pitched roofs. This information must be produced in a format that is easy to understand and distributed to all relevant personnel, including site supervisors, specialist subcontractors, and suppliers. Detailed documentation should include layouts of cut roofs and trusses, specifics of mono-pitch and lean-to roofs, bracing requirements, supports for equipment, insulation types and positions, ventilation methods, fire-stopping details, and all necessary fixing and trimming arrangements.
This section underscores that the precise dissemination of information is vital for ensuring that the construction process adheres to the planned design, thereby avoiding errors and ensuring structural integrity. Properly detailed plans and specifications help in achieving high standards of construction, compliance with regulatory requirements, and ensuring safety and efficiency in the building process. Providing this information also facilitates better communication among the construction team, leading to more efficient and error-free project execution.
7.2.3 Design of Pitched Roofs
Section 7.2.3 focuses on the design principles for pitched roofs, ensuring that the sizing and spacing of structural members provide adequate stability and prevent undue movement or distortion. The design must account for various loads, including dead, imposed, and wind loads, calculated in accordance with BS EN standards. The section specifies the need for coherent structural integrity, ensuring that all forces within the structure are resolved appropriately and that the design is suitable for the location, taking into account exposure and wind uplift.
Additionally, this section addresses specific requirements for trussed rafter roofs and traditional cut roofs, emphasizing the importance of compatibility and stability within the entire structure. It also highlights the need for engineered design in cases where standard configurations are exceeded, such as large spans or complex features. By adhering to these design guidelines, builders can ensure that pitched roofs are not only structurally sound but also capable of withstanding various environmental conditions, thereby enhancing the overall durability and safety of the building.
7.2.4 Protection of Trusses
Section 7.2.4 emphasises the importance of protecting trussed rafters from damage during storage, handling, and installation. Trusses should be stored vertically and propped with level bearers under the joints to avoid distortion and prevent damage. They must be protected against weather to prevent corrosion of truss plates and deterioration of the timber, and should be adequately ventilated during storage. Carrying trusses upright is recommended, as fasteners can loosen when carried flat. The guidelines also specify that trussed rafters should not be cut, modified, or repaired unless it is in accordance with written or drawn instructions issued and approved by the trussed rafter designer.
Proper handling and storage practices are crucial to maintaining the structural integrity of trussed rafters. By following these guidelines, builders can prevent common issues such as warping, splitting, and weakening of the trusses, which can compromise the safety and durability of the roof structure. Ensuring that trusses are protected from adverse weather conditions and physical damage will contribute to a longer lifespan and better performance of the roof system.
7.2.5 Durability
Section 7.2.5 addresses the durability requirements for timber used in pitched roof constructions. Timber members must be preservative treated or have adequate natural durability to ensure a long service life. The section outlines specific treatments and durability classes for different components, such as porch posts, tiling battens, soffits, barge boards, and fascias. For instance, timber selected for these applications must meet the durability standards set out in BS EN 350 and Chapter 3.3 of the NHBC Standards, which include specifications for natural durability classes of heartwood.
The section also highlights the increased risk of condensation in roofs that use fully supported weatherproofing membranes or continuous metal coverings. In such cases, the timber components must be preservative treated to prevent decay and ensure the roof’s longevity. By adhering to these durability guidelines, builders can ensure that the timber elements of the roof will withstand environmental stresses and provide reliable performance over an extended period.
7.2.6 Wall Plates
Section 7.2.6 details the requirements for wall plates and their role in distributing and transmitting loads from the roof structure. Wall plates must be bedded to line and level, fixed using nails or straps, and extend over a minimum of three joists, rafters, or trusses. They should be joined using half-lapped joints, including at corners, and typically measure 38 x 100mm or follow local practice. The guidelines specify that trussed rafters should only be supported at the junction between the ceiling tie and rafter, unless otherwise designed, to ensure stability and proper load distribution.
The section also addresses the fixings used to connect the roof structure to the wall plate, which should be specified according to the roof construction and site exposure. For roofs that are not subject to uplift, skew nailing with galvanized nails or truss clips is acceptable. However, where the roof must resist uplift, stronger metal straps should be used. These holding-down straps must be provided according to the geographical location and construction type, and installed to secure the roof structure effectively against wind forces and other loads. Proper installation of wall plates and fixings ensures the overall stability and integrity of the roof system.
7.2.7 Joints and Connections
Section 7.2.7 focuses on the design and construction of joints and connections to ensure structural stability without undue movement or distortion. It specifies that members should be accurately cut to fit tightly and should not be damaged or split. The section details the types of joints to be used at main connections for traditional cut roof members, such as nailed lapped joints for rafters to ceiling joists, birdsmouth joints for rafters to purlins, and scarf joints for purlins. Additionally, it describes the use of angle ties and dragon ties for hipped roof joints to prevent the wall plates and hip rafters from spreading under load.
Proper joint construction is critical for maintaining the overall integrity of the roof structure. Ensuring that each joint is correctly designed and executed helps distribute loads evenly and prevents structural failures. This section also emphasizes the importance of using appropriate fixing methods and materials to enhance the durability and stability of the connections, thereby extending the lifespan of the roof.
7.2.8 Restraint
Section 7.2.8 outlines the requirements for providing adequate restraint to support the structure, distribute roof loads, and prevent wind uplift. Restraint straps or gable ladders must be used where necessary to stabilize walls, and these must be installed in accordance with the design specifications. The section specifies the placement and fixing methods for lateral restraint straps, which should be located at maximum spacings of 2m for homes up to three storeys and 1.25m for taller buildings. The straps should be fixed to the roof structure using solid noggings or longitudinal bracing members, ensuring they are protected against corrosion.
Adequate restraint is essential to prevent lateral movement and ensure the structural integrity of the roof. By following these guidelines, builders can ensure that the roof structure can withstand various forces, including wind uplift, thereby maintaining the safety and stability of the building. Proper installation of restraint straps and adherence to design specifications help distribute loads effectively and prevent potential structural issues.
7.2.9 Bracing for Trussed Rafter Roofs
Section 7.2.9 addresses the requirements for bracing trussed rafter roofs to support applied loads and self-weight without undue movement. It covers general bracing requirements, specifying that roof bracing must be in accordance with the design and should not be altered without approval. The section provides guidelines for diagonal and longitudinal bracing at rafter levels, which may be omitted if sarking sheets or boards are used. It also details the necessary bracing for different types of roofs, including duo-pitched, mono-pitched, and attic roofs, emphasizing the importance of ensuring each truss is adequately braced and supported.
Proper bracing is crucial for maintaining the structural stability of trussed rafter roofs. This section ensures that roofs are designed to handle various loads and environmental conditions, preventing structural failures. By adhering to these bracing requirements, builders can ensure that the roof structure is secure, stable, and capable of supporting the loads imposed on it, thereby enhancing the overall durability and safety of the building.
7.2.10 Strutting for Site Formed Attic Trusses and Cut Roofs that Form a Floor
Section 7.2.10 specifies the need for adequate strutting in site-formed attic trusses and cut roofs that form a floor. Strutting is essential to support applied loads and self-weight without causing undue movement or distortion. The guidelines indicate that strutting should be installed according to the design specifications, particularly where the span between the node points forming the width of the floor exceeds 2.5 metres. For spans over 2.5 metres, the use of herringbone strutting (38mm x 38mm timber) or solid strutting (with a minimum of 0.75 times the depth of the floor and a minimum thickness of 38mm) is required.
The provision of strutting helps in distributing loads evenly and maintaining the structural integrity of the attic trusses and cut roofs. It ensures that the floor is stable and can support the weight of occupants and furniture without excessive deflection. By adhering to these guidelines, builders can prevent potential structural failures and ensure a safe and durable roof construction .
7.2.11 Support for Equipment
Section 7.2.11 highlights the requirements for supporting permanent equipment located in roof voids. Equipment such as water tanks and mechanical ventilation with heat recovery (MVHR) units must be adequately supported to prevent structural overload. The structure should be designed in accordance with PD 6693-1 and the recommendations of the truss manufacturer. Platforms for permanent equipment should be positioned with at least 50mm clearance above the insulation to ensure proper ventilation.
This section underscores the importance of designing roof structures to accommodate additional loads from equipment, ensuring they do not compromise the integrity of the roof. Proper support and clearances help in maintaining ventilation, preventing condensation, and ensuring that the equipment functions correctly without affecting the roof’s structural performance .
7.2.12 Access
Section 7.2.12 mandates that roof voids should be provided with suitable access for periodic inspection, maintenance, and removal of permanent equipment. Access should be available to the main roof space and any voids containing equipment such as heating systems, water storage, energy production, or ventilation units. The minimum opening size for access should be 520mm in each direction, and access should not be located over stairs or other hazardous locations.
Furthermore, the guidelines specify that securely fixed boarded walkways must be provided between the access opening and the permanent equipment. A minimum 1m² platform should be provided at each piece of permanent equipment to facilitate maintenance. Ensuring proper access and walkways helps in safe and efficient maintenance operations, preventing accidents and enabling easier inspection and servicing of roof-installed equipment .
7.2.13 Dormer Construction
Section 7.2.13 outlines the construction requirements for dormers, focusing on structure, ventilation, insulation, condensation control, and proprietary dormers. Dormers must be adequately constructed, with proper support for dormer cheek studs provided by either double rafters or double floor joists. Trimming members should be sufficiently sized to support the additional loads from the main roof members, dormer framing, and cladding. For larger or bespoke dormers, appropriate support should be designed by a qualified person or structural engineer. Framing and roofing timbers used in dormer construction must be preservative treated or possess adequate natural durability in accordance with Chapter 3.3.
The section also emphasizes the importance of ventilation and insulation in dormer construction to prevent condensation. Dormer roofs and cheeks should meet Building Regulations insulation standards and incorporate air and vapour control layers on the walls to limit interstitial condensation. Additionally, flat roof dormers should have falls to the front or sides and be constructed as either warm or cold decks, with appropriate ventilation in cold deck constructions. Proprietary dormers must hold a satisfactory assessment by an independent technical approval authority acceptable to NHBC, ensuring durability and compliance with condensation control requirements .
7.2.14 Underlay, Sarking Boards and Sheets
Section 7.2.14 details the requirements for underlay and sarking in pitched roofs to resist the passage of moisture. Underlay and sarking should comply with the manufacturer’s recommendations, considering the type and fixing of the roof covering. In areas of severe exposure, the use of rigid sarking sheets or boards with underlay is recommended. The section provides specifications for acceptable materials such as solid timber boards, plywood, chipboard, and OSB, along with the minimum thickness requirements for each material.
The guidelines also specify that underlay should be provided for all tiled and slated roofs, secured in accordance with the manufacturer’s instructions, and supported by a continuous fillet or proprietary eaves support tray to prevent sagging. Underlay must be fixed at vertical laps only over rafters, and at horizontal laps held in place by battens. It should be cut neatly to fit around surface penetrations and dressed into the gutter where exposed at eaves level. Additionally, underlay must be UV resistant or covered with a proprietary eaves guard where necessary to ensure long-term performance and weatherproofing .
7.2.15 Ventilation, Vapour Control and Insulation
Section 7.2.15 emphasizes the importance of adequate ventilation, vapour control, and insulation to prevent condensation and cold in pitched roofs. Roof ventilation must comply with BS 5250:2021 standards, with specific strategies selected based on the roof covering, underlay type, roof type, pitch, and ceiling type. Ventilation should ensure that pathways remain clear and prevent entry of birds or debris. The use of proprietary eaves ventilators, installed according to the manufacturer’s instructions, is recommended.
This section also covers the use of air and vapour control layers (AVCL), which should be placed on the warm side of insulation in warm and hybrid roofs to manage moisture effectively. These layers must be installed once framing timbers have a moisture content of less than 20% and the building is weathertight. Insulation should be laid to meet Building Regulations requirements, covering the entire loft and wall plate area. Additionally, pipework in roof voids should be insulated to reduce the risk of freezing or condensation, ensuring a comprehensive approach to managing moisture and maintaining the thermal efficiency of the roof structure .
7.2.16 Fire-Stopping and Cavity Barriers
Section 7.2.16 covers the crucial aspect of fire-stopping and cavity barriers within pitched roofs to provide adequate fire resistance and separation. Fire-stopping must be installed at the junctions between compartment walls and roofs, extending into any eaves to prevent the spread of fire. The materials used for fire-stopping should match the fire resistance level of the compartment wall. Additionally, gaps between compartments should be sealed using appropriate fire-stopping materials that allow for movement in roof timbers to prevent ‘hogging’ of the tiles. This ensures the integrity of the fire-stopping measures despite structural movements.
Cavity barriers are also required as per Building Regulations, to be installed at the edges and junctions of cavities. These barriers should be appropriately sized, tightly fitted to a rigid construction, and mechanically fixed in position. The guidelines further stipulate that combustible materials, such as roof timbers and sarking felt, should be kept away from heat sources to minimize fire risk. By adhering to these measures, the spread of fire and smoke within roof structures can be effectively controlled, enhancing the safety of the building.
7.2.17 Battens
Section 7.2.17 specifies the requirements for battens and counter battens used to support roof coverings. Battens must comply with BS 5534 and be accompanied by a delivery note indicating the supplier, origin, grade, and size. They should be preservative treated and must not be re-sawn, ripped, or planed after treatment, although they can be cut to length. Where battens are cut, especially at wet or dry verges, the ends should be treated with preservative. Battens must be at least 1.2 meters long and span a minimum of three rafters, set out in straight lines parallel to the ridge, and fixed with nails through each rafter they span.
The section also highlights the importance of correct spacing and jointing of battens. Battens should be set out to avoid joints over the same rafter whenever possible, with specific guidelines for batten gauge over and under 200mm. Properly sized and installed battens ensure that the roof coverings are securely fixed and able to withstand wind forces. The fixings should meet minimum requirements for nail penetration, with considerations for site exposure and location, ensuring the stability and weatherproofing of the roof system.
7.2.18 Roof Coverings
Section 7.2.18 details the standards and practices for selecting and installing roof coverings on pitched roofs. Roof coverings must be of suitable quality and durability to protect the building from weather. The selection of materials should align with established building practices and standards such as BS EN 1304 for clay tiles and BS EN 12326 for natural slates. Additionally, the use of recovered materials is permissible only with prior approval from NHBC, and independent certification of suitability may be required.
This section underscores the importance of compliance with design specifications and best practices to ensure long-term performance. Proper installation techniques, adherence to manufacturer recommendations, and suitable material selection are crucial for maintaining the roof’s integrity and weather resistance. The guidelines cover various materials including clay and concrete tiles, fibre cement slates, and proprietary products, ensuring that each type of roof covering is installed correctly to provide effective protection against the elements.
7.2.19 Fixing Tiles and Slates
Section 7.2.19 of the NHBC Standards details the requirements for properly fixing tiles and slates to ensure they provide adequate weather protection for the building. The section emphasizes careful setting out to improve the roof’s appearance and reduce the need for excessive tile cutting around abutments, chimneys, and similar obstructions. Tiles and slates should be fixed in accordance with the manufacturer’s recommendations and BS 5534 standards, with specific guidance on eaves, ridge, hip tiles, verges, and mansard roofs. Proper fixing methods include using appropriate nails, clips, or other mechanical fixings to secure tiles against wind uplift and other forces.
Furthermore, the section outlines that perimeter roof tiles or slates should be mechanically fixed using a minimum of two fixings, one of which can be a tile clip, adhesive, or dry verge capping system designed to resist uplift. For ridge and hip tiles, additional tail clips or two nails are required, depending on the tile type. The guidelines also specify the use of under-eaves courses for the bottom edges of double-lapped slate and plain tile roofs to ensure secure fixing and prevent wind uplift. Adhering to these standards ensures the roof coverings remain secure and weather-resistant throughout their lifespan.
7.2.20 Weathering Details
Section 7.2.20 focuses on the weatherproofing requirements at various roof junctions to prevent moisture ingress. It covers the need for suitable flashings at abutments, flat roof intersections, changes in roof slope, and projections through the roof. Flashing materials should be non-ferrous and installed in accordance with BS 5534 standards. This includes lead flashings, which should have a minimum upstand of 75mm and be tucked into bed joints and wedged securely.
Additionally, the guidelines specify proper weatherproofing measures for chimneys, pipes, and other roof penetrations. Flashings should be supported to prevent sagging, and joints should be adequately sealed to resist moisture penetration. The section also provides detailed instructions for installing raking copings and abutment gutters, ensuring they are secure and watertight. By following these guidelines, builders can ensure that all roof junctions are adequately protected from the elements, preventing water ingress and maintaining the building’s integrity.
7.2.21 Valleys and Hidden Gutters
Section 7.2.21 outlines the requirements for constructing valleys and hidden gutters to resist moisture ingress and ensure effective drainage. Valleys must be properly weathered, including the use of suitable flashings and compliant with BS 5534. The guidelines specify that valleys should be constructed using valley tiles, trough tiles, or non-ferrous metals, and should be fixed according to the manufacturer’s recommendations. Saddle flashings or lead flashings at the head of each valley are required to prevent water ingress.
The section also addresses the need for cutting and fitting tiles adjacent to open valleys. Tiles should be cut from double tiles, tile-and-a-half, or half tiles to ensure a neat and secure fit. For slate or plain tile roofs, laced, swept, or mitred valleys with soakers can be used. Additionally, horizontal valley gutters positioned over compartment walls should meet fire resistance standards and be properly fire-stopped. Proper construction and detailing of valleys and hidden gutters are essential for maintaining the roof’s water resistance and structural integrity.
7.2.22 Drainage
Section 7.2.22 of the NHBC Standards focuses on ensuring that roof drainage systems are adequate to handle rainwater efficiently. The guidelines state that drainage should be provided for roofs larger than 6 square metres, and consideration should also be given to smaller roofs such as dormers, porches, and balconies. The drainage system must be of sufficient size to accommodate normal rainfall and cope with concentrated flows, such as those from dormer roofs. The design and fitting should prevent erosion of lower surfaces where water discharges onto another surface and should be installed according to the design, using the correct type of fittings for internal and external angles, outlets, etc.
Additionally, drainage systems must be supported and jointed as per manufacturer recommendations, insulated when passing through a home, and installed to ensure gutters are provided with stop ends and laid with a sufficient fall towards the outlet. Discharge of rainwater from one roof to another should be avoided where possible, but if necessary, the quantity discharged must not exceed the normal drainage characteristics of the roof below. Gutters behind parapet walls should have an overflow to prevent water from entering the building, and downpipes should be fitted with shoes when discharging above ground level or drainage gullies .
7.2.23 Fascias and Trim
Section 7.2.23 outlines the standards for fixing fascias, bargeboards, and soffits to ensure they are adequately treated against decay and securely installed. Materials for fascia boards include marine grade plywood, exterior grade plywood, natural solid timber boards, high density fibre reinforced calcium silicate boards, and glass fibre reinforced cement boards. These materials should be preservative treated or naturally durable, and the treatment should comply with specified standards to ensure longevity and performance in various environmental conditions.
Installation of fascia boards and soffits should ensure that timber for external feature work is free from defects such as large knots and splits. When preservative treated timber is cut, the cut ends should be treated with preservative, and timber to be painted should be knotted and primed on all surfaces before fixing. Fascia boards should be fixed with two nails to each rafter and use splayed butt joints. Proper installation and material selection ensure that fascias and trims provide effective protection and aesthetic appeal while resisting weather-related wear and decay .
7.2.24 Spandrel Panels in Cold Roofs
Section 7.2.24 addresses the use of spandrel panels in cold roofs to create separation between dwellings or form the inner leaf of gable walls. Spandrel panels must be designed, manufactured, and installed to provide satisfactory performance in terms of fire resistance, acoustic transfer, and structural stability. Compliance with guidance from the Structural Timber Association or the Trussed Rafter Association ensures that spandrel panels meet the necessary standards for safety and durability.
Proper installation and design of spandrel panels are crucial to maintaining the integrity of fire and sound barriers within a building. By adhering to these guidelines, builders can ensure that spandrel panels effectively prevent the spread of fire and reduce noise transfer between adjoining properties, thereby enhancing the safety and comfort of residents. The use of compliant materials and installation practices guarantees that the panels will perform reliably over the building’s lifespan .
7.2.25 Roof Cassette Systems
Section 7.2.25 describes roof cassette systems, which are prefabricated roof panels often supplied with prefabricated wall panels, beams, and other supporting structures. These systems can be either open or closed panel and must be designed to safely transmit loads to the supporting structure without undue movement or deformation. Essential considerations include provision of detailed drawings, structural performance to support various loads, thermal and acoustic performance, fire safety, condensation and ventilation control, moisture protection, durability, and proper installation. The detailed design and specification information should be available on site, ensuring that work is carried out according to the design, and should include material specifications, fixing schedules, junction details, and manufacturer’s requirements for ancillary items like chimneys and dormers.
The section also emphasizes the need for adequate structural design to support dead, imposed, and wind loads in compliance with relevant standards. Thermal and acoustic insulation must meet Building Regulations, ensuring sound transmission between homes is adequately limited. Fire safety considerations include providing structural fire protection, adequate compartmentation, and necessary cavity barriers and fire-stopping. Roof cassette systems should be constructed to limit interstitial condensation and be ventilated according to BS 5250 standards. Durability is highlighted, with the structure expected to have a life expectancy of at least 60 years, using preservative-treated or naturally durable timber. Proper installation by competent operatives familiar with the system is crucial for ensuring performance and structural integrity.
7.2.26 Solar Roof Panels
Section 7.2.26 provides guidelines for the installation and integration of solar roof panels, ensuring they are securely fixed without compromising the building’s weather resistance. When solar panels form the roof covering, they must be of high quality and durability. Key considerations include weather tightness, proper fixing, ventilation and vapour control, and durability. Integrated solar roof panels must meet weather tightness criteria, and installers should consult the tile or slate manufacturer to ensure compatibility and proper sealing of connections that penetrate the weatherproofing layer. Appropriate flashings must be used to ensure the penetrations are weather tight.
Solar panels can be of the ‘on-roof’ type, sitting above the roof covering, or integrated into the tile or slate array. They should be secured to the roof framing or battens as per manufacturer requirements to resist wind uplift and snow loads. When forming the roof covering, integrated panels should be treated as air impermeable, requiring the whole roof to be ventilated unless proven otherwise by the manufacturer. A ventilated air space beneath the panel may be necessary to enhance ventilation and cooling. Durability of the panels, brackets, fixings, and flashings is essential, with appropriate materials selected for the specific environment, such as grade 316 stainless steel in coastal locations.
