7.2 Pitched roofs
SCOPE
DESIGN STANDARDS
Design that follows the guidance below will be acceptable for pitched roofs.
STATUTORY REQUIREMENTS
Designs should be in accordance with relevant Building Regulations and other statutory requirements.
LOADBEARING STRUCTURE
Items to be taken into account include:
(a) dead and imposed loads
Dead and imposed loads should be calculated in accordance with BS 6399 : Part 3. Dead loads include the self weight of the roof structure and the roof covering. Imposed loads include snow loadings and the weight of water tanks, insulation, etc.
Structural timber should be specified according to the strength classes in BS 5268 : Part 2. Timber specifications when using BS 4978 grading rules (eg GS) should also include the timber species. The strength class can then be determined from Table B1 in Approved Document A1/2 to the Building Regulations or Table 3 in BS 5268 : Part 2.
(b) wind loads
Wind loads appropriate to the site location should be calculated in accordance with BS 6399 Part 2. The roof should be designed to resist wind uplift. This resistance is often provided by the weight of the roof itself but holding down straps should be provided where the self weight of the roof is insufficient.
(c) holding down straps
Holding down straps are required in Scotland, Northern Ireland and other areas of severe exposure. Some roof covering manufacturers provide detailed guidance.
Where holding down straps are necessary, they should have a minimum cross section of 30mm x 2.5mm and be fixed at maximum 2m centres. Steel straps with a galvanized finish are normally acceptable. The design should detail how straps are to be fixed and what materials are to be used. The durability of fixings should be compatible with the straps.
(d) sizing and spacing of members
Sizing and spacing of rafters and ceiling joists should be as:
Generally, the spacing of rafters and ceiling joists should be not more than 600mm. The spacing of ceiling joists should suit the thickness and size of the plasterboard sheets or other ceiling finish. Plasterboard sheets may be fixed at the following joist centres:
| - up to 450mm spacing |
| - up to 600mm spacing |
Proprietary roof systems should be designed by an Engineer in accordance with Technical Requirement R5.
(e) size and spacing of tile battens
The size and spacing of tile battens should be in accordance with the roof covering manufacturer's recommendations, but not less than shown in the table in Appendix E.
Nails for fixing battens should be 3.35mm (10 gauge) x 65mm long. Galvanized smooth round nails are acceptable, except where the maximum basic wind speed is over 48m/s (from BS 6399 Part 2), when ring shank nails should be specified.
Items to be taken into account include:
(a) wall plates
Normally, trussed rafter roofs and traditional cut roofs should be supported on timber wall plates.
Wall plates should be as detailed in Clause D6(b) with regard to the table concerning minimum sizes. Fixings to connect the roof structure to the wall plate should be specified having regard to the roof construction and the exposure conditions of the site.
For trussed rafter roofs not subject to uplift, the minimum fixing should be two 4.5mm x 100mm long galvanized round wire nails, skew nailed, one from each side of the trussed rafter. Alternatively, truss clips can be used, fixed in accordance with manufacturers' instructions.
In Scotland, nails should be appropriate to wall plate dimensions.
(b) holding down straps
In situations where the roof is required to resist uplift, skew nailing is unlikely to provide sufficient strength. Appropriate metal straps should be used.
(c) lateral restraint straps
For dwellings of masonry construction, restraint should be provided at rafter level for gable walls. Larger gable or separating walls may also require restraint at ceiling level.
Guidance in assessing when ceiling restraint is needed can be found in Building Regulations.
Lateral restraint straps should have a minimum cross section of 30mm x 5mm and a minimum anchorage down-turn of 100mm. The level of galvanizing for straps and their fixings should be in accordance with BS 5628 : Part 3 (reference should be made to Appendix 7.2-A). Sheradizing is not acceptable in Northern Ireland and the Isle of Man. Straps should be of sufficient length to be fixed to three trusses and should be fixed with solid blocking.
In framed roofs, as an alternative, purlins and pole plates can also provide restraint if the timber abuts a gable construction. Where purlins are used to provide restraint they should not be spaced at more than 2 metre centres, unless the design shows they are adequate at greater spacing.
In trussed rafter roofs, an alternative is to provide restraint through gable ladder detailing.
(d) timber frame construction
For dwellings of timber frame construction, the designer should ensure stability in accordance with BS 5268.
TRUSSED RAFTER ROOFS
Items to be taken into account include:
(a) recognised design standards
Trussed rafters should be designed in accordance with BS 5268 : Part 3. Truss manufacturers may have their own computer programs for calculating truss designs in accordance with the British Standard.
(b) design information
To ensure that trussed rafters are correctly designed and fabricated, and are suitable for their intended purpose, an accurate specification is necessary.
BS 5268 : Part 3 gives a list of information to be supplied to the truss manufacturer, including the:
- height and location of building with reference to unusual wind conditions
- profile of the trussed rafter, including camber, if required
- span of the trussed rafter
- pitch or pitches of the roof
- method of support and position of supports
- type and weights of roof tiles or covering, including sarking, insulation and ceiling materials
- size and approximate position of any water tanks or other equipment to be supported on the trussed rafters
- overhang of rafters at eaves and other eaves details
- positions and dimensions of hatches, chimneys and other openings
- use of the building with reference to any unusual environmental conditions
- type of preservative treatment, where required
- spacing of trussed rafters and special timber sizes, where these are required to match existing construction.
The building designer should ensure that the design of the roof as a whole is satisfactory in achieving the overall stability of the complete structure. This includes its connection to, and compatibility with, the supporting structure and adjacent elements of the building.
(c) bracing
The building designer should specify all bracing. Trussed rafter roofs should be braced in accordance with Table 1 in Appendix 7.2-C, unless the roof is designed and braced in accordance with BS 5268 : Part 3.
All timber bracing to trussed rafters should be at least 100mm x 25mm in section and twice nailed to each trussed rafter. Nailing should be 3.35mm (10 gauge) x 65mm long galvanized round wire nails.
(d) spacing
Trussed rafters should not be spaced at centres greater than 600mm. Where this cannot be achieved, eg to accommodate hatch openings or chimneys, the spacing of trussed rafters may be increased as shown below provided that the spacing between centres of trimming trussed rafters does not exceed 2 times the design spacing of trussed rafters and that b is smaller than or equal to 2a - c, where:
a = design spacing of trussed rafters
b = distance between centres of trimming trussed rafter and adjacent trussed rafter
c = nominal width of required opening.
(e) mono-pitch trusses and girder trusses
Hipped roofs constructed with trussed rafters will generally require a series of diminishing mono-pitched trusses supported by a girder truss.
The bearing of mono-pitched trusses into shoes should be as follows:
| Span | Minimum bearing length | Minimum thickness of truss member |
| Less than 4m | 50mm | 35mm |
| 4m or more | 75mm | 35mm |
Alternative bearings should be designed by an Engineer in accordance with Technical Requirement R5.
(f) water tank support
Where water tanks are supported by roof trusses, their load should be transferred to the node points of the trussed rafter and spread over at least three trussed rafters in accordance with BS 5268 : Part 3.
A correct method of water tank support is shown in Appendix 7.2-D.
Proprietary tank support systems should be assessed in accordance with Technical Requirement R3.
(g) multiple trussed rafters
All multiple and reinforcing timbers to simple or multiple trussed rafters should be designed to be permanently fastened together. The timber members should be either fixed together during manufacture or, alternatively, fully detailed drawings and specifications showing the fixing method should be supplied to the site to enable the components to be assembled correctly.
(h) roofs incorporating valleys or other special features
Roofs with hips, valleys or other special features should be designed by an Engineer in accordance with Technical Requirement R5. Standard designs supplied by manufacturers which incorporate these features may usually be acceptable.
(i) combined trussed rafter and cut roofs
Particular care is needed where trussed rafters and a cut roof are combined in a roof design. The designer should provide details of the complete roof. Trussed rafters supporting traditional cut roof members should be designed by an Engineer in accordance with Technical Requirement R5.
(j) strutting to attic trusses
The part of an attic truss which forms a floor should have strutting in accordance with Appendix 7.2-F.
TRADITIONAL CUT ROOFS
Items to be taken into account include:
(a) recognised design standards
Sizes of certain roof members for basic pitched roofs are given in TRADA publications and BS 8103.
Where spans, sizes, spacing or strength classes of timber are outside the scope of the tables in the statutory regulations or where the form of roof is other than a basic pitched roof, the roof should be designed by an Engineer in accordance with Technical Requirement R5. Calculations should be based on BS 5268 : Part 2.
(b) member sizes
Unless the roof is designed by an Engineer in accordance with Technical Requirement R5, traditionally, nominal sizes of members would be as follows:
| Member | Minimum size (mm) |
| Struts and braces | 100 x 50 |
| Wall plates (Scotland) | 100 x 25 |
| Wall plates (Northern Ireland & the Isle of Man) | 100 x 38 |
| Wall plates (other) | 75 x 50 |
| Hips | rafter cut + 25 |
| Valleys | 32 thick |
| Ridges | rafter cut + 25 |
(c) triangulation
In the design of a cut roof, timber members should be triangulated or otherwise arranged to form a coherent structure. All forces inherent in the design should be resolved. The method of fixing or jointing members should be specified.
Particular care should be taken to ensure adequate triangulation when designs incorporate hips and valleys, and when cut roofs are used in conjunction with trussed rafters.
Details of all structural members should be provided.
(d) strutting to cut roofs
Any part of a cut roof which forms a floor should have strutting in accordance with Appendix 7.2-F.
TIMBER TREATMENT
Items to be taken into account include:
(a) treatment against House Longhorn Beetle
In some areas of the UK, treatment against House Longhorn Beetle is required. Reference should be made to relevant Building Regulations. Reference should also be made to Chapter 2.3 'Timber preservation (natural solid timber)' (Design).
(b) timber requiring treatment
The following timber members should be either naturally durable or suitably treated:
- fascias
- bargeboards
- soffits
- other trim.
For full protection, these timber members should also be painted or stained in accordance with the recommendations in Chapter 8.5 'Painting and decorating' (each section).
In pitched roofs with a fully supported weatherproofing membrane, the following timber members should be either naturally durable or suitably treated:
- rafters
- purlins
- ceiling joists
- bracing
- sarking
- wall plates
- battens for fixing vertical cladding.
The level of durability of all the above members can be achieved by natural durability or treatment with preservative. Reference should be made to Chapter 2.3 'Timber preservation (natural solid timber)' (each section) for guidance.
WEATHERTIGHTNESS
Items to be taken into account include:
(a) weathertightness of roof coverings
Roofs with a tile or slate covering should be designed in accordance with BS 5534 : Parts 1 and 2.
(b) tiles
For tiled roofs, the pitch, gauge and lap should be within the limits given in Table 1 of Appendix 7.2-B, unless the manufacturer specifies otherwise.
Fixings for single and double lap tiles should be designed in accordance with BS 5534 and BS 6399. Where tile manufacturers have computer programs based on these British Standards, their recommendations should be followed.
Tables 2 and 3 of Appendix 7.2-B contain minimum fixings for tiles. The tile manufacturer will be able to advise on any additional nails or clips required for a particular location.
(c) slates
Natural slates should be fixed in accordance with BS 5534 and BS 6399. Each slate should be nailed twice when centre nailed.
(d) roof coverings other than tiles or slates
Lead sheet roofing should be in accordance with BS 6915.
Other types of sheet roofing should be in accordance with the relevant parts of CP 143.
Roofs with the following traditional coverings should be designed in accordance with satisfactory custom and practice:
- natural stone
- shingles
- thatch.
Thatching should be as recommended by the Thatch Advisory Service or other appropriate authority in accordance with Technical Requirement R3.
Proprietary roof coverings should be assessed in accordance with Technical Requirement R3.
(e) roof underlays
An underlay should be provided for all tiled roofs.
The underlay may be felt to BS 747 or a proprietary sarking membrane complying with Technical Requirement R3. Some proprietary roofing underlays have a higher vapour resistance than bitumen felt and may need additional roof ventilation. Manufacturers' recommendations should be followed. Where the underlay is exposed, such as at the eaves, a type 5U felt should be used. A type 1F felt may be used for the remainder of the roof.
To prevent the underlay sagging at the eaves and forming a water trap behind the fascia, it is recommended that the underlay is supported by a continuous fillet. Where the pitch is below 30°, a continuous support fillet should be provided.
(f) rigid sarking
In areas of severe exposure, a rigid sarking with underlay is recommended and is normal practice in Scotland.
The choice of rigid sarking should take account of the type and fixing of the roof covering. The following materials are acceptable:
- tongued and grooved or square edged boarding to BS 1297
- bitumen impregnated insulating board to BS 1142 : Part 3 (sarking and sheathing grade)
- WBP or CBR plywood to BS EN 636, durability Class G
- type P5 chipboard to BS EN 312
- oriented strand board type OSB3 to BS EN 300
- proprietary products which have been assessed in accordance with Technical Requirement R3.
To avoid damage from condensation, proprietary insulation boards should be used strictly in accordance with the recommendations given in the independent assessment.
Where an underlay is fully supported and has a low vapour resistance (less than 5.7MNs/g), it should be used with counter battens, battens and roof covering.
Where underlay with a high vapour resistance is used (over 5.7MNs/g), the counter battens should be located between the sarking and the underlay. This is to allow ventilation below the underlay.
(g) flashings and other weathering details where a pitched roof abuts a vertical surface
Where a roof abuts a vertical surface, cover flashings, stepped cover flashings, soakers and back or parapet gutters should be provided as necessary. Where the roof is over an enclosed area the wall construction should include cavity trays linked to the flashings. Reference should be made to Sitework clause 7.2 - S12(f) for details.
Cover flashings should be tucked 25mm into a brick joint or chase not less than 75mm above the intersection with the roof.
Flashings and soakers should be of non-ferrous metal and of the same material to avoid electrolytic action.
Where lead is used, soakers should be at least Code 3 and flashings, gutters, saddles, etc should be Code 4 or better.
In the case of gutters behind parapet walls, provision should be made for an overflow in case the outlet becomes blocked.
(h) weathering details where a pitched roof intersects with a continuous waterproof membrane
For information on intersections with flat roofs, gutters or valleys, reference should be made to Sitework clause 7.2 - S12(e).
(i) pipes
Where soil pipes, vent pipes or other pipes penetrate roof tiling, a lead slate flashing, or a purpose-made accessory supplied by the roof covering manufacturer to form a weathertight joint, should be used.
If lead slates are used, they should be supported (eg using marine plywood) to prevent the lead sagging.
(j) chimneys
Flashings should connect with the chimney dpcs. The normal flashing components are shown in Sitework clause 7.2 - S12(g). Components will vary depending on whether the chimney intersects the roof at eaves or ridge level and the type of roof covering. Reference should be made to roof covering manufacturers' information sheets.
For more detailed information on the construction and weatherproofing of chimneys, reference should be made to Chapter 6.8 'Fireplaces, chimneys and flues'.
(k) ridges and hips
Ridge and hip tiles may be bedded on mortar or mechanically fixed. It is recommended that ridge tiles at gable ends and over separating walls are always mechanically fixed.
In areas of high exposure or where complex ridge features are involved, it may be necessary to mechanically fix all ridge tiles. The tile manufacturers' recommendations should be followed.
(l) valleys
A valley may be formed of purpose-made valley tiles or as an open valley lined with lead or other material acceptable under Technical Requirement R3.
Where slates or plain tiles are used, a laced valley, swept valley or mitred tiles with soakers may also be used.
Care should be taken to ensure that the true pitch of the valley is not less than the minimum allowed pitch.
(m) verges
Tiling at verges should project 40mm to 50mm beyond the gable wall or bargeboard.
Unless a proprietary dry verge system or cloaked verge is used, tiles should be bedded into mortar on an undercloak of plain tile, slate or cement-based board. Plain tiles should not be used as an undercloak below 30° pitch or on a bargeboard.
Purpose-made tile-and-a-half, or half-tiles, should be used at verges. Cut tiles are not acceptable.
Consider using an overhanging verge (by means of a gable ladder) to provide better weather protection to the gable wall, especially in exposed positions or where cavity insulation is used.
(n) proprietary roof coverings
Roof coverings not covered by a British Standard should comply with Technical Requirement R3.
Items to be taken into account include:
(a) moisture barrier
A moisture barrier should be provided behind all vertical tiling and slating.
Moisture barriers should be:
- underfelt or equivalent where the wall structure is brickwork or blockwork
- a breather membrane where the supporting structure is of timber construction.
For detailed information on the use of moisture barriers in association with timber frame construction, reference should be made to Chapter 6.2 'External timber framed walls'.
(b) batten size
Batten sizes should comply with Clause D3(e).
(c) fixing
Every tile or slate should be nailed twice and comply with the general requirements of BS 5534.
(d) weathering details
Bottom edges should be finished with an under-course tile. At dormer cheeks, the tiles or slates should be specified to be cut close to the slope of the roof, over a flashing fixed to the side of the dormer.
At internal or external angles, purpose made corner tiles or soakers should be used to form a weathertight joint.
Where pitched roofs abut masonry walls, a stepped flashing should be specified, turned behind the tiles. Details are shown in Sitework clause 7.2 - S12.
For information regarding vertical tiling or slating on walls, reference should be made to Chapter 6.1 'External masonry walls' (Design and Sitework) or Chapter 6.2 'External timber framed walls'.
INSULATION AND CONTROL OF CONDENSATION
The BRE Report 'Thermal insulation: avoiding risks' discusses aspects of insulation relevant to pitched roofs. In England and Wales account should be taken of Robust Details.
Insulation should be of sufficient thickness to meet the requirements of Building Regulations.
To reduce the risk of freezing, and condensation on pipework, the guidance in Sitework clause 7.2 - S14 should be followed.
Items to be taken into account include:
(a) ventilation of main roof spaces
Pitched roofs with insulation at ceiling level should always be ventilated to the outside air to minimise the risk of condensation.
Some proprietary underlays are highly impervious and special precautions are necessary to limit condensation, particularly during the drying out period.
Eaves ventilation should be provided on opposite sides of the roof to permit cross ventilation. Reference should be made to Sitework clause 7.2 - S11(a) for illustrations showing where ventilation should be provided.
Where the roof pitch is 15° or more, cross ventilation should be provided to the roof void equivalent to a 10mm slot running the full length of the eaves.
Where the ceiling follows the slope of a 'cold roof' regardless of pitch or where a cold roof has a pitch less than 15°, cross ventilation should be provided to the roof void equivalent to a 25mm slot running the full length of the eaves. At least 50mm clearance should be maintained between the insulation and the roof deck.
When the roof pitch exceeds 35° or when the span exceeds 10m, high level ventilation, equivalent to a continuous 5mm opening, should be used in addition to eaves ventilation.
The means of providing cross ventilation to mono-pitched roofs should be specified. BS 5250 indicates eaves ventilation together with the equivalent of a continuous 5mm slot at high level.
(b) position of vapour checks
Vapour control layers should be used in roof constructions where the ceiling board is fixed to the rafters and insulation is placed between the rafters.
In normal pitched roofs where insulation is placed at ceiling level and the void above is ventilated, a vapour control layer is not recommended. Moisture from the dwelling will be diffused through the ceiling and removed by roof space ventilation.
Vapour control layers, where required, should be placed on the warm side of insulation.
(c) ventilation of dormers
Pitched dormers should be ventilated from eaves to eaves or, where necessary, from eaves to ridge.
Flat roofed dormers of cold deck construction should be ventilated. The ventilation path should not be blocked by the timber structure, strutting, etc (reference should be made to Chapter 7.1 'Flat roofs and balconies' (Design and Sitework)).
(d) methods of ensuring unobstructed ventilation
Ventilation openings where the least dimension exceeds 10mm should be protected to prevent the entry of birds, etc.
Acceptable protection of openings can be provided by using materials complying with Materials clause 7.2 - M5(j).
A spacer in the eaves should be used so that ceiling insulation can be installed over and beyond the wall plate. This minimises the cold bridge without blocking the ventilation.
(e) other methods of reducing condensation
Methods of reducing condensation, such as draughtstripping loft hatches and sealing holes for services, are given in Sitework clause 7.2 - S14.
FIRE SPREAD
Items to be taken into account include:
(a) roof coverings
Slates and concrete or clay tiles are designated AA when tested to BS 476 : Part 3 and therefore can be used without limitation on any pitched roof. The use of some other materials is restricted by statutory requirements, particularly in relation to their distance from site boundaries (reference should be made to Building Regulations). These include:
- bitumen felt slates
- wood shingles
- thatch.
(b) chimneys and flue pipes
Combustible material, such as roof timbers and sarking felt, should be kept away from heat sources as described in Chapter 6.8 'Fireplaces, chimneys and flues' (Design).
The junction between a separating or compartment wall and a roof should be firestopped. If there are gaps, fire, smoke and flame can spread from one compartment to the next across the wall.
Mineral wool fire-stopping should be used to allow for movement in the roof timber, and avoid 'hogging' of the roof which is often associated with mortar fire-stopping.
Where a wall separates an integral garage from the rest of a dwelling, other arrangements are possible provided the principle of half-hour fire separation is maintained.
ACCESS
Access should be provided to:
- the main roof space, and
- roof voids that contain cisterns, tanks and the like.
SIZE OF OPENINGS
Access openings should be not less than 520mm in any direction. They should not be located directly over stairs or in other hazardous locations.
PROVISION OF WALKWAYS
Boarded walkways should be provided for maintenance purposes:
- between the access opening and any cistern located in the roof space
- around each cistern (at least 1m2).
Boarding should be securely fixed without compressing the loft insulation.
ROOF DRAINAGE
Items to be taken into account include:
(a) provision of gutters and downpipes
Roofs greater than 6m2 in area should be provided with rainwater gutters and downpipes. Consideration should also be given to the provision of rainwater drainage to roof areas less than 6m2, for example dormer and porch roofs.
(b) sizes
Gutters and downpipes should be of sufficient size to accommodate normal rainfall.
Care is needed in sizing gutters where dormer roofs interrupt the run-off from a pitched roof. The gutter should be sized to cope with the concentrated flows.
(c) discharge from one roof to another
Where water from a large roof surface discharges onto another surface, precautions should be taken to prevent erosion of the lower surface.
(d) discharge into drainage system
Unless designed otherwise, shoes should be provided to rainwater downpipes.
PROVISION OF INFORMATION
Full details of trussed rafter roofs should be available on site, including the following:
- layout drawing of trusses and associated items
- bracing requirements
- trimming around chimneys, access hatches, etc
- mono-pitch and lean-to roofs
- girder trusses, multiple trusses and diminishing trusses and how they are fixed together and supported on truss shoes, layboards or similar
- roof intersections (ie hips and valleys).
Assembly drawings are also important where there are complicated roof shapes or where trussed rafter and framed roofs are used in combination.
The drawings should show:
- the number and type of fixings for roof coverings
- means of providing eaves ventilation
- fire-stopping at separating wall and boxed eaves
- flashing details at abutments, chimneys, etc
- supports for water cisterns in the roof space
- restraint strapping
- position, thickness and limits of insulation.
Ensure that design and specification information is issued to site supervisors and relevant specialist subcontractors and/or suppliers.
