Also see:
BM TRADA Eurocode 5 span tables (4th edition)
BS 8103-3
7.2.3 Design of pitched roofs
The sizing and spacing of members shall ensure structural stability and provide restraint to the structure without undue movement or distortion. Issues to be taken into account include:
- trussed rafter roofs
- traditional cut roofs.
The design of pitched roofs should:
- have dead, imposed and wind loads calculated in accordance with BS EN 1991-1-1, BS EN 1991-1-3 and BS EN 1991-1-4
- be in accordance with PD 6693-1, and Technical Requirement R5, where appropriate
- be appropriate for the location, accounting for exposure and wind uplift
- ensure that the structure is coherent and that all forces are resolved
- ensure the deflection of floors formed by the bottom chords of attic trusses are in accordance with the requirements of Clause 6.4.9 ‘Timber joists’
- ensure that where raised collar trusses are used, as part of the room-in-roof construction, the horizontal deflection of the trusses at the supports should be no more than +/- 6mm
- ensure stability with the complete structure, including the connections and compatibility with the supporting structure and adjacent elements
- where trussed rafters and a cut roof are combined, the designer should provide details of the complete roof (particular care is needed in such circumstances).
Roofs should be designed by an engineer in accordance with Technical Requirement R5 where:
- the roof is not a basic pitched roof
- hips, valleys or other special features are included in a trussed rafter roof
- the spans, sizes, spacing or strength classes of the timber are outside the scope of authoritative tables
- trussed rafters support traditional cut roof members, or
- it is a proprietary system (designs supplied by manufacturers will generally be acceptable).
Structural timber components should be of a suitable strength class as specified by the designer to BS EN 338. Solid structural timber should be:
- machine graded to BS EN 14081, or visually graded to BS 4978 for softwoods or BS 5756 for hardwoods
- assigned a strength class based on BS EN 1912 when visually graded
- dry graded
- marked in accordance with BS EN 14081.
Further guidance on strength classes for certain timber species can be found in PD 6693-1.
Engineered wood products such as I-section or metal-web studs, joists and rafters should be assessed in accordance with Technical Requirement R3.
Trussed rafter roofs#
Trussed rafters should be:
- installed in accordance with the design, and the structure or spacing should not be altered without prior consent from the designer
- fixed to the wall in accordance with the design (eg using double skew nailing or truss clips)
- vertical and suitably located (where necessary, temporary bracing should be used to maintain spacing and to keep trusses vertical)
- evenly spaced at maximum 600mm centres.
Where the maximum 600mm spacing cannot be achieved, eg to accommodate hatch openings or chimneys, spacing may be increased to a maximum of twice the nominal spacing, provided that the spacing X is less than, or equal to, 2A-B where:
- X = distance between centres of trussed trimmed rafters and the adjacent trussed rafter
- A = design spacing of trussed rafters
- B = nominal width of opening.
Where multiple and reinforcing timbers to simple or multiple trussed rafters are used, they should be:
- designed to be permanently fastened together
- either fixed together during manufacture, or fully detailed drawings and specifications showing the fixing method should be supplied.
Hipped roofs constructed with trussed rafters typically require a series of diminishing mono-pitched trusses supported by a girder truss.
The bearing of mono-pitched trusses into shoes should be in accordance with Table 1, unless designed by an engineer in accordance with Technical Requirement R5.
Table 1: Bearing length of mono-pitched trusses into shoes
| Span | Minimum bearing length | Minimum thickness of trussed rafter |
|---|---|---|
| Less than 4m | 50mm | 35mm |
| 4m or more | 75mm | 35mm |
Ceiling finishes should be fixed according to the spacing of the support members and the thickness of the sheet. Plasterboard should be fixed as follows:
- 9.5mm plasterboard should be fixed at a maximum support spacing of 450mm
- 12.5-15mm plasterboard should be fixed at a maximum support spacing of 600mm
- additional members will generally be required to support coverings and finishes where trusses are spaced further apart.
Where the width of a gable ladder exceeds that of the trussed rafter centres, noggings should be used to reduce the span of the roofing tile battens.
Traditional cut roofs#
For traditional cut roofs:
- the design should specify the details of each structural member and the method of fixing or jointing
- the roof should be in accordance with the design and members accurately located
- members should be fully supported and tied together where necessary, particularly where the roof is not a simple triangle
- temporary support to long span members should be used until framing is complete
- purlins and binders should be built in where necessary
- framing should be completed before coverings are installed.
Table 2: Basic timber members
| Member | Notes |
|---|---|
| Valley rafter | Provides support for loads from both sections of the roof and should:
• be larger than ordinary rafters to take the additional load • provide full bearing for the splay cut of jack rafters • be provided with intermediate support where required |
| Hip rafter | Provides spacing and fixing for jack rafters and should:
• have a deeper section than the other rafters to take the top cut of the jack rafters. Purlins should be mitred at hips and lip cut to accept the bottom of the hip rafter |
| Ceiling joist or ties | Provides support for the rafters and should:
• stop the walls and roof spreading outwards • provide support to the ceiling finish and walkways, etc |
| Ridge | Provides fixings and spacing for the tops of the rafters |
| Purlin | Provides support to long span rafters to prevent deflection and increase stiffness |
| Struts | Provides support to purlins to prevent deflection and to transfer roof loads to the load-bearing structure below |
| Collar | Ties the roof together at purlin level |
| Ceiling binders and hangers | Provides support to long span ceiling joists |
| Pole plates | Similar to purlins, but used where ceiling joists are above wall plate level |
Generally sizes should be as Table 3, unless designed by an engineer in accordance with Technical Requirement R5.
Table 3: Typical sizes for timber members
| Member | Minimum size |
|---|---|
| Struts | 100mm x 50mm |
| Valley rafter | 32mm thick |
| Ridges and hips | Rafter cut + 25mm |
Last updated: 2nd January 2024