What This Requirement Covers
The roof structure must safely carry its own weight, imposed loads (snow, maintenance access), and wind loads, and transfer these loads to the supporting walls. Approved Document A provides some simplified guidance, but most roof structures are designed to BS EN 1995 (Eurocode 5: Timber Structures) or manufacturer-specific design for trussed rafters.
Key Requirements
Trussed Rafters
- Trussed rafter roofs are the most common roof form in UK housebuilding
- Each truss must be designed and manufactured to BS EN 1995 by the truss fabricator
- Trusses must be braced in accordance with BS 5268-3 (now withdrawn but still widely referenced) or the Trussed Rafter Association (TRA) guidance
- Diagonal bracing must be provided to prevent racking and buckling of trusses
- Longitudinal binders must be fixed to all truss members at maximum 2 metre intervals
- Trusses must be fixed to the wall plate with truss clips or straps
Cut Roofs (Traditional)
- Cut roofs using individual rafters, purlins, and ridge boards are designed to BS EN 1995 or, for simple domestic roofs, using span tables
- Rafter sizes depend on span, spacing, and imposed loads
- Purlins must be supported at intervals not exceeding the maximum span for the purlin size
- Collar ties or ceiling joists must provide triangulation to prevent the roof spreading
Wind Bracing
- All roofs must be designed to resist wind uplift and horizontal wind loads
- Roof trusses and rafters must be strapped to the wall plate to resist uplift: 30 mm x 5 mm galvanised steel straps at every truss or rafter, or at maximum 2 metre centres
- Gable walls must be strapped to the roof structure with straps extending over at least two trusses
- In exposed locations (coastal, hilltop), wind loads may require additional holding-down fixings
Roof Coverings
- Roof tiles and slates must be fixed in accordance with BS 5534 (Code of practice for slating and tiling)
- BS 5534 specifies the fixing method (nails, clips, or mechanical fixings) based on wind zone, building height, and tile type
- All perimeter tiles/slates, ridge tiles, and hip tiles must be mechanically fixed
- Underlay must comply with BS 5534 and be supported to prevent ponding
Practical Compliance Tips
- Order trussed rafters early; they are designed specifically for each project and have lead times of 2-4 weeks
- Ensure the truss layout drawing is available on site and followed precisely during erection
- Do not cut, notch, or alter trussed rafters without the manufacturer's written approval
- Install all bracing and binders before loading the roof with tiles or slates
- Check that wall plates are level and securely fixed to the wall before placing trusses
- In loft conversions, the existing roof structure may need to be significantly altered; always engage a structural engineer
- Keep the truss design certificate and bracing layout for the building file