Trussed rafters should be evenly spaced and vertical. Temporary bracing should be provided to control the spacing and keep trusses vertical. Most often, roof truss clips are included in your overall quote. However, should you need to buy these, you would be looking at a cost of around 25-40p per clip – a very minimal cost. This is a small price to pay to ensure your designs are validated and safe.

When inspecting trussed rafter roofs, be on the look out for the removal of any internal webs or any other alteration to the trusses where bolts, nails or screws have been used to fix members. Such an addition to a roof structure would generally indicate than some other member has been removed. Restraint straps should be fixed to solid noggings with a minimum of four fixings of which one should be in the third rafter. The fixings should be four steel screws or four 75mm x 4mm (8 SWG) round nails. Where bedded in 1:3 cement : sharp sand with plasticiser mortar the following tiles should also be nailed: For a given span, and a set load therefore, the pitch makes a huge difference to the member forces. The diagram above shows an example of the triangle of forces that apply at a support. The vertical component is the load passed onto the support (“the reaction”), the sloping line the top chord force, and the horizontal force. As the pitch flattens the forces are greatly magnified. Where a flat roof adjoins a pitched roof, or where valleys or gutters occur, the waterproof membrane should be carried up under the tiling to a height of 150mm above the flat roof, valley or gutter and lapped by the roofing underlay.

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However in many other instances trusses are simply laid flat upon the floor. Such a practice is satisfactory provided that the trusses have a firm and level support of the ground. However this again would involve some preparation. Provides support to purlins to prevent deflection and to transfer roof loads to the load-bearing structure below. Ventilation openings where the least dimension exceeds 10mm should be protected with mesh to prevent entry of birds, etc.

Celotex insulation fixings are a perfect solution when installing rigid insulation boards in between timbers. Trimming members around dormers should be large enough to take the extra load from the cut main roof members and dormer framing and cladding, as detailed in the design.The underlay should be turned up at least 100mm at all abutments to prevent rain and snow being blown into the roof space. All verge tiles and slates should be bedded on an undercloak. Alternatively, proprietary dry verge systems should be fixed in accordance with manufacturers' recommendations. Where proprietary eaves ventilators are used, they should be fixed in accordance with the manufacturer's instructions. A strip of underlay, at least 600mm wide, should be laid over hips and above the underlay to the main roof. Two clips are used per roof truss. One is placed on either side, where the truss is fixed to the wall plate. Usually, the roof truss clip is installed on the inside wall plate. However, you can also fit these on the outside wall plate if the detail of the truss heel allows.

The British Standard for trussed rafters, The Trussed Rafter Association and the NHBC handbook amongst other sources of information provide guidance on the bracing of trussed rafter roofs. Trussed rafters can almost be considered as a thin playing card in terms of their robustness about their weaker axis. Therefore for their stability and for the stability of the other roof a series of diagonal bracing and longitudinal bracing is required to ensure stability. A truss clip should always be fitted vertically to a wall plate, never lying flat or horizontally. You must make use of every hole on the face of the roof truss clip, and nail all of these in. Some installers may only use four of the thirteen holes which are on the clip – this is not utilising the strength of the clip fully. When using the following materials, the detailing and workmanship should follow the design and recognised good building practice: At valleys, a similar strip should be laid under the main roof underlay and held down by the valley battens, where used. The main roof underlay should be dressed over the valley battens.A truss is so much more than the sum of its parts. Any alteration to a truss once manufactured is likely to be involved and costly. Surveyors and Inspectors need to be vigilant in spotting unauthorised alterations. Sin No 2 Unauthorised alterations If a roof is not a simple triangle, all members should be fully supported and tied together. If necessary, temporary support to long span members should be used until the framing is complete. (c) valley and hip construction The design should specify the number of fixings for clay and concrete tiles. Tables 2 and 3 of Appendix 7.2-B contain minimum fixings for tiles. Additional nails and clips may be necessary in accordance with the design. A fixing schedule produced by the tile manufacturer, based on The Zonal Method, is acceptable. The webs are set out by considering the overall internal triangle. The intersection of the webs with the top chord will normally be at the quarter points. The intersection of the webs with the bottom chord will normally be at the third points of the internal triangle. For small span or for large span trusses, alternative internal configurations are used Connections

Trussed rafters should be supported only at the junction between the ceiling tie and rafter, unless specifically designed otherwise, eg as a cantilever. Where cheek framing does not extend to floor level, a double rafter will give necessary support to the cheek. The two rafters must be fixed together.

Dormers should be framed up so they are independent of the window frame, using a suitable lintel over the opening. All abutments should be weatherproofed using non-ferrous metal flashings. Lead flashings should be at least Code 4 (colour coded blue), while soakers are normally Code 3 (colour coded green). Normally, lead flashings should not exceed 1.5m in length, with laps of not less than 100mm. For any further information or prices please do call our national sales office. Buy Celotex Insulation fixings for fixing Pitched roof insulation currently unavailable Battens should be fixed with cut or wire nails. The nail shank can be smooth, annular ringed or helically threaded. Nails can be steel or aluminium. In coastal areas, steel nails should be hot dip galvanized. (d) slates and tiles