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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. 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 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.
Notes for specifiers and builders. The advice provided by trussed rafters manufacturers should be carefully adhered to and bracing in non standard situations should be applied. Sin No 4 Overloading / under loading
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Detailed guidance on the use and handling of trussed rafters is given in the International Truss Plate Association Technical Handbook available from trussed rafter suppliers. Where straps are fixed to masonry, hardened nails 4mm in diameter x 75mm long or No 12 wood screws x 50mm long into plugs should be used. Trusses are made by some forty four manufacturing companies dotted around the country. Trusses are engineered to suit the particular application requested. The software used by each manufacturer will be supplied by one of three companies who also make the connector plates used to fasten the members together (often referred to as “the system owners”). The major System Owners are Roof truss clips generally come in two different sizes, 38mm and 50mm. The most suitable size clip to use will depend on the size of the timber used to make the trusses. It is best to speak to your truss provider to make sure that you have the correct size clips. They can advise you further.
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. Ridge and hip tiles are bedded in 1:3 cement : sharp sand with plasticiser mortar or mechanically fixed using a proprietary dry ridge fixing system following the manufacturer's instructions. The method shown in the design should always be used. Conservative estimates would suggest there is somewhere in excess of 60 million trussed rafters in service in the UK. To date, there has been no known failure of a trussed rafter in its “ex factory” condition, a remarkable safety record. However, experience has shown that trussed rafters are not always well understood by the people who specify them or who erect them. Such misunderstanding can, in some circumstances, lead to poor installation or worse unsuitable alteration. Objective
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A purpose-made one-piece flashing and upstand should be used around pipes projecting through the tiling. 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 Ridge and hip tiles, for a distance of at least 900mm from the face of rigid masonry supports such as gables, abutments and separating walls, should be mechanically fixed.
Where a pitched roof abuts the wall at an angle, a stepped cavity tray linked to a stepped flashing should be used. On most dormers, the DORMER CHEEK STUDS should be supported either by a double rafter or by a double floor joist. 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. If the design specifies holding down straps to prevent the roof being lifted off the supporting structure, they should be at 2.0m centres (maximum). Angle ties should be used on hipped roof corners to prevent the wall plates spreading. For heavily loaded hip rafters, eg where they are carrying purlins, dragon ties or similar bracing should be used to prevent hip rafter spread.
Ventilation openings where the least dimension exceeds 10mm should be protected with mesh to prevent entry of birds, etc. Trussed rafters enabled Architects the freedom with the layout of the upper floor by avoiding the need for internal load bearing walls. Consequently, the ground floor layout and upper floor layout need no longer harmonise. Kitchens could be bigger than the bathrooms above them, and bedroom layouts could be independent of the lounge and dining room layout. The Seven Deadly Sins
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. If you are securing more than one roof truss, then you will need to use framing anchors. This is because roof truss clips are limited to being 38 or 50mm. If dealing with a set of two, three, or 4 trusses, then framing anchors can go either side of the truss at either end of the wall plate. Framing anchors, similarly to truss clips, provide a secure fixing. Where the width of gable ladders exceeds that of the trussed rafter centres, noggings should be provided to reduce the span of the roofing tile battens.Particular care is needed where pipes project through the underlay. Torn underlay around pipes can lead to the ceiling becoming wet and stained. Cut a small cross in the underlay and sleeve over the pipe so that the tongues turn up. RAFTERS to ceiling joists: nailed lapped joint. The rafter should be birdsmouthed over and skew nailed to the wall plate It is essential when installing celotex rigid insulation boards in cut to fit roof applications to ensure they are secured firmly. Insulation boards falling down on your head is no fun and also you really don’t want the weight of the boards resting on your plasterboard finish. Celotex insulation clips are and ideal purpose made fixing that ensures the PIR insulation boards are kept firmly in their place. The advantage of these clips is that if you have uneven timbers or if you are unable to cut the insulation to fit tightly in between the timbers. Then these fixings will work perfectly holding the insulation in position. Dormers should be framed up so they are independent of the window frame, using a suitable lintel over the opening.
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