If a roof is not a simple triangle, all members should be fully supported and tied together. The design details for sizes of timber members should be followed.Īll framing should be completed before roof coverings are laid.
(b) prevention of distortion and overloading Positions of standard structural members are shown in the diagrammatic representation below: POLE PLATES: similar to purlins but used where ceiling joists are above wall plate level.CEILING BINDERS and HANGERS: support long span ceiling joists.COLLAR: ties the roof together at purlin level.The following are extra members which may be used on large roofs: STRUTS: give support to purlins to prevent deflection and transmit roof loading to loadbearing structure below.PURLIN: supports long span rafters to prevent deflection and increase stiffness.RIDGE: provides fixing and spacing for the tops of rafters.CEILING JOIST or TIE: triangulates the rafters, stopping the walls and roof spreading outwards supports the ceiling finish and any walkways, etc.RAFTER: carries the weight of the roof finish, eg tiles, tile battens and underfelt.In a typical traditional roof, the basic timber members are: Purlins and binders should be built in, where necessary. Items to be taken into account include: (a) location of membersĪll members should be accurately located. the soffit board is cut carefully and then fixed securely so as to restrain the outer leaf.there is blocking between the last trussed rafter and the inner leaf (at a maximum of 2m spacing), and.The gable ladder can be used to provide restraint to the external wall if: 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. The spacing or structure of trusses should not be altered without the designer's approval. using double skew nailing or truss clips.Īvoid damaging the metal truss plates, trussed rafters or wall plates.Trussed rafters should be fixed to the wall plates either: Temporary bracing should be provided to control the spacing and keep trusses vertical. Trussed rafters should be evenly spaced and vertical. Trussed rafters should be supported only at the junction between the ceiling tie and rafter, unless specifically designed otherwise, eg as a cantilever. The number of fixings should be in accordance with design requirements and the lowest fixing should be within 150mm of the bottom of the vertical strap.ĭetailed guidance on the use and handling of trussed rafters is given in the International Truss Plate Association Technical Handbook available from trussed rafter suppliers.ĭetailed drawings should be available on site to show the layout of the trussed rafters, especially at hips, valleys and trimmings to chimneys, etc. 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. 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). The turn-down should be on a substantial piece of blockwork, preferably the centre of an uncut block. Straps should go under rafters and over ceiling joists. Rafters should not be notched to make the straps flush with the rafter. The fixings should be four steel screws or four 75mm x 4mm (8 SWG) round nails.
Restraint straps should be fixed to solid noggings with a minimum of four fixings of which one should be in the third rafter. They should be installed as shown in the design. Restraint straps, or a restraining form of gable ladder, are required to provide stability to walls.
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