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

RAFTERS to ceiling joists: nailed lapped joint. The rafter should be birdsmouthed over and skew nailed to the wall plate 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. In roofs with plain tiles, purpose-made valley coursing tiles should be used. Adjacent roof tiles should be cut neatly to form a smooth junction, preferably cutting from tile-and-a-half tiles.

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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. Restraint straps, or a restraining form of gable ladder, are required to provide stability to walls. They should be installed as shown in the design.

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. 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. Cavity trays should be linked to the flashing to prevent water penetrating into an enclosed area where a:

Definition

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. Trussed rafters should be evenly spaced and vertical. Temporary bracing should be provided to control the spacing and keep trusses vertical. The guidance above will assist in reducing the risk of condensation occurring but is not acceptable as an alternative to cross ventilation of the roof space.