Party or separating walls

The other form of gable occurring in roof structures is that unseen gable constructed at the division between dwellings in pairs or terraces of houses. The wall in that position must be continuous to the underside of the tiles to ensure adequate fire break, and generally it is not acceptable to have timber members built into this wall, unless they are adequately separated between the houses with a material which will impart one hour fire resistance. Good practice would therefore dictate that timber in general is not built in to separating walls.

A problem which occurred in the early days of truss and trussed rafter use in this country was that of 'hogging' over these separating walls between buildings; this is illustrated in Fig. 8.15. To overcome this problem, the party wall brickwork or block-

Fig. 8.15 'Hogging' at compartment wall.

work must be kept down below the top of the rafter line by some 25 mm. BS 5268: Part 3 limits deflection to 12 mm for roofs up to 12 m span. Consequently at maximum deflection of the trussed rafter roof there is still a 12 mm gap between the top of the rafter and the top of the brickwork. This gap must be filled with compressible yet non-combustible material (usually mineral wool), the tile battens themselves being the only timber item to pass from one building to the other. The detail described avoids the problem of hogging and its unsightly effect on the roofs of terraced houses. Figure 8.16 illustrates the correct detail.

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