Civil Engineering Reference
In-Depth Information
section of these panels may have to be left off until the upper rafters J are nailed in posi-
tion on the purlin and lapped to the top of rafters A.
Finally rafters J and their ridge can be fitted, the rafters being birdsmouthed over the
purlin in the usual manner and well lapped and nailed to the top of rafters A. Racking
sheathing E can now be completed. Diagonals K and L can now be fitted to the underside
of the rafters and to the top of the ceiling collars C.
All timbers mentioned above for bracing can be of 22 mm × 97 mm section, with the spe-
cification for nailing and lapping, if necessary, as set out in Chapter 5.
The final brace for an attic roof is of course the floor boarding or sheeting applied on top
of the floor joists between the uprights G. This floor diaphragm is subjected to more load-
ing by the structure above it than with a conventional two-storey building. It is essential
therefore that the floor joists are solidly bridged between them, or fitted with herringbone
strutting, and that supports are provided to carry the floor boarding, particularly at the
junction with the wall G. Correct nailing of any boarding used for the floor must be used;
if chipboard is used this would mean annular ring shanked nails and gluing of the tongued
and grooved joint between the boards.
The Effect of Openings on Bracing
All attic roofs will have some openings in the sloping part of the ceiling, either in the form
of roof lights or dormers. The effect of these openings on the bracing will naturally de-
pend upon the frequency and size of opening in either or both roof's slopes in relation to
the overall length of the building. Clearly the provision of adequate bracing in the form of
the panels E must be considered and if this cannot be provided immediately adjacent to a
gable wall, then continuity between the panels near the wall and those repositioned be-
cause of an opening must be maintained to allow the two separate areas of bracing to act
as one.
Stairwell openings within the main body of the building are unlikely to cause any signific-
ant decrease in the effectiveness of the floor diaphragm. Those staircases constructed par-
allel with the floor joists and immediately adjacent to a gable wall, however, may present
stability problems for that gable wall which will invariably be supporting one of the pur-
lins carrying the roof. A means of providing an alternative to brace H must be found and
additional attention may be needed to gable end restraint in such instances, above and bey-
ond that dealt with in the section on gable wall restraint in this chapter.
