Civil Engineering Reference
In-Depth Information
Having surveyed the roof void, it is also necessary to survey the building below accur-
ately, right down to ground level. It is most important to establish which of the internal
walls are load bearing, i.e. which have a full foundation below them, in order that the ex-
isting supporting structure can be clearly established. In relatively modern houses it
should be simple to acquire plans of the building, either through deeds, a builder (if
known) or the local authority building control department which approved the building
originally. On older buildings, a good guide is to lift floor coverings to find the direction
of the floor boarding; the floor joists will run at right angles to the floor boarding and the
walls onto which the floor joists are supported will be load bearing. Partition walls
between rooms on the upper floors may not be of load bearing construction and this must
be clearly established before considering them for possible support for the new floor to the
attic.
Typical Roof Types
The Terrace
The roof void in a typical terraced house is illustrated in Fig. 3.2; the distance between the
party or compartment walls, i.e. those dividing the houses, is often relatively small and
there are frequently load bearing internal walls to be found on the ground floor, supporting
the first floor joists. As can be seen from the figure, by removing the 'collars' a useful
room void would be achieved. The collar is, of course, there to tie the rafters together by
stiffening the purlins and bracing the rafters. If the collar is removed some alternative re-
straint must be found. If room height is adequate, extra collars could be fitted to act as at-
tic ceiling joists and to lighten the load on the purlins; an additional ridge purlin can be fit-
ted, supported on steel shoes built into the party wall. It should be noted at this point that
in some terraces no party wall will be found as has been stated earlier, and if this is the
case then one must be constructed using methods adequate to provide both fire and sound
performance to the adjoining properties.
Traditional Hip and Valley Roofs
Traditionally constructed hip roofs will generally be of substantial construction, but they
frequently defy engineering analysis to prove their stability. Traditionally, much reliance
was placed on 'tosh' or 'skew' nailing, something the engineer is unable to prove but the
passage of time has proven to work surprisingly well (see Fig. 8.6).
