Civil Engineering Reference
In-Depth Information
The additional load imposed by the dormer on the main roof is relatively small, bearing in
mind that the most significant weight on a roof is the tiles, and this tile area of course is
not increased. The additional load must be allowed for.
Dormers, depending on the architecture of the area in which the house is to be built, will
have a variety of roof shapes. Whilst Fig. 3.17 illustrates a flat, mono-pitch or 'catslide'
and a conventional symmetrically pitched roof with gable ends, hip end dormers are not
uncommon.
Dormer Framework
The construction of the dormer is relatively simple. The critical part of the design is the
forming of an adequately strong framed opening within the main roof. In forming the hole
for the dormer, the continuity of roof slope support is removed and provision must be
made to carry the load both above and below the opening formed. If an upper and lower
purlin are used in the attic structure, then these members may be used to support the
rafters both above and below the opening. The perimeter of the hole formed in the main
roof will provide the foundation for the dormer framework itself. Figure 3.18a illustrates
the method of imposing simple dormer framework onto a trimmed opening. Figure 3.18b
illustrates the rules for trimmer numbers.
Roof Lights and Roof Windows
Roof lights generally will require much smaller openings within the main roof structure
than a dormer described above, therefore a similar method to that illustrated in Fig. 3.18
will be more than adequate. However, as the roof lights may not extend up the roof slope
the full distance between two purlins, separate secondary purlins or trimmers may have to
be introduced. If this is the case the rafters onto which the trimmers are fixed must be re-
inforced by attaching an additional rafter to each side of the opening, these additional
rafters extending from the lower to the upper purlin (see Fig. 7.26).
The term 'roof window' is a term recently introduced to the building industry to describe
what is in effect an opening roof light. The roof light is normally fixed, of course, and
provides only light to the building. Roof windows are most commonly of proprietary man-
ufacture with the manufacturers providing detailed guidance on the method of fitting the
roof window to both existing and new roof structures. Reference should be made to the
manufacturer's instructions if such a roof window is to be fitted.
