Major roof openings are those that interrupt the normal run of
rafters or other roof framing. Such openings may be for ventilators, chimneys,
trap-door passages, or skylight or dormer windows.
Figure 7-22 shows roof-opening construction.
Roof openings are framed by headers and trimmers. Double headers are
used at right angles to the rafters, which are set into the headers in
the same way as joists in floor-opening construction. Trimmers are actually
double rafter construction in roof openings. Nailing strips
may be added if needed.
Asphalt and asbestos-cement roof coverings are most frequently used
on pitched-roof structures. Built-up roofing is used mainly on flat or
nearly flat roofs.