Figure 3-31.-Laying a five-ply built-up roof.
approximately 400 pounds per square and the slag 325
pounds per square. Other aggregates would be applied
at a rate consistent with their weight and opacity. The
surface aggregate protects the bitumen from the sun and
provides a fire-resistant coating.
CAP SHEETS.A cap sheet surface is similar to
gravel-surfaced roofings, except that a mineral-surface is
used in place of the flood coat and job-applied gravel.
Cap-sheet roofing consists of heavy roofing felts (75 to
105 pounds per square) of organic or glass fibers.
Mineral-surfaced cap sheets are coated on both sides
with asphalt and surfaced on the exposed side with
mineral granules, mica, or similar materials. The cap
sheets are applied with a 2-inch lap for single-ply
construction or a 19-inch lap if two-ply construction is
desired. The mineral surfacing is omitted on the portion
that is lapped. The cap sheets are laid in hot asphalt
along with the base sheet. Cap sheets are used on slopes
between 1/2: 12 and 6:12 where weather is moderate.
emulsions, cutback asphalts, or patented products can be
applied over the top ply of a hot-mopped roof or as an
adhesive between plies. If emulsified asphalt is to be
used as art adhesive between plies, special plies (such as
glass fiber) must be used that are sufficiently porous to
allow vapors to escape. Decorative and reflective
coatings with asphalt-emulsion bases have been
developed to protect and decorate roofing.
DRAINAGE.When required, positive drainage
should be established before the installation of built-up
roofing. This can be achieved by the use of lightweight
concrete or roofing insulation placed as specified with
slopes toward roof drains, gutters, or scuppers.
APPLICATION PROCEDURES.Built-up roofing
consists of several layers of tar-rag-felt, asphalt-rag-felt,
or asphalt-asbestos-felt set in a hot binder of melted
pitch or asphalt.
Each layer of built-up roofing is called a ply. In a five-
ply roof, the first two layers are laid without a binder;
these are called the dry nailers. Before the nailers are
nailed in place, a layer of building paper is tacked down
to the roof sheating.
A built-up roof, like a shingled roof, is started at the
eaves so the strips will overlap in the direction of the
watershed. Figure 3-31 shows how 32-inch building
paper is laid over a wood-sheathing roof to get five-ply
coverage at all points in the roof. There are basically
seven steps to the process.
1. Lay the building paper with a 2-inch overlap.
Spot-nail it down just enough to keep it from
2. Cut a 16-inch strip of saturated felt and lay it
along the eaves. Nail it down with nails placed 1
inch from the back edge and spaced 12 inches OC.