Figure 3-14.-Roofing underlayment: A. Single coverage;
B. Double coverage.
the roof with atop lap of at least 2 inches at all horizontal
points and a 4-inch side lap at all end joints (fig. 3-14,
view A). Lap the underlayment over all hips and ridges
6 inches on each side. A double underpayment can be
started with two layers at the cave line, flush with the
fascia board or molding. The second and remaining
strips have 19-inch head laps with 17-inch exposures
(fig. 3-14, view B). Cover the entire roof in this manner.
Make sure that all surfaces have double coverage. Use
only enough fasteners to hold the underpayment in place
until the shingles are applied. Do not apply shingles over
In areas where moderate-to-severe snowfall is
common and ice dams occur, melting snow refreezes at
the cave line (fig. 3-15, view A). It is a good practice to
apply one course of 55-pound smooth-surface roll
roofing as a flashing at the eaves. It should be wide
Figure 3-15.-Protection from ice dams A. Refreezing snow
and ice; B. Cornice ventilation.
enough to extend from the roof edge to between 12 and
24 inches inside the wall line. The roll roofing should
be installed over the underpayment and metal drip edge.
This will lessen the chance of melting snow to back up
under the shingles and fascia board of closed cornices.
Damage to interior ceilings and walls results from this
water seepage. Protection from ice dams is provided by
cave flashing. Cornice ventilation by means of soffit
vents and sufficient insulation will minimize the melting
(fig. 3-15, view B).
ASPHALT FELT. Roofing felts are used as
underpayment for shingles, for sheathing paper, and for
reinforcements in the construction of built-up roofs.
They are made from a combination of shredded
wood fibers, mineral fibers, or glass fibers saturated
with asphalt or coal-tar pitch. Sheets are usually
36 inches wide and available in various weights from 10
to 50 pounds. These weights refer to weight per square
ORGANIC FELTS. Asphalt-saturated felts
composed of a combination of felted papers and organic