HomeMy WebLinkAboutLow pitch roof codeR905.2.6 Attachment. Asphalt shingles shall have the mini-
mum number of fasteners required by the manufacturer. For
normal application, asphalt shingles shall be secured to the roof
with not less than four fasteners per strip shingle or two fasten-
ers per individual shingle. Where the roof slope exceeds 20
units vertical in 12 units horizontal (20:12), special methods of
fastening are required. For roofs located where the basic wind
speed per Figure R301.2(4) is 110 mph (177 km/h) or greater,
special methods of fastening are required. Special fastening
methods shall be tested in accordance with ASTM D 3161,
modified to use a wind speed of 110 mph (177 km/h).
Shingles classified using ASTM D 3161 are acceptable for
use in wind zones less than 110 mph. Shingles classified using
ASTMD3161 modified to use awind speed of 110mph are ac-
ceptable for use in all cases where special fastening is required.
Commentary: In all cases, the minimum number of fasteners for as-
phalt shingles must be the number required by the
shingle manufacturer. It is typical for asphalt strip
shingles to be attached to the roof sheathing by at leastfour fasteners for each strip shingle. If individual
shingles are installed, at least two fasteners are re-
quired per shingle. For very steep roof slopes, the
manufacturerÂ’s installation instructions should be re-
viewed for any special fastening methods.
Where the basic wind speeds in accordance with
Figure R301.2(4) are greater than or equal to 110 mph
(177.1 km/h), special testing is required to determine
how such roofing should be fastened. The fastening
methods must be specifically tested to ASTM D 3161,
but the test must be modified to accommodate the
wind speed of 110 mph (177.1 km/h).
ASTM D 3161 deals with several ranges of wind
speeds. More basic wind speeds are considered up to
110 mph (177.1 km/h). Therefore, areas prone to high-
er winds must comply with the modified test.
R905.2.7 Underlayment application. For roof slopes from
two units vertical in 12 units horizontal (17-percent slope), up
to four units vertical in 12 units horizontal (33-percent slope),
underlayment shall be two layers applied in the following man-
ner. Apply a 19-inch (483mm)strip of underlayment felt parallel
with and starting at the eaves, fastened sufficiently to hold in
place. Starting at the eave, apply 36-inch-wide (914mm) sheets
of underlayment, overlapping successive sheets 19 inches (483mm),
and fastened sufficiently to hold in place. For roof slopes
of four units vertical in 12 units horizontal (33-percent slope) or
greater, underlayment shall be one layer applied in the following manner.
Underlayment shall be applied shingle fashion,
parallel to and starting from the eave and lapped 2 inches (51
mm), fastened sufficiently to hold in place. End laps shall be
offset by 6 feet (1829 mm).
Commentary: This section specifies the underlayment requirements
for asphalt shingles installed on both low-slope and
high-slope roofs. Low-slope roofs present a potential
problem because water drains slowly, and this creates
the opportunity for water back-up. Wind-driven rain
can also pose a problem. Therefore, a special under-
layment applicationmethod is used so that the roof re-
mains weathertight. All portions of the roof will be pro-
tected by a minimum of two layers of underlayment if
the installer follows the instructions found in this sec-
tion. Commentary Figure R905.2.7 illustrates the re-
quired application of underlayment for a low-slope ap-
plication. Only one layer of underlayment, applied in
shingle fashion in accordance with this section, is
mandated for high-slope roofs of asphalt shingles.