Metal open valley recommendations | Technical ElementsMaciek Rupar
Of the three basic types of valley construction for asphalt
shingle roof systems—open, closed-cut and woven—only
open construction relies on metal valley lining for
weatherproofing. With the other two construction types, shingles
line the valley's center. However, recommendations differ regarding
open valley installation on asphalt shingle roof systems.
NRCA recommends
open valley lining for use with asphalt shingle roof systems be
formed from a minimum 18-inch-wide metal and include a center rib
or splash diverter no less than 1 inch high. This configuration
affords 8-inch flanges on either side of the center rib, which
allows asphalt shingles to lap onto a flange at least 4 inches.
Open valley metal thickness should be selected to match asphalt
shingles' expected life.
In contrast, the Asphalt Roofing Manufacturers Association's
(ARMA's) ARMA Residential Asphalt Roofing Manual, 2006
edition, recommends open valley flashing material be a 26-gauge
galvanized metal or an equivalent corrosion-resistant, nonstaining
material (such as copper or aluminum) and a minimum of 24 inches
wide.
ARMA recommends having shingle ends lap the metal valley evenly
on either side, leaving an open center channel 6 inches wide at the
top of the valley and widening toward the bottom by 1/8 of an inch
per foot of valley length. The minimum shingle overlap on each side
of the metal valley should be 6 inches. Shingles trimmed to less
than 12 inches in length should not be used to finish a
course...
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