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.
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
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