Valley flashing is required for steep-slope wood shake roof
systems where two adjoining roof slopes intersect. Detail SHK-7
"Closed Valley" in The NRCA Roofing and Waterproofing Manual,
Fifth Edition, provides information for constructing a closed
valley with a metal liner for a wood shake roof system.
First, a valley underlayment is installed as secondary
protection against water intrusion. NRCA recommends using an
18-inch-wide valley metal to line the valley and provide the
primary watershedding protection. Shingle-fashion overlap of the
individual valley metal pieces is 8 inches minimum. Two beads of
sealant or two-sided butyl tape are required to help prevent water
infiltration at the overlap joint.
One key element to implementing this detail successfully is the
location of the nails within the valley metal. Nails will penetrate
the valley metal; it is impossible to install a wood shake roof
system without the nails penetrating the 18-inch-wide valley metal.
The farther away the nails are from the center of the valley and
the steeper the valley slope, the less likely moisture will be able
to infiltrate the nail holes.
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