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.