Valley Flashing for Asphalt Shingles

Open valleys are a rare sight on asphalt shingle roofs and should be formed with roll roofing in two layers. The first layer should be 18 inches wide laid face down in the valley, and the second layer should be 36 inches wide laid face up. Both plys should be cemented down, and the shingles cut back to 3 inches from the centerline; andmetal flashing a minimum of 24 inches wide with a center rib at least 1 inch high, nailed at 18 inches on center, and not more than 1 inch from the outer edges. Ideally, this would also have a layer of ice and water shield installed under the flashing, extending the flashing protection to 18 inches on either side of the valley. In both cases, it is good practice to also cut an angle on the top of the shingle to help stop water from migrating across the top of the shingle. Closed and closed-cut valleys are more common with asphalt shingle roofs. A fully closed valley is when the shingles from both planes are interwoven across the valley and extend a minimum of 12 inches onto the adjacent plane. A closed-cut valley is when the shingles from one roof plane cross over to the next, but the covering overlaid from the other plane is cut back 2 inches from the centerline of the valley. In both cases, the valleys should still have additional protection with one of the following applied, in addition to the normal felt or tar paper underlayment: one layer of 36-inch roofing felt or tar paper; one layer of 36-inch roll roofing; or one layer of 36-inch ice and water shield. The shingles should not be nailed within 6 inches of the valley centerline, and the un-nailed areas should be cemented down.