Valley treatments — open, closed-cut, and woven valley construction methods for asphalt shingle roofs

A valley is where two roof planes meet at a downward seam — the channel that concentrates water runoff from both slopes. Three construction methods exist for asphalt-shingle valleys, each with different visible signatures and different performance trade-offs. The open valley uses a metal liner — typically galvalume, aluminum, or copper — installed full-width over the valley underlayment, with the field shingles on each slope held back from the centerline so the metal channel is visible from above. NRCA's "In the Valley" technical reference (Tom Bollnow, Professional Roofing, March 2013) recommends valley metal formed with a centerline rib at least 1 inch high, 24 inches wide minimum, and installed in lengths no greater than 10 feet to control thermal expansion. NRCA recommends open valleys for high-profile architectural asphalt shingles and for any valley with a slope of 4:12 (18 degrees) or shallower. The closed-cut valley — sometimes called "California valley" when the cut is made on a specific slope — runs one slope's shingles fully across the centerline and trims the opposite slope's shingles parallel to and just past the centerline, exposing the run-across slope's shingle rather than metal at the visible seam. The closed-cut method has no visible metal from above; cut-line water integrity depends on workmanship and on the quality of the underlayment beneath. The woven valley interlaces shingles from both slopes across the centerline; NRCA does not recommend woven valleys for modern asphalt-shingle roof systems because they are more prone to debris accumulation, ice and water bridging in cold climates, and material bridging that can lift shingles off the deck. Major architectural-shingle manufacturers (GAF, Owens Corning, CertainTeed) prohibit woven valleys in their install specs for laminated shingles; using a woven valley on architectural shingles typically voids the manufacturer system warranty. In Houston storm exposure, open valleys with quality metal generally outlast closed-cut valleys because the concentrated water flow has no shingle granule surface to abrade and no cut-line sealing dependency. Valley treatment selection is a contractor decision per the manufacturer's install spec; the homeowner-relevant visual signal is what is visible from the ground (metal channel = open valley; shingle-continuous diagonal seam = closed-cut; interlaced shingles with no clear seam = woven, which on an architectural shingle roof flags a warranty-disqualifying install). [Source: NRCA "In the Valley" technical reference, Professional Roofing March 2013; NRCA Roofing Manual valley chapter; GAF Lifetime Shingle install instructions; Owens Corning Duration install instructions; CertainTeed Landmark install instructions; ASTM D7158 wind resistance for steep-slope roofing]