Valley Flashing

What Is Valley Flashing

Valley flashing is the waterproofing material installed in the channel where two sloping roof planes meet and converge. Roof valleys are natural water highways. Every raindrop that lands on either of the two converging planes is directed into the valley, making it the highest-volume water path on the roof. Valley flashing can be exposed metal (open valley), hidden beneath overlapping shingles (closed or woven valley), or a combination of metal and self-adhering membrane. The most durable configuration is an open metal valley with ice and water shield on the deck beneath it. Valley metal is typically 16 to 24 inches wide, made from 26-gauge galvanized steel, aluminum, or copper, and features a center crimp or ridge that prevents water migrating from one side to the other.

Valley Flashing Methods

There are three standard valley methods for shingled roofs. An open metal valley uses a W-shaped or flat metal trough with shingles trimmed back at least 3 inches from the centerline on each side, leaving the metal exposed. This is the most inspectable and longest-lasting method. A closed-cut valley extends the shingles from the larger or steeper roof plane across the full valley, while the opposing side's shingles are cut along a straight line 2 to 3 inches from the center. No metal is exposed, but ice and water shield must be present underneath. A woven valley interlaces shingles from both planes alternately across the valley. Woven valleys are the fastest to install but the most prone to failure because the layered shingles create ridges that can channel water sideways.

Common Valley Flashing Failures

The number one cause of valley leaks is debris accumulation. Leaves, pine needles, and granule buildup create a dam in the valley channel that forces water laterally under the shingle edges. Regular valley cleaning is essential roof maintenance, particularly on homes surrounded by trees. The second most common failure is nail penetration through the valley metal. Shingle nails should never be driven within 6 inches of the valley centerline, but careless installation frequently violates this rule. On roofs where two planes of unequal length or slope meet at the valley, cross-wash occurs: water velocity from the larger plane pushes a wave over the shingle edge on the smaller plane's side. A center crimp in the valley metal helps prevent this. In cold climates, valleys are the first place ice dams form because concentrated snowmelt refreezes at the colder eave.

Valley Flashing in Insurance Claims

Valley flashing is a priority inspection area after any storm event. Hail strikes on open metal valleys leave clear, measurable dents that are strong evidence of storm impact on the roof. Wind-blown branches or debris can puncture or displace valley flashing. Because valleys handle more water than any other roof area, even minor valley damage can cause disproportionate interior damage. When documenting valley flashing for a claim, note the valley method (open, closed, or woven), measure the total length of each valley run, and photograph any dents, punctures, or debris displacement. Xactimate line items for valley flashing are measured in linear feet. If ice and water shield under the valley also needs replacement, that is a separate line item and should be included in the initial estimate.