ROOF VALLEY FLASHING DETAILS

Most roof leaks can occur where water is channelled off the roof or where the roof abuts a vertical wall or chimney. At these points, metal valleys and flashing are used to assist the shingles or shakes in keeping the structure sound and dry. Structural members that protrude through a roof should also be flashed at all intersecting angles to prevent leakage. Step flashing should extend under the shingles or shakes and up the vertical surface and should be covered by a second layer of flashing (counterflashing). (Figure 13).

Flashing should be pre-painted both sides using a good metal or bituminous paint. Flashing strips which must be bent to sharp angles and should be painted after bending. Metal flashing with baked-on enamel coating is available in some areas. Different flashing metals are available in different areas depending on climatic variations. It is good practice to use metals that have proven their reliability under the specific conditions to be encountered. It is important that metal flashing have the same longevity as Western Red Cedar.

Valley - Shingles

For roofs with slopes of 1:1 or greater, valley flashing should extend not less than 7" (177.8 mm) on each side of the valley centerline. For roof slopes less than 1:1, flashing should extend not less than 10" (254 mm) each side. Valley flashing should be center-crimped, painted, galvanized steel or aluminum. Valley metal should be underlayed with No. 15 roofing felt( minimum). Shingles should not be applied with their grain parallel to the valley centerline and those extending into the valley should be cut at the correct angle (Figure 14, page 14). Joints between shingles must not break into the valley.