Roof Valley Framing and Flashing

A roof valley forms where two sloping roof planes meet at an inside angle, creating a channel that concentrates water flow. Valleys handle significantly more water than any other part of the roof, making proper construction and waterproofing critical. Valley rafters, like hip rafters, run diagonally and use compound angle cuts.

Valley Rafter Calculations

When both intersecting roofs have the same pitch, the valley rafter angle mirrors the hip rafter — its effective run is √2 times the common rafter run. For a 6:12 main roof, the valley rafter pitch is approximately 6:17. When the two roofs have different pitches (an irregular valley), the calculations become more complex and each side has different jack rafter lengths and angles.

Valley Flashing Methods

Open valleys use a visible metal channel (typically 24-gauge galvanized steel or copper) at least 24 inches wide. The metal sits on ice-and-water shield underlayment, and shingles are trimmed back 3-6 inches from the centerline. This is the most durable method and is recommended for steep roofs and heavy rainfall areas.

Closed-cut valleys weave shingles from one plane across the valley while the opposing side is trimmed along a chalk line. Woven valleys interlace shingles from both sides alternately. These methods hide the valley but trap more debris and are harder to maintain.

Common Valley Problems

Most valley leaks stem from inadequate underlayment, improperly trimmed shingles, or insufficient flashing width. Ice dams form in valleys before anywhere else because of concentrated water flow. In cold climates, extend ice-and-water shield at least 36 inches from the valley centerline on both sides. Check valleys during maintenance for accumulated debris, granule buildup, and flashing corrosion.