Roof Valley

A valley is the inward-facing junction where two roof slopes meet, the opposite of a hip. Because water from both planes converges there, a valley carries more water than any other part of the roof, and it is also where snow, ice and leaves accumulate. Valleys are therefore detailed carefully and usually given extra underlayment, such as a strip of self-adhered membrane along their full length.

Valley types on shingle roofs

• Open valley: a continuous metal or membrane valley liner is left exposed, and the roof covering on each side is cut back to leave a visible channel. Open metal valleys are durable and handle heavy flow and debris well.
• Closed-cut valley: shingles from one slope (usually the lower-pitched or smaller one) are run across the valley and up the other side; shingles from the second slope then overlap them and are cut in a straight line a short distance from the valley centreline.
• Woven valley: shingles from both slopes are interlaced across the valley, course by course. It is used mainly with three-tab shingles and is generally not recommended for thick laminated shingles.

Valleys on tile, slate and metal roofs

• Open valleys with lead, metal or GRP (glass-reinforced plastic) liners are common under tiles in the UK and Europe, sometimes with dry valley systems.
• Laced, swept and mitred valleys are traditional British slate and plain-tile techniques in which tiles or slates are cut and shaped to turn the corner without exposed metal; a mitred valley uses concealed soakers.
• Metal roofs use formed valley pans with the panels terminated over a closure.

Framing

In stick-framed roofs a valley rafter runs diagonally along the junction, with jack rafters from each slope landing on it. In truss roofs, one roof is often framed on top of the other's sheathing using a "California fill" or valley set.

Common problems

Debris build-up, nails driven too close to the centreline, undersized or corroded metal, and shingles cut too short are typical causes of valley leaks.