How do you roof a valley?
Short answer. Roofing a valley starts with a valley lining, usually self-adhering ice and water membrane centered in the valley, followed by one of three methods: an open valley with exposed metal, a closed-cut valley where one side's shingles run through and the other side's are cut back 2 inches from the centerline, or a woven valley where shingles from both sides interlace. Laminated architectural shingles generally use open or closed-cut valleys, because they crack when woven.
Step 1: the lining
The International Residential Code requires a valley lining under shingles. Open valleys lined with metal need metal at least 24 inches wide; closed valleys can use a 36-inch roll of smooth roll roofing or self-adhering polymer modified bitumen membrane. In practice most roofers run self-adhering ice and water membrane down every valley, smooth and centered, then lap the field underlayment over it.
Step 2: pick a method
- Open metal valley. Prefinished metal, often with a center crimp, laid over the membrane and held only at its outer edges. Shingles on both sides are cut back to leave an open channel. It handles heavy water and debris well.
- Closed-cut valley. Shingles from one slope run across the valley and at least 12 inches up the other side. Shingles from the other slope are laid over them and cut on a chalk line 2 inches back from the centerline, with the upper corner of each clipped so water does not run along its top edge. GAF's detail keeps nails at least 6 inches from the centerline.
- Woven valley. Shingles from both sides alternate across the valley. It works with 3-tab shingles, but GAF does not allow it with its laminated Timberline shingles, and other makers discourage it too.
Details that make or break it
- No nails near the center of the valley.
- Follow the shingle maker's instructions on which slope runs through and which slope is cut.
- Embed the clipped, cut shingle ends in a bead of roofing cement where the manufacturer calls for it.
- Keep valleys clear of debris; a dammed valley leaks no matter how well it was built.
See ice and water shield in Georgia and roof anatomy explained.