How to Flash a Roof Valley
Installing open valley metal flashing correctly — the most durable and leak-resistant method for where two roof planes meet.
Step-by-Step
- 1
Apply ice and water shield in the valley
Before any metal or shingles, apply ice and water shield the full length of the valley, centered on the valley centerline. It should extend at least 18 inches on each side. Smooth it flat — bubbles create stress points.
- 2
Cut and fit valley metal
Use pre-bent W-profile or V-profile valley metal sized for your conditions. The center rib divides the two drainage channels and prevents water from crossing from one slope to the other. Cut sections to length and overlap by 12 inches, upper lapping over lower.
- 3
Position valley metal
Center the valley metal in the valley. The metal should extend at least 6 inches on each side of the valley centerline, tucked under the underlayment on each slope. Nail only on the outer edges — nails in the water channel leak.
- 4
Nail outer edges only
Fasten the valley metal through the outer flanges only, at 12 inches OC, at least 1 inch from the edge. Never nail through the center channel. Use roofing nails the correct length to penetrate the deck without going through the soffit.
- 5
Lap upper sections over lower
At any valley section joint, the upper piece always laps over the lower piece by at least 12 inches. Seal the overlap with roofing cement under the flap.
- 6
Snap chalk lines for shingle cuts
Snap chalk lines along each side of the valley to mark where shingles will be cut. For an open valley, the lines are typically 3 inches from the valley centerline at the ridge, widening to 3.5 inches at the eave to direct water flow.
- 7
Cut and install shingles
Run shingles across the valley and cut along the chalk line. Cut the top corner of each shingle at 45 degrees to divert water into the channel. Clip the top corner of each shingle at 45 degrees to prevent water from being trapped under the tab. Do not nail within 6 inches of the valley centerline.
Pro Tips
- → Valleys collect twice the water of a standard roof section — size and install them with extra care.
- → Widen the valley lines slightly at the eave (3 inches at ridge, 3.5 inches at eave) to prevent water buildup in heavy rain.
- → Apply a thin bead of roofing cement under the edge of each shingle where it crosses the valley flashing — not as a substitute for flashing, but as an additional seal.
Watch Out
- ! Never nail through the center of valley flashing — the nail penetrates the highest-traffic water path and will leak.
- ! Do not use caulk as a substitute for valley flashing. Caulk fails within a few years and is not a waterproofing solution.