Understanding your result
For an open valley the headline is the pieces of valley metal; for a closed-cut or woven valley it is the rolls of lining, since there is no metal to buy. The stats show the valley length, the lining area and the metal count. The breakdown adds the run-out and waste, then counts metal, lining and fasteners.
The figure to remember is the length to line with its run-out: it is the same length that goes into the ice and water shield calculator if you are buying the membrane with the eave courses, and it is the length the shingle crew will be cutting along. A closed-cut valley does not reduce the shingle order; the cut-off triangles are the valley part of the 15 to 20 percent waste in the shingle calculator.
How we calculate this
The step that surprises people is the run-out. A valley measured on the plan ends at the eave line, but the metal and the lining must run past it to the drip edge and be trimmed there, and the lining should extend onto the eave membrane; a foot per valley covers it. The other one is the lap: valley metal pieces overlap at least 4 inches (6 in snow country), so a 10-foot stick covers 9.5 feet, not 10.
Valley length itself comes from the geometry of the two roof planes. For two planes of the same pitch meeting at a right angle in plan, the valley is the horizontal distance from the inside corner to the ridge multiplied by the hip and valley factor, √(rise² + 12² + 12²) ÷ 12, which is 1.50 at 6/12; the hip and valley rafter calculator gives it for any pitch and the pitch factor table lists the factor.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Open valley metal width | 24 in | IRC R905.2.8.2 (minimum 24 in for open valleys) |
| Metal stick length | 10 ft | Standard pre-formed W-valley and flat valley stock |
| Metal end lap | 6 in | IRC and manufacturers: 4 in minimum; 6 in common in snow regions |
| Lining width | 36 in | IRC R905.2.8.2 (36-in mineral-surfaced roll roofing or self-adhering membrane) |
| Lining roll length | 66.7 ft (membrane) or 36 ft (roll roofing) | Standard roll sizes |
| Edge fasteners | every 12 in, both edges | Manufacturers’ instructions for valley metal (cleats or nails within 1 in of the edge) |
| Run-out at the eave | 1 ft per valley | Installation practice |
| Waste | 10% | Estimating practice |
Assumptions last reviewed October 7, 2026.
The calculator does not size valleys for unequal pitches (where the valley line is not at 45 degrees in plan and the metal needs a wider leg on the shallower side), does not count the sealant some closed-cut details call for along the cut edge, and does not know the manufacturer’s rules for your shingle. The guide to drip edge, step flashing and valleys shows the three valley details in order of installation and where each is allowed.
Two worked examples
Two closed-cut valleys on an L-shaped house
Two valleys totalling 40 feet, closed-cut architectural shingles, lined with 36-inch self-adhering membrane in 66.7-foot rolls, 10% waste.
- Length to line: 40 + 2 × 1 = 42 ft
- With waste: 42 × 1.10 = 46.2 ft
- Lining: 46.2 ft × 3 ft = 139 sq ft
- Rolls: 46.2 ÷ 66.7 = 0.69, rounded up to 1 roll
- Valley metal: none
One roll of membrane and no metal. The cost of the valleys on this roof is in the shingle waste and the cutting time, not in materials.
Four open metal valleys in snow country
Four valleys totalling 90 feet, open W-valley 24 inches wide in 10-foot sticks with 6-inch laps, membrane lining.
- Length to line: 90 + 4 × 1 = 94 ft
- With waste: 94 × 1.10 = 103.4 ft
- Effective piece: 10 ft − 6 in = 9.5 ft
- Metal pieces: 103.4 ÷ 9.5 = 10.9, rounded up to 11 pieces
- Edge fasteners: 94 × 2 = 188
- Lining: 103.4 ÷ 66.7 = 1.55, rounded up to 2 rolls
Eleven sticks of W-valley and two rolls of membrane. The lining goes down first, centred on the valley, then the metal, then the shingles cut back to leave the 4- to 6-inch channel open on each side of the rib.
Where to find your inputs
Valley length. From the ridge (or the point where the valley starts) to the eave along the valley line. From the ground, measure the horizontal distance from the inside corner of the building to the point under the ridge and multiply by the hip and valley factor; or measure a valley on the plan.
Number of valleys. Count them on the plan or from the ground: every inside corner where two roof planes meet makes one. An L-shaped plan has one; a T has two; a cross has four. Dormers add one on each side.
Metal width and stick length. On the supplier’s rack: 24 inches is the code minimum and the common size; some W-valley is 20 inches and does not meet the code for open valleys.
Lining roll length. 66.7 feet for a standard 2-square membrane roll, 75 feet for some brands, 36 feet for mineral-surfaced roll roofing.
Common mistakes
- Measuring the valley in plan. The plan length is the horizontal projection; the real valley is 1.45 to 1.75 times longer depending on the pitch. Use the hip and valley factor.
- No run-out. Metal and lining cut flush with the eave line leave the drip edge unprotected where the valley discharges most of its water. Add the foot.
- Nailing through the metal. Fasten the edges only. A nail in the middle of a valley is a leak that appears in the first storm.
- Shingle nails near the centre line. Keep them 6 inches away; the lining is not a substitute for the metal or the shingles over it.
- Woven valleys with architectural shingles. Laminated shingles are too thick to weave and crack at the bend. Use closed-cut or open metal.
- Skipping the lining under an open valley. Metal expands, laps open a hair, and wind-driven rain gets under it; the membrane beneath is the second line.
Questions people ask
- What are the three types of shingle valley?
- An open valley has a strip of metal down the centre, with the shingles on each side cut back to leave it exposed. A closed-cut valley has no exposed metal; the shingles of one plane run across the valley and the shingles of the other are cut in a straight line 2 inches from the centre. A woven valley interlaces the two planes course by course and is only possible with flexible three-tab shingles.
- How wide should valley metal be?
- The International Residential Code (R905.2.8.2) requires open-valley metal at least 24 inches wide, in a corrosion-resistant metal of a minimum thickness that depends on the metal (for example 26-gauge galvanized steel or 0.024-inch aluminum). Most pre-formed W-valley is 24 inches with a centre rib that stops water from one plane washing under the shingles of the other.
- How many pieces of valley metal do I need?
- Measure each valley from the ridge to the eave, add a foot of run-out, add 10 percent for waste and divide by the effective length of a stick, which is its length less the lap: 9.5 feet for a 10-foot stick with a 6-inch lap. Four valleys totalling 90 feet are 94 feet with run-out, 103.4 with waste, and 11 pieces.
- What goes under a valley?
- The code requires a valley lining for closed valleys: one ply of mineral-surfaced roll roofing at least 36 inches wide, or a self-adhering membrane. Open metal valleys also need underlayment or membrane beneath the metal. Most roofers use ice and water membrane for both, centred on the valley line for its full length; it also serves as the ice barrier.
- How is valley metal fastened?
- Only along its edges, with nails or cleats within an inch of the edge every 12 inches or so, never through the centre, and the pieces lap at least 4 inches with the upper piece over the lower. Shingle nails must stay at least 6 inches from the valley centre line. Ninety-four feet of open valley takes about 190 edge fasteners.
- Which valley type is best?
- Open metal valleys shed water and debris fastest and are easiest to inspect, which is why they are preferred in snow country and under trees. Closed-cut valleys are cheaper and look continuous, and they are what most architectural shingle roofs use. Woven valleys are the oldest method and are limited to three-tab shingles; the manufacturer's instructions say which are allowed for your shingle.