Valley Flashing Length Calculator
Find the true diagonal length of a roof valley from the horizontal run and pitch, plus how many flashing pieces to buy.
How it works
Slope = pitch ÷ 12 · Valley factor = √(2 + slope²) · Length per valley = run × factor + overlap · Pieces = ceil(total length ÷ piece length)
A valley formed where two equal-pitch roof planes meet at a right angle runs diagonally in plan and also climbs the slope, so its true length is longer than either the run or a simple diagonal. The valley factor √(2 + slope²) captures both effects at once: the √2 term is the 45° plan diagonal and the slope² term adds the vertical rise. Multiply the horizontal run by this factor, then add a little for overhang at the eave and the lap under the ridge.
Worked example
For a 12 ft run at a 6/12 pitch, the slope is 0.5, so the factor is √(2 + 0.25) = 1.500. Each valley is 12 × 1.5 = 18 ft plus 1 ft overlap = 19 ft; two valleys total 38 ft, needing 4 ten-foot pieces.
Frequently asked questions
Why is the valley longer than the roof run?
The valley cuts diagonally across the plan (adding the √2 factor of a 45° line) and simultaneously rises with the slope. Both add length, which is why a valley is always noticeably longer than the level distance from eave to ridge.
Does this work for unequal roof pitches?
The √(2 + slope²) factor assumes two equal pitches meeting at 90°, which covers most homes. For very different pitches or non-square intersections the valley shifts off 45° and you should measure the actual diagonal on site.
How much overlap should I add?
Metal valley pieces lap several inches at each joint, and the flashing should run a bit past the eave and under the ridge. Adding about a foot per valley covers typical laps and overhang; add more for long valleys with several joints.
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This calculator is for educational purposes. Double-check important results before acting on them.