What Is a Roof Valley Length Calculator?
A valley rafter forms the sloped inside corner where two roof planes come together — common wherever an addition, dormer or ell roof meets the main roof. Geometrically it's a mirror image of a hip rafter: both cross a building corner diagonally at 45° in plan while climbing the roof's pitch, so both use the same 3D right-triangle formula. This calculator gives you the raw valley length plus a version padded with a cutting allowance for seat cuts and field trimming.
How to Read Your Results
Valley Rafter Length (with Allowance)
The board length to cut, including the percentage allowance you entered for waste, seat cuts and trimming at the ridge.
Raw Valley Length
The exact theoretical length before any allowance — the true top-edge distance from the wall plate intersection to the ridge.
Horizontal Valley Run
The plan-view (horizontal) distance the valley travels, always √2 (≈1.414×) times the common rafter's run because it crosses the corner diagonally.
Total Rise
The vertical rise over that run, based on your entered roof pitch.
Real-World Example
A valley intersection with a 10-foot common run, a 6:12 pitch and a 5% cutting allowance:
| Value | Result |
|---|---|
| Horizontal valley run | ≈14.1 ft |
| Total rise | 5.0 ft |
| Raw valley length | 15.0 ft |
| Length with 5% allowance | ≈15.75 ft |
The 15-foot raw length comes out to a clean number here because 10×√2 and the rise happen to form a 3-4-5-style right triangle at this pitch — in most real cases you'll see a less round number.
Tips for Framing Valleys
- Valley rafters typically need to be sized deeper than common rafters since they carry jack rafters from both roof planes.
- This formula assumes equal pitches on both intersecting roof planes — unequal pitches (a "bastard valley") need custom layout.
- Order extra length for double valleys or California-style framing where the valley also supports sheathing runs.
- Flash valleys carefully — they concentrate more water runoff than any other roof plane intersection.
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Last updated: August 2026 · Reviewed by: Simple Calculator Tools Editorial Team