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Dormer Rafter Length Calculator

Calculate common rafter length for a dormer roof, including overhang, from the dormer's own width and pitch.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax
6.56 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Total dormer rafter length

5.13 ft

High confidence
Rafter length to ridge
3.94 ft
Overhang tail
1.18 ft

At the values currently entered, the total dormer rafter length works out to 1.56. The largest intermediate quantity is rafter length to ridge, at 1.20 m — check that step first if the total looks off. Figures are shown for United States, where IRC 2024 is the governing residential reference; switch the market above if you are building elsewhere.

Estimated cost — your price

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Standard carpentry rafter-length formula applied to a dormer's own (typically narrower, steeper) roof: rafter length = run × √(1+(pitch/12)²)

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Frequently asked questions

Why use the dormer's own width instead of the main roof's dimensions?
A dormer has its own small roof, typically steeper and narrower than the main roof, so its rafter run is half the dormer's own width — not related to the main roof's span or pitch.
Why is overhang length calculated with the same slope factor as the rafter?
The rafter tail continues along the same roof plane past the wall, so its along-slope length is found the same way as the rafter itself — the horizontal overhang distance multiplied by the same √(1+(pitch/12)²) slope factor.
Does this account for the ridge board thickness or birdsmouth cut?
No — this is theoretical rafter length along the roof plane only. Actual cut length will be adjusted slightly for the ridge board thickness and the birdsmouth seat cut at the wall plate.