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Mansard Roof Rafter Geometry Calculator

Calculate total rafter length for a mansard roof's steep near-vertical lower segment plus shallow upper segment.

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
32.81 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Total mansard rafter length

21.43 ft

High confidence
Lower segment length
6.98 ft
Upper segment length
14.45 ft

Running these inputs gives 6.53 as the total mansard rafter length. Upper segment length carries the most weight in this calculation, at 4.40 m. Currently reading for United States under IRC 2024 — pick a different market above and the figures re-cast accordingly.

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

  • The steep lower mansard segment is sized by its slope angle and vertical height (length = height/sin(angle), run = height/tan(angle)); the shallow upper segment uses the standard rise/12 rafter-length formula over the remaining half-width

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Frequently asked questions

Why does the lower segment use an angle while the upper segment uses a rise-per-12 pitch?
Mansard lower slopes are typically specified as a steep angle from horizontal (often 60-75°) since a rise/12 pitch notation becomes awkward at near-vertical slopes, while the shallow upper segment is conventional enough to use the standard rise-per-12 carpentry pitch notation.
What happens if the lower segment's run is longer than half the building width?
That geometry is invalid — the steep lower slope would need to travel past the ridge centerline before reaching the break line. This calculator still computes a result in that case but flags it as low confidence so you catch the input error.
Does this account for the double layer of framing typical at a mansard's break line?
No — this calculates rafter-length geometry only. The structural transition, blocking, and any additional framing members at the break line between the two segments are not included.