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Broken Gable (Mansard) Roof Calculator

Calculate upper and lower slopes for Mansard and Gambrel roofs. Optimizes attic space and determines critical transition points for builders.

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Category Roofing
ft
ft (Distance from wall to break)
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About

A broken gable roof, often found in Gambrel or Mansard styles, utilizes two distinct slopes to maximize the usable volume of the upper story. The lower slope is steep, acting almost like a wall, while the upper slope is shallow. The transition point, or 'knuckle', supports significant structural loads. This calculator helps architects and builders determine the run and rise for both sections. It is critical for ensuring that the upper rafter and lower rafter meet at the correct height to provide adequate headroom. Incorrect transition calculations can result in structural sagging or code violations regarding ceiling height.

roofing mansard gambrel truss architecture

Formulas

The roof is divided into two right triangles. The total span S is split into the lower run r1 and upper run r2.

rtotal = r1 + r2

Rafter lengths L are calculated independently.

L = √Run2 + (Run × Pitch)2

Reference Data

StyleLower PitchUpper PitchTypical UseSpace Gain
Classic Gambrel20:124:12Barns / Dutch ColonialHigh
Mansard60°10°French EclecticMaximum
Modern Farmhouse12:123:12ResidentialMedium
Shed Dormer8:122:12RetrofitVariable

Frequently Asked Questions

The lower slope is typically very steep, often between 12:12 and 20:12, or even 60-70 degrees, to serve as a wall for the living space inside.
The knuckle location is usually dictated by the desired interior room width. A common rule is to set the lower run to provide a specific interior wall height (e.g., 8 feet).
No. While it provides the geometry for the rafters, the structural gusset plate design at the transition joint requires a structural engineer's approval for load bearing.