gambrel roof calculator

Gambrel Roof Calculator

A gambrel roof estimate treats each side as two sloped roof planes, then doubles the upper and lower plane areas across the building length.

Inputs

Results

Immediate estimate

Gambrel roof area1,284.41 sq ft12.84 roofing squares including waste

Lower rafter length9.22 ft

Upper rafter length6.71 ft

Base roof area1,146.8 sq ft

Estimated bundles39 bundles

This assumes a simple symmetric gambrel roof with four rectangular roof planes. Dormers, overhangs, valleys, trim, and transitions need separate takeoff.

Shopping list

39 bundles Asphalt shingles — Approximate; verify product coverage and roof-plane takeoff.

Diagram

Gambrel roof planes

Upper and lower slope segments are converted to roof area and roofing squares.

Lower slope
Upper slope
Roof area

How this calculator works

Enter building length and gambrel slope segments to estimate roof area, rafter segment lengths, roofing squares, and shingle bundles.

Segment geometry

The engine calculates upper and lower rafter segment lengths from run and rise using the Pythagorean theorem.

Area model

Roof area is two times the building length times the sum of the upper and lower rafter segment lengths.

Shape caution

Dormers, valleys, overhangs, transitions, trim, and non-symmetric gambrels need roof-plane takeoff outside this MVP.

Rafter segmentssqrt(run squared + rise squared) for lower and upper slopes

Roof area2 x building length x (lower rafter + upper rafter)

Purchase arearoof area + waste allowance

Assumptions

Shape: Symmetric gambrel roof with two matching lower planes and two matching upper planes.
Shingles: Bundle estimate assumes three bundles per roofing square.

Worked examples

Barn-style gambrel

Simple symmetric gambrel roof estimate.

1,284.41 sq ftGambrel roof area

Steeper lower slope

Shows larger roof-plane area and waste.

2,148.83 sq ftGambrel roof area

FAQ

Does this work for a barn roof?

Yes, for a simple symmetric gambrel or barn-style roof with two matching slopes per side.

Should I include overhangs?

Include overhangs in the slope run dimensions if you want them counted in the roof plane area.

Does this calculate framing strength?

No. This is a geometry and material estimator, not a structural roof design.

Sources and review

Roof measurement and shingle planning, GAF. Accessed 2026-07-16.

Reviewed 2026-07-20. Planning estimates only; confirm project-specific requirements.