Gambrel Roof Calculator
Rafter lengths, angles and area for a barn-style roof.
Enter your values
ⓘ Live results
Results
- Lower rafter
- 8.4 ft
- Upper rafter
- 9.007 ft
- Total ridge height
- 11.778 ft
- Break point height
- 7.275 ft
- Roof area
- 1,253 sq ft
- Shingle squares
- 12.5
Quick answer
A gambrel roof has two slopes each side — typically a steep 60° lower rafter and a shallow 30° upper rafter meeting at the ridge.
rafter = √(rise² + run²) per slope
How to use the Gambrel Roof Calculator
- 1Enter the building span wall to wall.
- 2Set the lower and upper slope angles — 60° and 30° is traditional.
- 3Move the break point to change the loft headroom.
Worked example
Using the values the calculator loads with:
Inputs
- Building span24 ft
- Lower slope angle60 °
- Upper slope angle30 °
- Break point from wall35 % of half span
- Building length36 ft
Results
- Lower rafter8.4 ft
- Upper rafter9.007 ft
- Total ridge height11.778 ft
- Break point height7.275 ft
- Roof area1,253 sq ft
- Shingle squares12.5
Frequently asked questions
What angles are used on a gambrel roof?
Most barns use 60° for the lower slope and 30° for the upper, which makes an even semicircle of the truss.
Why choose a gambrel roof?
It maximises usable loft space under the roof compared with a plain gable of the same span.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.