Gambrel Roof Calculator

A gambrel (barn) roof has two pitches per side — a steep lower and shallow upper section.

How the Math Works

The Gambrel Roof Calculator uses right triangle trigonometry to determine rafter lengths by treating each section of the roof as a separate pitched triangle. The lower (sawn) section and upper (dead) section are calculated independently using the Pythagorean theorem, where the horizontal run is divided at the break point and each section's rise is proportional to its pitch angle. The ridge height is then found by subtracting the upper section's rise from the total building height, ensuring the two sections meet precisely at the break point to create the characteristic barn-like silhouette with its distinctive curved or angled transition.

Practical Applications

To use this calculator, first measure your building's width and determine where you want the break point along the wall plate, which defines where the lower and upper roof sections meet. Input the building height, the desired pitch angles for both the lower and upper sections, and the break point location. The calculator will output the exact rafter lengths for each section and the required ridge height, allowing you to cut rafters to precise measurements and ensure proper fit during construction without costly rework or guesswork.

Day-to-Day Use

This calculator simplifies home improvement projects by providing accurate measurements for gambrel roof construction, whether you're building a barn, shed, or adding a roof extension. It eliminates the frustration of trial-and-error cutting, saves money on materials by preventing waste, and ensures your roof will have the proper structural integrity and aesthetic appeal. The precise calculations also help you communicate effectively with contractors or suppliers, making your DIY projects more successful and professional-looking.

Worked example

24 ft span, 8 ft lower run, 60°/30° → ridge ~16.2 ft.

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