🏚️ Gambrel Roof Calculator
Work out the rafter lengths, roof height, and slope angles for a barn-style gambrel roof with a steep lower slope and shallow upper slope.
What this calculator does
The gambrel roof calculator finds the rafter lengths, overall roof height, and slope angles for a classic two-slope barn (gambrel) roof, using the building width and your chosen break point between the steep lower slope and shallow upper slope.
The formula
Each side of the roof spans half the building width, split into a lower run (the steep section, near the eaves) and an upper run (the shallower section, near the ridge). Pitch is expressed as rise per 12 inches of run — for example, a 24/12 pitch rises 24 inches for every 12 inches of horizontal run. Rise for each section is (pitch / 12) x run, and rafter length is found with the Pythagorean theorem from the run and rise. Total roof height is the sum of both sections’ rise.
Classic gambrel barns often use a steep lower pitch (around 60-70 degrees) for extra loft headroom and a shallower upper pitch (around 25-30 degrees). This is a geometric estimate for planning — have a structural professional verify rafter sizing and connections for your specific building.