Gambrel Roof Calculator

A gambrel roof has two slopes on each side: a steeper lower pitch and a shallower upper pitch. This design creates more attic space than a simple gable roof and is common on barns and Dutch Colonial homes.

In the half-circle method, the gambrel roof fits inside a semicircle whose diameter equals the building width. The lower roof pitch is always 45 degrees greater than the upper pitch.

Calculate each roof segment area as length plus gable overhang times rafter length (including eaves overhang on the lower segment). Multiply the sum of both segments by two for both roof sides.

Typical lower pitches range from 50 to 70 degrees and upper pitches from 10 to 25 degrees. The lower run length must be less than half the building width so both segments fit.

Gambrel roofs offer significantly more usable attic volume than triangular gable roofs because the lower steep slope creates vertical wall space near the eaves. Use the attic volume result to plan storage or living space.