Gambrel Roof Calculator

Use this gambrel roof calculator to determine the constructional dimensions of a gambrel roof, given its upper and lower roof pitch, building width and length, and other parameters.

Ridge height above the plate12 ft12 ft — break fitted to a semicircle

Lower rafter length9 ft 2.21 in29 in 12 over 3.515 ft of run

Upper rafter length9 ft 2.21 in5 in 12 over 8.485 ft of run

Break point8 ft 5.82 in in from the ridge3 ft 6.18 in in from the eave

Height at the break8 ft 5.82 in

Lower slope angle67.5°

Upper slope angle22.5°

Angle at the break135°the interior angle the purlin plate has to make

Total rafter run per side18 ft 4.43 in

Roof area1,322.55 sq ftall four slopes over 36 ft

Roofing squares13.226

Attic floor area gained55.68 sq ft per foot of lengththe width between the break points, which is the point of a gambrel

The method

two pitches per side; the classic form inscribes a semicircle

Headroom is the whole point

A gambrel breaks each side into a steep lower slope and a shallow upper one, which pushes the walls of the attic outwards and turns an unusable triangle into a room. That is why barns and Dutch colonial houses use the form.

The classic proportion inscribes all four rafters in a semicircle of radius half the span, which puts the break at 45° and gives a lower slope of about 22 in 12 and an upper of about 5 in 12. It looks right because it is a circle, and the rafter lengths come out equal.

The break needs a purlin plate to carry it, and that plate has to be supported — usually by a knee wall that conveniently becomes the attic wall.