Gambrel Roof Calculator
Last reviewed How we calculate
A gambrel roof, the classic barn shape, has two slopes on each side: a steep lower slope and a shallower upper one. That shape gives more headroom in the attic, but it makes the roof harder to measure. This works out the area, the slope lengths and the height from the building size and the two pitches.
Gambrel roof area and dimensions
Roof area, slope lengths, angles and height for a barn-style gambrel roof.
This calculator needs JavaScript to work. The measurements below are the ones you would need to do the sum by hand.
Roof area
- Squares to order, with waste
- Lower slope, along the roof
- Upper slope, along the roof
- Ridge height above the wall plate
- Lower slope angle
- Upper slope angle
Overhangs are not included; add them to the building dimensions if the roof covers them. Very steep lower slopes usually need extra nails and hand-sealed shingles, so follow the manufacturer steep-slope instructions.
How the calculation works
Each side of the roof is split at the break. The lower slope covers a horizontal distance you choose from the wall line, and the upper slope covers the rest of the way to the middle of the building. Each is multiplied by the pitch multiplier for its own pitch to get its sloped length. Add the two, double it for both sides, and multiply by the length of the building for the roof area.
A worked example
A building 30 ft wide and 40 ft long with lower slopes 5 ft wide at 18 in 12, and upper slopes at 5 in 12, has a lower slope 9.01 ft long and an upper slope 10.83 ft long on each side. The roof area is about 1,588 sq ft, and the ridge stands about 11.67 ft above the wall plate. The lower slope is at about 56 degrees and the upper at about 23.
Things to watch
Overhangs are not included, so add them to the building dimensions if the roof covers them. The upper slope of a gambrel can be shallow enough to need a doubled underlayment, and the lower slope steep enough to need extra nails and hand-sealed shingles. Check both pitches against the shingle manufacturer instructions.
Frequently asked questions
What pitches does a gambrel roof use?
Usually a steep lower slope and a much shallower upper one, but there is no single standard. The calculator takes any pair of pitches you enter.
Can shingles go on the steep lower slope?
Yes, but very steep slopes usually need extra nails and hand-sealed shingles. Shingle manufacturers publish steep-slope instructions for exactly this.
Does the roof area include the gable walls?
No, it covers the roof only. The gable end area calculator works out the wall area for siding or paint.