CalcQuarry
Menu

Gambrel Roof Calculator

Updated How we build and test

Enter the building footprint, where the roof breaks and the two pitches to get the area of a gambrel roof, the shingles to order and the length of both rafters.

Horizontal distance from the wall to the break in the roof.

in 12
in 12
in

Measured horizontally.

in
%

Roof area to order

Enter the building size and both pitches to size the roof.

How to use the gambrel roof calculator

  1. Enter the width (the gable side) and the length (the eave side) of the building.
  2. Enter the lower slope run: how far in from the wall the steep lower section ends, measured level.
  3. Enter both pitches as rise in 12. Barn-style roofs are often around 20 in 12 below and 7 in 12 above.
  4. Add the overhangs and a waste allowance, then read the area, squares and rafter lengths.

Gambrel roof formulas

Upper run = width ÷ 2 − lower run
Rafter length = run × √(1 + (pitch ÷ 12)²)
Roof length = building length + 2 × gable overhang
Roof area = 2 × (lower rafter with overhang + upper rafter) × roof length
Squares = area ÷ 100
Height = lower run × lower pitch ÷ 12 + upper run × upper pitch ÷ 12
Gable end area = (width − lower run) × lower rise + upper run × upper rise

Rafter lengths are measured along the slope from the wall line to the break and from the break to the ridge centerline. Take off half the ridge board thickness and allow for your seat cuts and gusset layout when you cut them.

Worked example: a 24 × 32 ft barn

  1. Upper run: 24 ÷ 2 − 4 = 8 ft.
  2. Lower rafter: 4 × √(1 + (20 ÷ 12)²) = 4 × 1.944 = 7.78 ft, plus a 12 in overhang that adds 1.94 ft along the slope: 9.72 ft.
  3. Upper rafter: 8 × √(1 + (7 ÷ 12)²) = 8 × 1.158 = 9.26 ft.
  4. Roof length: 32 + 2 = 34 ft.
  5. Area: 2 × (9.72 + 9.26) × 34 = 1,291 ft².
  6. With 10% waste: 1,420 ft², or 14.2 squares, which is 43 bundles at three per square.
  7. Height above the walls: 6.67 + 4.67 = 11.33 ft.

Questions

What are the angles of a gambrel roof?

There is no single standard. A 20 in 12 lower slope is about 59° and a 7 in 12 upper slope is about 30°. Kit barns and sheds vary, so measure or check the plans.

How do I measure the pitch of each section?

Hold a level against the roof, mark 12 in along it and measure straight down to the roof surface. That number is the rise in 12 for that section.

Why is a gambrel roof bigger than a gable roof on the same building?

The steep lower section adds slope length. In the example above the roof covers 1,291 ft² over a 768 ft² building plus overhangs; a 7 in 12 gable on the same building would be about 1,020 ft².

Related calculators