How to Figure a Gambrel Roof

Gambrel roofs, like gable roofs, have two sides that meet in a peak. Unlike gable roofs that form an inverted V, gambrel roofs have two distinct rectangular surfaces on each side. The upper rectangle on each side has a shallow slope, and the lower rectangle on each side has a steep slope. Gambrel roofs are often associated with barns, where they are used for the extra storage room they provide. They are sometimes even called barn roofs. If you plan a building with a gambrel roof, you need to determine the size of the roof so you can purchase the appropriate building materials.

Calculator

Instructions

- 1
  Label the sides of the house that the lowest parts of the roof run along as the “A” sides. Label the other two sides, which have the peak in the middle, as the “B” sides.
- 2
  Divide the length of the B sides by two. The result is the horizontal run of one side of the roof.
- 3
  Divide the horizontal run by two if you are planning to build an English style gambrel, where the slope changes about halfway from the edge to the peak. Multiply it by two and divide it by three if you are planning a Dutch colonial style, where the lower slope is much larger than the upper. Label this result the lower run. Subtract the lower run from the horizontal run of the side and label the result the upper run.
- 4
  Multiply the lower run by 0.707 if you are building an English gambrel roof. The result is the length of the sloped side of the lower rectangle. The English style has a slope of 45 degrees; 0.707 is the cosine of 45 degrees. Multiplying the cosine by the lower run gives the length of the slope formed by a specific base at that angle. Multiply the lower run by 0.5, which is the cosine of 60 degrees, if you are building a Dutch colonial style roof.
- 5
  Multiply the length of the lower slope by the length of the A side. The result is the total area of the lower rectangle.
- 6
  Multiply the upper run by 0.906, which is the cosine of 25 degrees, to get the length of the upper slope for an English style roof. Multiply the upper run by 0.927 to get the upper slope length of a Dutch colonial style, which has a 22 degree slope.
- 7
  Multiply the upper slope length by the length of side A to get the area of the upper rectangle. The lower slope area plus the upper slope area is equal to the size of one side of the whole roof. Adding the areas of both sides gives you the total size of the roof.

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