How to Figure the Lengths of Gambrel Roof Top Chords

When you design a roof truss, you need to know the lengths of the chords, or rafters, because they bear the main portion of the roof's weight. A gambrel roof, or a roof with two pitches to a side such as a barn roof, allows you to have more storage space. You can use basic math to determine the length of the uppermost chord in a gambrel roof.

Calculator
Graph paper
Pencil
Ruler

Instructions

- 1
  Draw the gambrel roof truss design on graph paper to scale. For example, assume each block in the grid to represent 1 square foot.
- 2
  Draw a horizontal line separating the two halves of the roof from one another and note the measurement. For the purpose of this tutorial, assume this dimension is 14 feet.
- 3
  Draw a vertical line down from the peak of the roof, bisecting it perpendicular to the horizontal line you just drew. Note this measurement. Assume the vertical line is 4 feet long as an example.
- 4
  Take the square of these measurements and add them. 7 squared is 49, and 4 squared is 16; adding the two gives you 65.
- 5
  Take the square root of the sum of this measurement per the Pythogorean theorem. The square root of 65 is 8.062. This is the length of your upper gambrel chords.

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