Calculating the Length of a Roof Rafter

wooden rafters
  • 1-40 hours
  • Beginner
  • 1-10,000
What You'll Need
Framing square
Circular saw
Hand saw
Rafter stock
Safety equipment

When framing a roof system, it is important to calculate the length of a roof rafter correctly. Doing this will save time and material. It will also ensure the structural integrity of the roof. This how-to will guide you through the steps of calculating your roof rafter correctly.

Step 1 -Determine Rise, Run, and Pitch

The run is the only constant in this equation. It is the horizontal distance of the structure, divided by 2. The rise and pitch can be determined by how high and how steep you want your roof. Let's assume you have a structure that is 24 feet wide and you would like to build a roof that is 10 feet high. This gives you a rise of 10 feet (120 inches) and a run of 12 feet (144 inches). For every 12 inches of run, the roof will rise 10 inches. This is called a 10-inch pitch.

Use the Pythagorean Theorem to calculate the rafter length. a2 x b2 = c2 (your rafter length)

14400" + 20736" = 35136" The square root of which = 187.446" (your rafter length in inches)

Step 2 -Lay out your Rafter

Align your framing square so the 10" mark and 12" mark meet the edge of your rafter stock. Scribe your pencil along the 10" edge of the framing square. This line will serve as your ridge plum cut. Hold your tape measure on the long point of that line. Measure and mark 187.446." Using your framing square, again align the 10" and 12" marks on the edge of the rafter. Scribe along the 10" edge of the square. This will serve as your heel cut.

Step 3 -Cut your Rafter

Using the Circular Saw, carefully cut along each like. Fit and adjust the first two rafters. Once you are satisfied with the fit, use your rafter as a pattern and cut the remaining rafters.