How to Figure the Size of a Rafter

Rafters are the framework of a roof.

Rafters are large beams that form the internal framework of a roof. They extend from the ridge or hip of the roof to the eave. This ranges from the front of the roof where it overhangs to the top of the roof. Before you build a roof with rafters, it is necessary to figure out how long the rafters will need to be. Rafter length is based on the size of the roof and can easily be figured out with a rafter calculator.

Things You'll Need

• Rafter calculator

Instructions

1. Determine what size the roof you are building is going to be. This will help determine the size of the rafters. Draw a small outline of the design of the roof on a piece of paper with a pencil.

2. Write down the measurement for the width of the roof in feet from one side to the other. Divide the width in half to get the run of the roof. If the width is 24 feet, the run will be 12 feet. This is the horizontal distance the rafter travels to meet the rafter from the other side.

3. Determine the spacing of the rafters. Most roof rafters are spaced out every 16 inches on center. Decide what pitch you will be using for your roof. The pitch is equal to the rise of the roof over the run.

4. Put these three measurements into a rafter calculator. This will calculate the length the rafters will need to be for your roof.

References

• APB Pole Barns; Roof Rafter Calculator; 2010
• Carpentry-Pro-Framer: Basic Roof Framing Instructions