How do you find roof pitch with a speed square?

Know the parts of the square

• Lip or fence: the raised edge that hooks over a board.
• Pivot point: the corner at one end of the lip, marked PIVOT.
• Degree scale: along the long sloped edge, 0 to 90.
• COMMON scale: numbers along the same edge giving rise per 12 for common rafters. Many squares also have a HIP/VAL scale.

Reading an existing pitch

1. In the attic, hold the square flat against the side of a rafter with the pivot point touching the rafter's top or bottom edge.
2. Hold a torpedo level against the square's straight ruler edge, the one running from the pivot at a right angle to the lip.
3. Rotate the square around the pivot until that edge is plumb.
4. Read the COMMON scale where the rafter's edge crosses it. That is the pitch: 8 means 8/12. You can read degrees on the degree scale at the same spot.

Marking a plumb cut for a known pitch

1. Set the pivot point on the rafter's top edge.
2. Rotate until the desired number on the COMMON scale lines up with the same edge.
3. Draw along the ruler edge. That line will be plumb when the rafter is installed.

Tips

• Check two rafters; framing is not always perfect.
• The HIP/VAL scale is for hip and valley rafters on equal pitch roofs.

For the measurement method with a plain level, see how roofers measure a roof, and for rafter vocabulary, roof anatomy explained.