New construction such as an addition or rebuilding a damaged roof might require you to cut roof rafters. Precise measurements and careful calculations are essential in order to learn how to cut roof rafters correctly. Unless you have strong trigonometry skills, you should find an online roofing calculator for managing your measurements. There are some phone apps that can do this for you as well.
Before diving into the directions, it’s good to know basic roof rafter terms. The ridge cut, also known as the plumb cut, is the top rafter cut where the boards meet at the peak of the roof. The birdsmouth cut is the notch cut on the other end of the board where the rafter sets against the wall. Finally, the tail cut is the angled cut on the lower end of the rafter.
Tools You’ll Need
- Roofing calculator
- Tape measure
- Pen and paper
- Stair gauges
- Framing square
- Circular saw
How to Cut Roof Rafters Step by Step
- Measure the width of the building three times to make sure you have the best measurement.
- Try to get the most precise measurement as possible down to the nearest 1/8 or 1/16 of an inch.
- Write down the width.
- Measure the width of the ridge beam where all rafters will be attached at the roof’s peak.
- Subtract the width of the ridge beam from the building width.
- Divide the result of building width minus ridge beam width by 2.
- The result of that calculation is called the RUN.
- Write down the run figure and enter it into your roofing calculator section labeled run.
- Now measure from the top of the wall to the top of the roof and write down the figure.
- Take a second measurement down from the ridge beam to the edge of the wall and write down the figure.
- Enter both figures into the roofing calculator’s PITCH formula. Pitch will be expressed as a ratio like 6/12.
- Use the DIAGONAL function on the roofing calculator to see the result for rafter length.
- Write down the diagonal length.
- Add the desired length for the rafter overhang beyond the wall to the calculated rafter length. Write this figure down.
- Select the RISE function on the roofing calculator to produce the height of the ridge beam.
- Attach stair gauges to a framing square.
- Apply the numbers from the roof pitch ratio to adjust the stair gauges.
- Use the first pitch ratio figure for the vertical tongue of the framing square.
- Move the horizontal body of the framing square to 12.
- The framing square now shows the cutting angle for the ridge cut.
- Place the tongue of the framing square so it faces the top of the rafter board.
- Follow the edge of the square to mark the cutting line for the ridge cut.
- Clamp the rafter board across sawhorses and make a ridge cut with the saw.
- Hook the tape measure at the top of the ridge cut.
- Extend the tape measure down the board and mark the rafter length WITHOUT overhang.
- Place the framing square’s tongue (shortest leg) at the mark.
- Trace down the longer leg of the square’s body across the board to mark the tail cut.
- Go to the wall and measure its total width and write down the figure. It is usually 4 inches.
- Multiply this measurement by 2.
- Place the framing square’s body (longest leg) at the mark on the rafter.
- Move the square until the ruler on the body crosses the line for the tail cut at the dimension that is double the wall width.
- Trace along the edge toward the tail cut mark to make the guide for the birdsmouth cut.
- Now slide the framing square down the rafter for the number of inches that you want the rafter to overhang.
- Stop the square once when its ruler intersects the mark for the birdsmouth cut at the length of the overhang.
- Trace the tongue (shortest leg) for the actual tail cut that will create the lower end of the rafter.
- Clamp the board to sawhorses and carefully cut the triangular notch marked for the birdsmouth cut.
- Cut the outer tail cut where the line includes the rafter overhang.
- Measure and cut all remaining rafter boards in the same way.
Get Help Installing Shingles From PJ Fitzpatrick
Once you have learned how to cut roof rafters you might be inclined to get to work. Installing new rafters is a big job. Consider having a professional roofing team move in and help add the shingles. PJ Fitzpatrick has shingled thousands of roofs throughout the Delaware Valley. Contact our roofing company for an estimate today.