How to Cut Wood Rafters

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Rafters form a roof. In a typical gable roof, which slopes on two sides from a center peak, opposing rafters support a ridge board at the top. Rafters are covered, or sheathed, with a solid material like plywood or oriented strand board (OSB) before waterproofing material and shingles are applied. Rafters must be cut accurately to make the proper pitch, or slope angle, of the roof and fit solidly between the ridge board and the top of the walls on each side.

Things You'll Need

  • Framer's square
  • Pencil
  • Circular saw
  • Tape measure
  • Determine the span, run and pitch for the rafters. The span is the width of the building, the run is half that distance or the space each rafter must cover and the pitch is the angle of slope. A house plan will provide these dimensions, or measure the span with a tape measure and divide by two to get the run. The pitch will vary with house design, local climate and other factors, including personal preference. Most houses are 5/12 or 6/12, meaning the roof rises 5 or 6 inches for every foot from the wall to the peak.

  • Use a framer's square, which has a 90-degree angle from a point (the heel), with a thin 16-inch side (the tongue) and a wide 24-inch side (the blade). The blade and tongue are marked in inches and fractions; the blade on one side has a table marked "length of common rafters per foot of run." This square, a pencil, a tape measure and a circular saw will enable you to cut perfect rafters.

  • Get a long and straight 2-by-4 rafter board to make a pattern. Put the heel of the square at the bottom of one end, with the 5-inch mark on the tongue and the 12 mark on the blade at the top of the rafter. That will give you the angle for a top ("plumb") cut at the top of the rafter (this is for a 5/12 roof; use the 6 mark for a 6/12 roof, etc.). Cut that angle with a circular saw.

  • Figure the length of the rafter to the wall cap using the table. Look under the 5 on your square; it will show 13, meaning a rafter must be 13 inches long for every foot of run. For a 24-foot wide building, that run would be 12 feet, meaning a rafter must be 13 feet long, or 156 inches, to reach the wall cap. Use a tape measure to measure that length from the plumb cut and make a mark on the opposite end of the board, on the bottom of the rafter. Draw a triangle from that mark, from one inch up from the bottom of the board and down 3 1/2 inches (for a 2-by-4 wall) from the bottom of the mark. Cut that notch (called a "birdsmouth") with a circular saw, to fit over the wall cap.

  • Add any overhang needed for an eave, generally 18 inches, and mark that point. For a 5/12 rafter that totals 177 1/2 inches -- 156 from plumb cut to birdsmouth, 3 1/2 for birdsmouth and 18 for eave. Cut the bottom of the rafter at that point, either a square cut or the same angle as the plumb cut but reversed. Go back to the plumb cut and take off another 3/4-inch to allow for a ridge board between rafters.

  • Calculate the number of rafters needed from the length of the building, using a tape measure or architect house plan to get that distance. Divide that by 24 -- rafters are spaced 24 inches apart -- to get the number needed. For a 30-foot building, that would be 15 rafters on each side or a total of 30. Test that pattern rafter to make sure it fits properly. Once it fits, cut all other rafters to the same pattern.

Tips & Warnings

  • Get help to cut rafters. Cutting long boards and lifting test rafters into position requires at least two people.
  • Some houses use 2-by-6-inch rafters; the angle cuts are the same but may need adjustment to fit the wider boards.

References

  • Photo Credit Jupiterimages/BananaStock/Getty Images
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