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How to Build Roof Rafters

You will need to know how to build roof rafters when putting up a permanent structure on your property. The carpentry is very dependent on accurately measuring the roof pitch and span. These dimensions determine what figures to follow on a framing square when cutting the rafters.

How to Build Roof Rafters Steps

  1. Measure width of the building from exterior wall edges to determine roof span.
  2. Subtract width of the ridge beam from roof span.
  3. Divide the adjusted figure by 2 to establish measurement for the building run.
    1. Enter this figure as the “run” in the roofing calculator.
  4. Calculate the roof pitch.
    1. It is expressed X/12 and may be included in your building plans.
    2. For example, a roof pitch of 7/12 rises 7 inches per every 12 inches of height.
    3. Enter this figure in the calculator for pitch.
  5. Have the calculator figure the “diagonal” measurement for the roof rafter.
  6. Adjust stair gauges on framing square to the numbers for the roof pitch.
    1. In this example, put a stair gauge at the 7 on the short tongue of the square.
    2. Place the other gauge at the 12 on the longer body of the square.
  7. Set framing square on the board with the 7 mark near the end.
  8. Rest the portion of the framing square with the 12 farther down the board.
  9. Mark a line from the 7 down the outer edge of the square.
  10. Saw the line to create your plumb cut for the rafter’s top.
  11. Measure along the top of the rafter board and mark the point where you reach the “diagonal” length.
  12. Position the 7 on the framing square at this mark and draw a line down the edge.
  13. Measure width of the wall.
    1. Subtract that figure from 12.
  14. Flip square around and place the long side of the square across the line on the number that represents 12 minus wall width.
  15. Make a second line that intersects with first line.
  16. Shift the square down the board for the number of inches you want the overhang to be.
    1. Mark that line.
  17. Saw out the notch where the rafter meets the wall.
  18. Cut the rafter’s end.
  19. Use this board as a template to cut other rafters.

Tools & Materials

Locate a roofing calculator tool online to perform the calculations necessary for determining rafter length. You may also choose to purchase the specialty calculator from a building supply store.
  • Roofing calculator
  • Framing square
  • Square gauges
  • Tape measure
  • Circular saw

Contact the Roofing Experts at PJ Fitzpatrick

Taking on home improvement projects yourself is extremely satisfying, but fast results from professional roofers have their appeal as well. Ask us for an estimate for your roofing project.

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