Notchline
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Notchline

Rafter calculator

Fill in any two of span or run, rise, pitch and rafter length. The page works out the rest, the tail of the overhang, and the birdsmouth notch against a quarter of the rafter depth.

Enter any two of run or span, rise, pitch and rafter length. The rest is worked out.

Units
You know the
ft-in
Span: outside of wall to outside of wall. Run: half of it.
Pitch shown as
ft-in
ft-in
Along the top edge
Overhang, birdsmouth, heights
ft-in
Level distance from the building line to the tail
ft-in
ft-in
Level cut on the plate, usually the plate width
The rafter bears on

Result

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Worked out in your browser. Nothing you type leaves this page.

How it is worked out

The common rafter is the long side of a right triangle: the run along the bottom, the rise up the side. Its length is the run times the slope factor, √(144 + r²) ÷ 12 for a pitch of r in 12. A 6 in 12 roof has a factor of 1.1180, so a 12 ft run gives a 13 ft 5 in rafter.

Any two of the four sides and angles fix the triangle. If you type three that do not agree, the page says by how much and changes nothing.

The length is measured along the top edge of the rafter, from the building line to the ridge centre. Take off half the ridge board, measured level, before you cut.

The birdsmouth check

The seat cut sits on the wall plate; the heel cut is the plumb face above it. The notch depth, measured square to the rafter, is the seat times the sine of the pitch. The IRC (R802.7.1 pointing to R502.8.1) keeps a notch at the end of a sawn rafter within a quarter of its depth, and R802.6 asks for 1 ½ in of bearing on wood or 3 in on masonry. The page checks both.

Many carpenters use one third of the depth as a rule of thumb. The page shows it too, labelled as a rule of thumb, not a code. The structure itself, rafter size and span, is for your engineer or the span tables.

Pitch in 12ths, degrees and percent

4 in 12 is 18.4° or 33.3 %, 6 in 12 is 26.6° or 50 %, 12 in 12 is 45° or 100 %. The page converts as you switch.

Sources

  1. ICC, International Residential Code 2021, R802 Wood roof framing
  2. NIST SP 811, Appendix B.9: inch, foot

Results are design aids. Check the current rules with your local building authority before you build.

Checked against the sources on November 2, 2026.

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