Rafter Length Calculator

A common rafter is the diagonal of a right triangle, so its line length equals the square root of (run squared plus rise squared). With pitch in 12, rise equals run times pitch divided by 12, so the rafter equals run times the square root of (12 squared plus pitch squared) divided by 12. For a 144 inch run at 6 in 12, that is 144 times 1.118, about 161 inches before any ridge deduction.

Run is the horizontal distance from the outside of the wall plate to the center of the ridge, which is half the building span. Rise is the vertical height the roof climbs over that run. Pitch ties them together as rise in inches per 12 inches of run, so a 6 in 12 pitch rises 6 inches for every 12 inches of run. The calculator lets you enter pitch directly or enter rise and have the pitch derived for you.

The full run reaches the center line of the ridge, but the rafter actually stops at the face of the ridge board. Standard framing practice deducts half the ridge thickness, measured horizontally, then carries that shortened run up the slope. For a 1.5 inch ridge the run drops by 0.75 inch before the line length is computed, which trims roughly an inch off a steep rafter.

The birdsmouth is the notch where the rafter sits on the top plate. It has a level (seat) cut that rests flat and a plumb (heel) cut that bears against the plate. The plumb cut angle equals the roof angle, atan of the slope, and the level cut is 90 degrees minus that. The heel depth equals the seat cut length times the slope, so a 3.5 inch seat at 6 in 12 gives a 1.75 inch heel.

The headline line length is plate to ridge only. The overhang, also called the tail or eave projection, is added separately as its horizontal run times the same rafter multiplier. The total length shown is the line length plus the sloped overhang, which is the rough stock to cut before you trim the tail and ridge ends.

It sizes the common rafter, the straight run from plate to ridge on the main roof slope. Hip and valley rafters travel diagonally across the plan, so they use a 17 unit run instead of 12 and have different multipliers. Use the common rafter result for the bulk of a gable or shed roof, and treat hips, valleys, and jacks as a separate layout.