Rafter Calculator

A rafter calculator works out the common rafter length for a gable roof from just the building span and the pitch — then adds the overhang, the plumb-cut angle and how many rafters you'll need for the whole roof. It saves stepping off rafters or wrestling with the rafter tables on a framing square.

Your measurements

Result

Enter the span (the full width the rafters cross), the pitch, your overhang and the building length, and it returns everything to lay out and order the framing.

How it is calculated

Run = span ÷ 2. Slope factor = √(pitch² + 12²) ÷ 12. Line length = run × slope factor; the overhang adds its run × slope factor. The plumb-cut angle = arctan(pitch ÷ 12). Rafter count = (building length ÷ spacing + 1), doubled for both roof slopes.

Steps

Enter the building span — the full width the roof crosses.
Enter the pitch as rise per 12 (e.g. 6).
Add the overhang (eave projection) you want.
Enter the building length and your rafter spacing (16 or 24 in OC).
Read the rafter length, plumb-cut angle and the number of rafters.

Worked example

A 24 ft span at 6/12 with a 12 in overhang gives a common rafter about 14.5 ft long (13.4 ft to the ridge), a 26.6° plumb cut, and 62 rafters for a 40 ft-long building at 16 in OC.

Common questions

How long should my rafters be?

Multiply the run (half the span) by the slope factor for the line length, then add the overhang. The calculator does both.

What is the plumb cut angle?

The angle of the cut where the rafter meets the ridge — equal to the roof's pitch angle, arctan(rise ÷ 12).

What's a birdsmouth?

The notch cut into the rafter where it sits on the top wall plate, so the rafter bears flat. Cut it after the plumb and tail cuts.

16 or 24 inch spacing?

16 in on-centre is the common residential default; 24 in uses fewer rafters but needs heavier members and sheathing. Follow your design or code.