Work out common rafters for a gable roof. Enter the building span, roof pitch or angle, overhang and ridge board thickness to get the rafter length, run and rise, plumb and seat cut angles, birdsmouth depth, board length to buy and how many rafters you need.
Last updated:
Units
ft
Decimal feet: 24.5 = 24 ft 6 in
m
Roof slope given as
/ 12
°
in
mm
in
1.5 in for 2× lumber, 0 if rafters meet
mm
Birdsmouth and quantity (optional)
in
3.5 in for 2×4 walls, 5.5 in for 2×6
mm
Feet (or meters) along the ridge, to count rafters
Enter your values to see the result.
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How to use the Rafter Length Calculator
Enter the building span, measured across the outside of the walls.
Enter the pitch (inches of rise per 12 of run) or the roof angle.
Add the overhang measured horizontally, and the ridge board thickness.
Optionally set the wall plate width and rafter size to size the birdsmouth, and the building length to count rafters.
Read the rafter length and cut angles, then mark the plumb cut at the ridge and the birdsmouth at the wall.
Birdsmouth depth (plumb) = seat cut width × pitch ÷ 12, kept at or below 1/3 of the rafter depth
The rafter length is measured along the top edge from the long point of the ridge plumb cut to the tail. Lay it out from the ridge end, mark the birdsmouth where the line length ends, then add the tail.
Pitch, angle and slope factor
Pitch
Angle (plumb cut)
Seat cut
Slope factor
Rafter per ft of run
3/12
14.04°
75.96°
1.0308
12.37 in
4/12
18.43°
71.57°
1.0541
12.65 in
5/12
22.62°
67.38°
1.0833
13.00 in
6/12
26.57°
63.43°
1.1180
13.42 in
7/12
30.26°
59.74°
1.1577
13.89 in
8/12
33.69°
56.31°
1.2019
14.42 in
9/12
36.87°
53.13°
1.2500
15.00 in
10/12
39.81°
50.19°
1.3017
15.62 in
12/12
45.00°
45.00°
1.4142
16.97 in
Check rafter sizes against the rafter span tables for your snow load and roof weight. Hip, valley and jack rafters need different lengths and side cuts.
Frequently asked questions
How do you calculate rafter length?
Find the run (half the building span minus half the ridge board thickness), then multiply it by the slope factor √(1 + (pitch ÷ 12)²). Add the overhang multiplied by the same factor. For a 24 ft span at 6/12 with a 1.5 in ridge and 12 in overhang: run = 143.25 in, factor = 1.118, rafter = (143.25 + 12) × 1.118 ≈ 173.6 in, about 14 ft 5 9/16 in.
What is the plumb cut angle for a rafter?
The plumb cut angle equals the roof angle, atan(pitch ÷ 12). A 6/12 roof is 26.57°, 4/12 is 18.43°, 8/12 is 33.69° and 12/12 is 45°. With a speed square, pivot on the pivot point and line up the pitch number on the common scale with the rafter edge.
What is a seat cut?
The seat cut is the level (horizontal) cut of the birdsmouth that sits on the wall top plate. It is at 90° minus the roof angle from the plumb line, and its length is usually the width of the wall plate, 3.5 in for a 2×4 wall or 5.5 in for a 2×6 wall.
How deep can a birdsmouth be?
A common rule, and the requirement in many codes, is that the birdsmouth must not cut more than one third of the rafter depth. If the seat is too deep, shorten the seat cut so it bears on at least part of the plate, or use a deeper rafter.
Why subtract half the ridge board thickness?
Rafters stop at the face of the ridge board, so each side loses half of its thickness. With a 1.5 in ridge, each run is 0.75 in shorter, which shortens the rafter by 0.75 in × the slope factor.
Is the overhang measured level or along the rafter?
This calculator asks for the horizontal overhang (from the outside of the wall to the end of the rafter, measured level), which is how eaves are usually specified. It converts it to the length along the rafter using the slope factor.
Is my data sent anywhere?
No. Everything is calculated in your browser.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.