Free Calculator · Published November 2026
Rafter Calculator for Line Length and Tail
A rafter calculator takes half the span, then subtracts half the ridge thickness. That remainder is the run. Rise equals run times pitch over 12. Line length is the square root of run squared plus rise squared. A horizontal overhang is multiplied by the slope factor. It is not added flat.
Built by SubcontractorHub — the software roofing contractors use to quote, finance, and run every job.
Book a DemoHow the Rafter Calculator Splits the Board
One length runs from the ridge plumb cut to the outside of the plate. The tail runs from that plate line to the fascia plumb cut. Both ends are plumb, so they are parallel, and the top-edge length matches the triangle.
1.5 inches is an example ridge, the dressed thickness of a nominal 2-inch board. Set 0 to ignore the ridge. 3.5 inches is an example seat, the dressed thickness of a nominal 4-inch plate. It is not a code minimum, and it is not added to the length.
Run 11.9375 ft. Rise 5.9688 ft. Line length 13.3465 ft.
Slope factor 1.1180. Tail 1.1180 ft. Board from ridge plumb to tail plumb: 14.4646 ft.
Pitch angle 26.57 degrees. Seat 3.50 in, level, not added to the board.
All calculations are estimates based on historical information and should be verified by the user.
Important: this is layout math
All calculations are estimates based on historical information and should be verified by the user. This tool is provided as a free service for planning purposes only and is not a substitute for a professional assessment, a supplier quote, or applicable local code requirements.
What the Rafter Calculator Keeps Apart
Pitch ratio, degrees, and roof area belong on the roof pitch calculator. Squares of roofing belong on the roofing squares calculator. This rafter calculator is the board length. It does not multiply the building footprint by a slope factor.
Worked example
Span 24 feet, outside of plate to outside of plate. Ridge 1.5 inches, which is 0.125 feet, so half the ridge is 0.0625 feet. Run is 12 minus 0.0625, which is 11.9375 feet. Pitch 6/12 means rise is half the run, 5.9688 feet. Line length is the square root of those two squares, 13.3465 feet at four decimals. The slope factor is 1.1180 at four decimals. One foot of horizontal overhang times that factor is a tail of 1.1180 feet. The board from the ridge plumb cut to the tail plumb cut is 14.4646 feet. The pitch angle is 26.57 degrees at two decimals.
| Step | Feet |
|---|---|
| Run | 11.9375 |
| Rise | 5.9688 |
| Line length at four decimals | 13.3465 |
| Tail at four decimals | 1.1180 |
| Line plus tail at four decimals | 14.4646 |
Birdsmouth and overhang
The birdsmouth is the notch on the underside. The seat cut is level when the rafter is up. The plumb cut at the outside of the plate is vertical. Those two cuts meet at a right angle. The seat length is how much bearing you want on the plate. The example uses 3.5 inches because a nominal 4-inch plate is dressed to 3.5 inches. That is an example, not a minimum this page found in a code book. The seat does not make the board longer.
Height above the plate depends on how deep the rafter is and where you place the seat. This page does not compute that height. Mark it on the board. The overhang you type is horizontal. Adding that same number of feet onto the line length would treat the tail as a flat stick. On a 6/12 pitch the tail is longer than the horizontal measure by the slope factor 1.1180.
The plumb cut
When the rafter is in place, the ridge cut and the tail cut are vertical. The angle between the rafter and a level line is the pitch angle, 26.57 degrees for 6/12. A saw set that many degrees off square, with the shoe on the face of the rafter, cuts that plumb line. The level seat is the other side of the right angle. No span table was fetched for this page, so no maximum span is printed. A collar tie, a ridge beam, and a rafter size are a different design.
How should you write the rafter?
- Span, and whether it is outside of plate to outside of plate.
- Ridge thickness, or a zero if you ignored it.
- Pitch as rise per 12, the run, and the rise.
- Line length, then the horizontal overhang and the sloped tail.
- The sum, which is the top edge from ridge plumb to tail plumb.
- Seat length as a bearing you chose, and a note that no span table was used.
Stairs use the same triangle
A stair stringer is also a square root of rise squared plus run squared. The cuts repeat at every step, and the nosing stays out of the run. That layout is the stair stringer calculator. Do not drop a rafter by a tread thickness. The birdsmouth is the only notch on this board.
Frequently Asked Questions
How does the rafter calculator get line length?
Take half the span and subtract half the ridge thickness. That is the run. Rise is run times pitch over 12. Line length is the square root of run squared plus rise squared. A 24 foot span, a 1.5 inch ridge, and a 6/12 pitch have a run of 11.9375 feet and a line length of 13.3465 feet at four decimals.
How does an overhang change the rafter length?
A horizontal overhang is multiplied by the slope factor, the square root of 1 plus pitch-over-12 squared. On a 6/12 pitch that factor is 1.1180 at four decimals. One horizontal foot of overhang adds 1.1180 feet of board, not 1.000 foot.
What is the birdsmouth on this rafter calculator?
The birdsmouth is a level seat cut and a plumb cut. The seat is horizontal when the rafter is in place. The plumb cut is vertical, parallel to the ridge cut. They meet at a right angle. The seat length is the bearing you choose. It is not added to the line length. This page does not set a code minimum for that bearing.
Is this the roof pitch calculator?
No. The roof pitch calculator owns the pitch ratio and the area multiplier. This rafter calculator separates the ridge-to-plate line from the sloped tail and subtracts half the ridge. It does not compute roof area or squares.
Save the Rafter Length on the Cut List
The run, the line, the tail, and the seat you chose belong on the same card.
Get a Free DemoAll calculations are estimates based on historical information and should be verified by the user. This tool is provided as a free service for planning purposes only and is not a substitute for a professional assessment, a supplier quote, or applicable local code requirements.