Rafter Length Calculator
Get the rafter length from the pitch, the run and the overhang.
Works in your browser. Nothing you type is sent to us.
What you know
Rise is how far the roof goes up over the run, the horizontal distance. Fractions work: 5 1/2.
A 6:12 roof rises 6 inches for every 12 inches it runs. Type the 6.
Grade is rise divided by run, as a percentage. 100% is 45 degrees.
Rafter and roof area optional
The run is measured flat, from the outside of the wall to the centre of the ridge: half the building's width for a gable roof.
Rafter and roof area
Type the run and the ridge length to get the rafter length and the area to cover.
Worked out in your browser. Nothing you type is sent to us.
Rafter Length Calculator at a glance
- Price
- Free
- Data sent to us
- None, it runs in your browser
- Sign-up
- Not needed
- Takes
- Rise and run, pitch, degrees or percent
- Gives
- Pitch, angle, grade, slope factor, rafter length, roof area
- Units
- in, ft, cm, m, with fractions
How to use Rafter Length Calculator
- Pick what you knowChoose Rise and run if you measured with a level, Pitch if you know the number out of 12, or Angle or Grade from a drawing.
- Type the numbersEnter the measurements in any one unit. Fractions such as 5 1/2 work. The pitch, angle, grade and slope factor update as you type.
- Add the run and ridge for the areaType the horizontal run from the wall to the ridge, the ridge length and any overhang to get the rafter length, the roof area and the roofing squares.
Type the pitch, then the horizontal run from the outside of the wall to the centre of the ridge and the overhang, and read the common rafter length in feet and inches or metres. The length is the run plus overhang times the slope factor; the calculator also gives the rise at the ridge and the roof area for the ridge length you type. It gives the line length along the rafter; take off half the ridge board's thickness and add the plumb and seat cuts when you mark the timber.
How it works
Rafter Length Calculator is a web page that does its work on your own device. Everything you type or choose is worked out by your browser and never sent to a server, so nothing is stored and there is nothing to delete. There is no account, no sign-up and no limit on how often you use it.
What the length is measured to
The result is the theoretical line length from the outside of the wall plate to the centre line of the ridge, plus the overhang. On the timber, shorten it by half the ridge thickness, and remember that the overhang here is measured horizontally, as on a plan, not along the rafter.
Questions
Does it work for hip and valley rafters?
No. Hip and valley rafters run diagonally and need a run of the common run times 1.414 for a square-cornered roof; use that run here for a first estimate, and check the cuts with a framing square or rafter tables.
Other shortcuts
Roof Pitch Calculator · Roof Pitch to Degrees
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