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Rafter length calculator

Find the theoretical rafter length from span and pitch.

Unit

One unit for the span and the overhang.

ft

The outside width the rafter crosses. Half of this is the horizontal run.

Rise of the pitch triangle. Use 6 with a run of 12 for a 6-in-12 pitch. Use 0 for a flat line.

Run of the pitch triangle. Must be more than 0. A common pitch run is 12.

ft

Extra horizontal run past the wall. Leave blank for none. It is not a tail cut.

Result

Rafter length calculator

Unit
Feet

Rafter length

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Enter a span and a pitch rise and run.

Verify with your local code or a qualified professional. This is a theoretical line length, not a cut list.

Estimate for planning only. This is not tax, legal, accounting or financial advice. Results depend on the figures you enter. Check all numbers, rates and rules with the official source or a qualified professional before relying on them.

How this works and common questions

About this tool

The length is the sloping line from half the span, plus any horizontal overhang, at the pitch you type. Verify with your local code or a qualified professional.

How it works

Horizontal run is half the span, half up, plus the overhang. The length is that run times the pitch triangle’s hypotenuse, divided by the pitch run, half up to 0.01. A 24.00 span, rise 6.00, run 12.00, and no overhang is 13.42. A 20.00 span, rise 4.00, run 12.00, and 1.00 overhang is 11.60.

Rules

The pitch run must be more than 0. A rise of 0 is a flat length equal to the horizontal run. This does not deduct a ridge board or add a plumb cut. Verify with your local code or a qualified professional.

Questions

What length is this?

The theoretical sloping line. It is not a finished cut length.

How is 13.42 worked out?

Half of 24.00 is 12.00. A 6 on 12 pitch makes that line 13.416…, which rounds half up to 13.42.

Does this follow the building code?

No. Verify with your local code or a qualified professional.

Do I need an account?

No. The tools work without one. You only need to sign in to save your work.

Is what I type saved?

No. Nothing is saved unless you sign in and choose Save. Otherwise it stays on the page.

Do you show ads or sell my information?

No. We show no ads and we do not sell your information.

Is this tax or legal advice?

No. Results are estimates for planning. Check rates and rules with the official source or a qualified professional.

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Estimate for planning only. This is not tax, legal, accounting or financial advice. Results depend on the figures you enter. Check all numbers, rates and rules with the official source or a qualified professional before relying on them.