Rafter Length Calculator

How this is calculated

A rafter is the hypotenuse of a right triangle. The horizontal leg is the RUN, which is half the span for a standard gable roof. The vertical leg is the RISE, which is the run multiplied by the pitch over 12. The rafter is the square root of run squared plus rise squared. Roof pitch is written as rise per 12 inches of run - 6/12, 8/12 and so on - which is why the 12 appears in the formula. It is a ratio, not a measurement, so it works whatever units the span is in. The overhang is added ALONG THE SLOPE, not horizontally. If you want a specific horizontal overhang past the wall, the rafter has to be longer than that figure by the same ratio the rafter bears to the run - which is what the slope factor output is for. Multiply your desired horizontal overhang by the slope factor to get the length to add. ⚠️ Everything here is a THEORETICAL line length, measured from the centre of the ridge to the outside of the wall. Two deductions apply in practice and neither is included, because both depend on your timber and your cuts: First, the ridge board. Half its thickness comes off each rafter, since the theoretical lines meet at a point but two rafters plus a ridge board cannot occupy the same place. A 1.5 inch ridge board means 0.75 inches off every rafter. Second, the birdsmouth. Cutting a seat over the wall plate changes where the rafter bears and shortens the effective run - the exact amount depends on the seat cut depth and the plate width. So treat the number here as the line to lay out from, not the length to cut to. The standard practice is to mark the theoretical length, deduct half the ridge thickness, then cut and test-fit ONE rafter as a pattern before cutting the rest. Roof framing also has real code requirements for span, size and spacing depending on your load, timber grade and location, which no calculator can supply.

Common questions

What exactly is 'pitch'?

Rise per 12 inches of horizontal run, written as a fraction like 6/12 or 8/12. A 6/12 roof climbs 6 inches for every 12 it travels sideways. It is a ratio rather than a measurement, which is why the same pitch works on a shed and a house, and why the 12 shows up in the formula. Note that the run is HALF the span on a standard gable - the commonest mistake here is entering the run where the span is asked for, which halves every answer.

Why does the overhang go along the slope?

Because the rafter runs along the slope, so any length added to it is added in that direction. If what you actually want is a specific HORIZONTAL overhang past the wall, multiply it by the slope factor - a 12 inch horizontal overhang on a 6/12 roof needs about 13.4 inches of rafter. The slope factor is shown for exactly this, and it works for any horizontal distance you need to convert.

Can I cut to this length?

Not directly, and this is the part that catches people. The figure is a theoretical line from the CENTRE of the ridge to the outside of the wall. In practice half the ridge board's thickness comes off each rafter - the theoretical lines meet at a point, but two rafters and a ridge board cannot all occupy it. A 1.5 inch ridge means 0.75 inches off each side. The birdsmouth seat cut also affects where the rafter bears. Mark, deduct, then cut and test-fit ONE as a pattern before doing the rest.

What is the slope factor for?

Converting any horizontal measurement into its length along the roof. Multiply a horizontal distance by it and you get the sloped distance. That makes it useful well beyond rafters - the roof AREA is the footprint area times the slope factor, which is how you work out how much sheathing or roofing material a roof needs from a plan view.

Does this work for a hip roof?

Not for the hip rafters themselves. Common rafters on a hip roof use exactly this calculation, but the hips and valleys run diagonally and take a different, longer calculation based on a 17 rather than 12 unit run. If you are cutting hips, this gives you the commons and you will need to work the hips separately.

Is a steeper roof much more material?

Yes, and the slope factor tells you how much. A 4/12 roof has a slope factor of about 1.054, so a little over 5 percent more area than the footprint. A 12/12 roof is about 1.414 - over 40 percent more. That is 40 percent more sheathing, felt, shingles and rafter length, which is worth knowing before choosing a pitch on aesthetics alone.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the Rafter Length Calculator at https://www.bfcbrilliance.com/tools/rafter-length-calculator.

What I entered:
- Span (full width of the building) (ft): ___
- Roof pitch (rise per 12) (/12): ___
- Overhang along the slope (ft): ___

What it calculated:
- Rafter length including overhang: ___
- Line length (ridge to wall, no overhang): ___
- Run: ___
- Rise: ___
- Slope factor: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

Where the job touches structure, gas, or electrics, tell me to check local building code and whether a permit or a licensed trade is required.

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