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MaterialCalc — Rafter Length & Cut Calculator

Estimate prepared 09/01/2026. Estimates only; verify against your plans before ordering.

Rafter Length & Cut Calculator

Outside to outside. The run is half of this on a gable roof.

A 6/12 roof rises 6 in for every 12 in of run.

The ridge direction — sets how many rafters you need.

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Result

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A rafter is the hypotenuse of a triangle whose base is half the building width and whose height is set by the pitch. That makes the length easy; what catches people is the ridge deduction and the overhang, both of which change the board you have to buy. This gives you the length, the plumb cut angle, the rafter count and the stock size.

Assumptions used

These are the numbers behind the result. Where one of them is soft, the cost range widens rather than pretending to a precision the data does not have.

Pitch run base
12 in

Roof pitch is always written as rise over a 12 in run.

Standard roofing convention — Published product data, checked 09/01/2026

Stud spacing, 16 in on center
16 in o.c.

International Residential Code (2021) — Published product data, checked 09/01/2026

Eave overhang
12 in

Typical residential detail — Industry standard, varies by supplier, checked 09/01/2026

Prices verified 09/01/2026 (US national average, big-box retail shelf price and regional ready-mix quotes). Regional adjustment: Blended from RSMeans city cost index bands and BLS construction wage differentials.

The formula

Run = building width ÷ 2. Rafter length = run × √(1 + (pitch ÷ 12)²). The overhang is calculated the same way on its own run and added. Plumb cut angle = arctan(pitch ÷ 12). Rafter count = (roof length ÷ spacing + 1) × 2 for a gable roof.

Worked examples

24 ft wide building, 6/12 pitch, 12 in overhang

Building width: 24 Roof pitch (rise per 12): 6 Eave overhang: 12 Roof length: 40 Waste: 10

12 ft of run gives a 13.42 ft body plus 1.12 ft of overhang — 14.54 ft total, so 16 ft stock. Plumb cut 26.6°.

28 ft wide, 8/12 pitch

Building width: 28 Roof pitch (rise per 12): 8 Eave overhang: 16 Roof length: 48 Waste: 10

A steeper pitch lengthens every rafter — 14 ft of run becomes 16.8 ft of rafter. Pitch costs lumber, not just headroom.

20 ft wide shed roof at 4/12

Building width: 20 Roof pitch (rise per 12): 4 Eave overhang: 12 Roof length: 24 Waste: 10

A shallow pitch is the cheapest roof to frame but sits right at the limit for conventional asphalt shingles.

Choosing materials

  • Deduct half the ridge board thickness from the run before you cut — 3/4 in for a standard 1.5 in ridge. Every rafter is otherwise 3/4 in too long, and the roof ends up wider than the walls.
  • Cut one rafter, test-fit it against the actual ridge and wall, then use it as the pattern. Buildings are never as square as the plan.
  • The plumb cut at the ridge and the seat cut at the wall are both set by the same angle. A framing square or a speed square with the pitch marked does both without arithmetic.
  • Pitch drives everything downstream. Above 4/12 you can use asphalt shingles conventionally; below that you need a low-slope underlayment or a different roofing material entirely.
  • Buy the next stock length up, not the exact figure. Rafters need the tail cut after the seat cut is fitted, and there is no recovering from a board cut short.

Common questions

How do I calculate rafter length?

Take half the building width as the run, then multiply by the square root of 1 plus (pitch ÷ 12) squared. For a 24 ft building at 6/12 that is 12 × 1.118 = 13.42 ft, before the overhang and before deducting half the ridge thickness.

What is the plumb cut angle for a 6/12 roof?

26.6 degrees from vertical, which is arctan(6 ÷ 12). The same angle applies at the ridge plumb cut and in laying out the seat cut at the wall plate.

Do I subtract the ridge board from the rafter?

Yes — half its thickness, so 3/4 in for a standard 1.5 in ridge board. Measure the run from the outside of the wall to the centerline of the ridge, then take off that 3/4 in before cutting.

How far apart should rafters be?

Sixteen or 24 inches on center. Twenty-four uses a third fewer rafters but needs 5/8 in sheathing rather than 1/2 in, and gives a less rigid roof under snow load. Span tables set the maximum for your rafter size and load.

How many rafters do I need?

Divide the roof length by the spacing, add one for the closing rafter, then double it for a gable roof with rafters on both sides. A 40 ft roof at 16 in on center is 31 pairs — 62 rafters.

What size rafters do I need?

It depends on span, spacing, species and snow load. As a rough guide, 2x8 at 16 in on center spans about 13 ft in southern pine under moderate snow. Check the IRC rafter span tables for your actual conditions rather than guessing.

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