📐 Rafter Length Calculator: Span & Pitch

Span, run, and rafter length — the three numbers people mix up

Nearly every rafter mistake starts here, so it is worth being precise. These are three different measurements and only one of them is the board you cut.

• Span — the full width of the building, outside wall to outside wall. On a 24 ft wide house, the span is 24 ft.
• Run — the horizontal distance one rafter covers. On a symmetrical gable roof, run is half the span: 12 ft.
• Rafter length — the actual sloped board, always longer than the run. At 6:12 over a 12 ft run, it is 13 ft 5 in.

If you order lumber off the span you will be roughly double what you need. If you order off the run you will be short by the pitch multiplier — between 5% and 41% depending on how steep the roof is.

Rafter length = (span ÷ 2) × √(rise² + 12²) ÷ 12, before overhang and ridge deduction.

Span to rafter length chart

Common building spans converted to rafter length. These are line lengths from the ridge to the outside of the wall plate — add your overhang and subtract half the ridge board thickness to get the finished cut.

Assumes a symmetrical gable roof where run is exactly half the span. Rounded to the nearest inch.

How far can a rafter actually span?

This page calculates geometry — how long the board is. That is a different question from how far a given board can safely span, and we want to be clear about the difference because getting it wrong is a structural failure, not a material overage.

The maximum allowable span for a rafter depends on variables this calculator does not ask for and cannot infer:

• Species and grade — Southern Pine No. 1 and Hem-Fir No. 3 do not span the same distance in the same dimension.
• Member size — 2×6 versus 2×8 versus 2×10.
• Spacing — 12, 16, 19.2, or 24 inches on centre.
• Ground snow load — the dominant factor in most of the country, and highly regional.
• Dead load — a slate or tile roof loads the framing far more than asphalt shingles.
• Whether the ceiling is attached to the rafter bottom, which changes the deflection limit.

Published allowable spans live in the IRC rafter span tables (R802.5), indexed by all of the above. We deliberately do not reproduce them here: they run to dozens of pages, they are amended locally, and an out-of-context number is worse than no number.

How Rafter Length Is Calculated

A common rafter is just the hypotenuse of a right triangle. The "run" is the horizontal distance from the outside wall to the center of the ridge. The "rise" is how much the roof climbs over that same distance, set by your pitch. Once you know both, the rafter length is simple Pythagorean math: length = √(run² + rise²).

Don't Forget the Overhang

The overhang (eave) extends past the wall at the same slope as the rest of the roof, so it adds length along the same angle — not just a flat addition. Our calculator factors this in automatically so your total cut length is accurate, not just the distance to the wall line.

Hip & Valley Rafters: A Quick Note

This calculator handles common rafters (the standard rafters running from wall to ridge on a gable or shed roof). Hip and valley rafters run diagonally across the corners and use a different run — roughly your common rafter's run multiplied by 1.414 (√2) before applying the same pitch — because they're covering the diagonal of a square, not a straight side. If your project includes hip or valley framing, budget extra length and confirm exact cuts on-site, since the birdsmouth and ridge cuts on hip rafters are compound angles best laid out with a speed square or framing calculator at the actual ridge.

Frequently Asked Questions

How do I find the run for a gable roof?

Measure the total width of the building from outside wall to outside wall, then divide by two. That's your run for each common rafter, since the roof slopes up symmetrically from both walls to the ridge in the middle.

What length rafter do I need for a 6/12 pitch and 12 foot run?

With a 6/12 pitch, the rise over a 12 foot run is 6 feet. Using the Pythagorean theorem, the rafter length is √(12² + 6²) = √180 ≈ 13.4 feet, before adding overhang.

Should I buy lumber the exact length of my rafter, or longer?

Always round up to the next standard stock length (8, 10, 12, 14, 16, 18, or 20 feet) plus a few extra inches for the ridge cut and any adjustment during layout. Cutting a rafter too short means starting over.

What's the difference between rafter length and "span"?

Span is the total horizontal distance a roof covers between supports (usually twice the run for a gable roof). Rafter length is the actual diagonal board length needed to cover that span at your chosen pitch — span is always shorter than the rafter itself once pitch is added.