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The overall width of the building, wall to wall.
The rafter run is half the building width, from the exterior wall plate to the ridge centerline.
The roof's slope, expressed as inches of rise per 12 inches of horizontal run.
Rise per 12 of RUN, which is horizontal — not per 12 along the rafter, and not an angle. A 6/12 roof rises 6 for every 12 it travels in plan, which is 26.6°, and entering 26.6 here would give a roof rising 26.6 in 12. If you have only an angle, the rise per 12 is twelve times its tangent. If you have an existing roof, measure the rise over a level 12 units set out from the rafter with a spirit level rather than trusting a drawing that may have been stretched.
The horizontal distance the rafter tail extends past the wall plate.
Measured in PLAN, out from the outside face of the wall plate — the tail's actual length along the slope is longer and is what this calculates. Take it to where the rafter is cut, not to the fascia face or the edge of the gutter, and note whether your drawing dimensions the overhang from the plate or from the wall's outer skin: on a cavity wall those differ by the full cavity and the outer leaf.
Total rafter length (incl. overhang)
16.03 ft
- Rafter length to ridge (no overhang)
- 14.53 ft
- Overhang tail length
- 1.49 ft
They open the calculator with your figures already in it
Common Gable Rafter Length Calculator: 16.03 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Standard carpentry rafter-length formula: rafter length = run × √(1 + (rise/12)²), where run is the horizontal distance from wall plate to ridge
Inputs used
- Building Width
- 26 ft
- Roof Pitch (rise per 12 run)
- 6
- Overhang (Horizontal)
- 16 in
Intermediate steps
- Rafter length to ridge (no overhang)
- 14.53 ft
- Overhang tail length
- 1.49 ft
What this calculation does not cover
- The run is taken as exactly half the building width, so the length measures to the ridge centreline of a symmetric gable with equal pitch each side and level wall plates. A real common rafter stops at the face of the ridge board, so half the ridge thickness must be deducted along the slope: for a 38 mm ridge at 6/12 that is about 21 mm off every rafter, and the deduction grows with pitch.
- Pitch here is rise per 12 units of run, not an angle in degrees. Entering a measured roof angle produces a far steeper roof than intended — 22.5 read as a ratio is a 61.9 degree roof, whereas a 22.5 degree roof is a pitch of about 5 in 12.
- The overhang input is the horizontal projection past the wall plate, and the calculator multiplies it by the slope factor itself. Entering the sloped length of the tail measured along the rafter overstates the total by that factor again, about 12 per cent at 6/12. The figure also assumes the tail runs to the plumb cut, with no setback for fascia thickness or a soffit return.
- The result is a line length, not a cut list. The plumb cuts, the birdsmouth and the tail are drawn at full size in the rafter cut templates on this page, with every length marked from the heel point. Neither adds anything for saw kerf, waste or rounding up to a stock timber length, and neither tests whether the birdsmouth leaves enough rafter over the wall plate — that limit is the building code's, not this page's.
- Nothing here checks structural adequacy. The calculator never asks for rafter section, timber grade, spacing or load, so it cannot say whether that span works under snow, wind and imposed loads, or whether purlins, struts, collars or a structural ridge beam are needed. Span tables set that limit, and a long run can exceed any common section. The result is one rafter's length, not a rafter count or a timber order.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-17 · v1.0.2
Regulatory standards & verification citations1
- Standard carpentry rafter-length formula: rafter length = run × √(1 + (rise/12)²), where run is the horizontal distance from wall plate to ridge
Cite this page
Your workspace
Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.
Tools and safety for this job
To cut framing lumber. Generic types, no brands, no prices.
Protection this work requires
- Hardwood dust is a recognised carcinogen: extract at the tool and wear a P2/N95 respirator when cutting or sanding indoors.
- Saws, grinders and breakers run above 85 dB, where hearing damage accumulates and does not recover: defenders or plugs for every cut, not just the long ones.
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Essential
Circular saw
Combination or framing square
Tape measure
Recommended
Mitre saw
Optional
Hand saw
Also needed as materials: circular saw blades, mitre saw blades.
what each carpentry tool is for, and the spec that decides which to buy where one does.
Full-size rafter cut templates
Print the tail, the birdsmouth and the ridge plumb cut of this rafter to trace onto the first one, with every length marked from the heel point — and a scale check on each sheet, so a shrunken print never becomes a cut.