A gambrel — the classic barn roof — is two roofs stacked: a steep lower slope that behaves almost like a wall, and a shallow upper slope that caps it. Each segment is an ordinary rafter on its own run and pitch, joined at the knuckle. This calculator takes the half-span, both pitches and the split point (how much of the half-span the steep lower segment covers) and returns both segment lengths, both rises, and the angles at the wall, the knuckle and the ridge.
Work a gambrel the way it is built: as two separate rafters per side. The lower segment runs from the wall plate up to the knuckle on a steep pitch — 18/12 to 24/12 is typical, and traditional barns go steeper still. The upper segment runs from the knuckle to the ridge on a shallow pitch, usually 5/12 to 8/12. Each segment has its own horizontal run, and the two runs add up to the half-span. The point where the runs divide is the split point, and it is the gambrel's master dimension: it fixes the knuckle's position, both rises, and the proportions of the whole roof.
Because each segment is computed on its own run with its own multiplier, a gambrel needs no special mathematics — just the discipline of treating the knuckle as both a ridge (for the lower segment) and a wall (for the upper one). The lower segment's length comes from the lower run and lower pitch; the upper's from the upper run and upper pitch; the ridge deduction applies at the top of the upper segment, and the overhang tail at the bottom of the lower one.
The example is a 24-foot-wide barn: half-span 12 ft, split evenly, so each segment covers 6 ft of run. Lower pitch 18/12, upper pitch 6/12.
Rise = 6 × 18/12 = 9 ft. Multiplier = √(1 + 1.5²) = 1.803. Line length = 6 × 1.803 = 10 ft 9 13/16 in, before the tail is added.
Rise = 6 × 6/12 = 3 ft. Multiplier = √(1 + 0.5²) = 1.118. Line length = 6 × 1.118 = 6 ft 8 1/2 in, before the ridge deduction.
The knuckle stands 9 ft above the plate and the ridge 12 ft. Per side, the two lines total about 17 ft 6 in of rafter — the price of that near-vertical lower wall of attic.
The lower pitch does most of the visual work. Pairings below 18/12 on the lower segment start to look like a gable with a kink; pairings above 24/12 read as a true barn, with a lower slope you could nearly stand a ladder against. The upper pitch is chosen more practically — it still has to shed weather like any shallow roof, so it rarely drops below 5/12 where shingles are planned.
| Lower / upper pitch | Lower rise | Upper rise | Total rise | Lower line | Upper line |
|---|---|---|---|---|---|
| 12/12 · 4/12 | 6 ft | 2 ft | 8 ft | 8 ft 5 13/16 in | 6 ft 3 7/8 in |
| 18/12 · 6/12 | 9 ft | 3 ft | 12 ft | 10 ft 9 13/16 in | 6 ft 8 1/2 in |
| 18/12 · 8/12 | 9 ft | 4 ft | 13 ft | 10 ft 9 13/16 in | 7 ft 2 1/2 in |
| 24/12 · 6/12 | 12 ft | 3 ft | 15 ft | 13 ft 5 in | 6 ft 8 1/2 in |
The steep lower segment is where the framing bites back. Its plumb and seat cuts are worked out at the lower pitch, so at 18/12 the seat cut is the steep one (56.3° from the board's edge) and the plumb cut the shallow one (33.7°) — and a birdsmouth cut into that steep rafter removes depth fast, because the notch's perpendicular depth grows with the pitch angle. Check the notch on the lower segment against the one-quarter limit before assuming your usual seat length works.
Headroom is the other trap. Moving the split point outward raises the knuckle and buys standing room at the walls, but it also lengthens the steep segment and shortens the shallow one, pushing the ridge up and the roof's cost with it. Run two or three split points through the calculator and compare total rise and segment lengths before fixing the design — small moves at the knuckle change the whole building.
A gambrel is the right tool when the building wants a room in the roof — barns, loft conversions, and anywhere a near-vertical lower slope earns its keep. If the same width and height would serve as a single steep pitch instead, that roof is a plain gable, and the main rafter calculator will size its rafters in one step. And if the gambrel is going on a freestanding barn or workshop where the "walls" are two different heights rather than a knuckle, that's a single-slope problem — the shed rafter calculator handles it directly.
Every result on this page comes from the method described above, using standard framing geometry and the published data credited below. Found a number that looks wrong? Use Report an Error — every report is read.
Anywhere the roof wants a knuckle: traditionally the lower segment covers roughly the outer third to half of the half-span. Moving the split outward makes a taller, boxier attic (more headroom at the walls); moving it inward makes a more barn-like, peaked profile. It is a design input, not a code figure.
Almost always steeper — that's what defines the gambrel. Common pairings run a lower pitch of 18/12 to 24/12 (some go to 30/12) with an upper pitch of 5/12 to 8/12. A gambrel with equal pitches is just a gable roof with a pointless joint in the middle.
In the middle: the chain runs lower rafter from wall plate to knuckle, then a short horizontal ceiling joist (collar) at knuckle height tying the two sides, then the upper rafters to the ridge. The collar joist is structural — it stops the knuckle spreading — and the calculator's two segment lengths assume the segments meet at a point on that line.
Yes — each segment is its own small roof plane, so a birdsmouth at the wall plate uses the lower segment's plumb and seat angles, and the upper segment's foot is cut at the angle where the two planes meet. Cut and test one side's pair before batching.
For the same span, yes — noticeably. The two-segment line hugs a taller profile, so total rafter length per side runs longer than a gable's single rafter, and you buy two shorter boards (or cut both segments from one long one) instead of one. The worked example's 12 ft half-span needs about 17 ft 6 in of rafter per side against a gable's 13 ft 5 in at 6/12.
Usually for headroom, not span tables: the lower, near-vertical segment puts full-height wall close to the eaves, which is why gambrels dominate barn and loft designs. Structurally each segment is checked against the span tables for its own horizontal run, like any other rafter.
The full common rafter: length, angles, birdsmouth and materials.
Line and cut lengths and angles for the diagonal rafter on a hip roof.
Where two roof planes meet — valley length and its cut angles.
First jack length and the common difference for your spacing.
Single-slope rafters between two different wall heights.
Seat cut, heel height and HAP, checked against the IRC one-quarter limit.
Plumb, seat and cheek angles — plus the saw settings to cut them.
Convert pitch, rise/run, degrees and percent, both ways.
How far a board can span — IRC tables by species, grade and load.
The lumber size a span actually needs. The inverse of the span tool.
The full pitch × run table, printable for the workshop wall.
Figure any rafter by hand — formula and framing-square methods.