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The overall building width the gambrel roof spans.
The full width the roof spans, wall face to wall face — the calculation halves it, so entering the half width produces a roof for a building half the size. On a barn-style roof this is usually the outside of the frame rather than of the cladding. Both slopes are worked from this one figure, so an error here is inherited by every rafter on both segments.
The steeper lower roof segment's pitch, as rise per 12 units of run.
The lower, steep segment — typically somewhere near 20/12, which is about 59°, and steeper than most pitch charts go. That is normal for a gambrel: the lower slope is doing the work of a wall. If you are copying an existing barn, measure this one on the building rather than estimating it, because at this angle a small error in the pitch is a large error in the rafter.
The shallower upper roof segment's pitch, as rise per 12 units of run.
The upper, shallow segment that closes the roof to the ridge — often near 7/12. It is measured the same way as any pitch, but check the direction: both segments rise towards the ridge, so both are entered as positive rise per 12, and the change between them is what makes the break. The upper pitch also decides the ridge height, so if headroom at the apex is the constraint, this is the number to adjust.
The fraction of the half-building-width covered by the lower segment's run before transitioning to the upper segment.
Where the two slopes meet, as a fraction of the half width — 0.5 puts the break halfway out from the ridge, which is the traditional proportion and gives the familiar barn outline. Lower it and the steep segment gets shorter and the roof flattens; raise it and the steep part climbs, buying floor space in the loft at the cost of a taller, more exposed elevation. It is a plan-view fraction: the break point's height comes out of the pitches, not out of this.
Total gambrel rafter length
21.8 ft
- Lower segment length
- 14.53 ft
- Upper segment length
- 7.27 ft
They open the calculator with your figures already in it
Gambrel Roof Rafter Geometry Calculator: 21.8 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 rafter-length trigonometry applied independently to each of the gambrel roof's two slope segments: segment length = segment run × √(1+(segment pitch/12)²)
Inputs used
- Building Width
- 26 ft
- Lower (Steep) Segment Pitch (rise per 12)
- 24
- Upper (Shallow) Segment Pitch (rise per 12)
- 6
- Break Point (fraction of half-width, lower segment)
- 0.5
Intermediate steps
- Lower segment length
- 14.53 ft
- Upper segment length
- 7.27 ft
What this calculation does not cover
- The figure covers one side of the roof only, from wall to ridge, and it is the sum of two separate members rather than one stick of timber. Order and set out from the two breakdown lengths, never from the total: one stick cut to the total length fits nothing on the roof. The layout is assumed symmetrical, with the ridge on the centreline and the same pair of pitches on both sides.
- Nothing is added or taken off for the cuts. The run is half the overall building width, so the length is measured to the theoretical apex on the centreline with no deduction for half the ridge board thickness, and no allowance for the birdsmouth or seat cut, the plumb cut at the ridge, the bevel where the two pitches meet, or the tail beyond the plate. If the wall plate sits inboard of the outer wall face, the true run is shorter than half the overall width by that offset.
- The break point is entered as a fraction of the half-span and is independent of the two pitches you type, so the default 0.5 is a free choice rather than the traditional barn setting-out. The classic method inscribes half an octagon in a semicircle drawn on the span, which puts the break at roughly 0.29 of the half-span with segment slopes near 29/12 and 5/12. Keeping 0.5 while entering those pitches describes a different roof profile and a different ridge height.
- This is geometry with no structural content. It does not size the rafters, set their spacing, check the span against a load table, or account for snow and wind, and it says nothing about the break line, which behaves as a hinge and normally needs a purlin plate, a collar or a gusseted truss joint to hold the two segments in line.
- Only length is returned, never rise or angles. The ridge height above the plate, which governs headroom in the storey below and any height limit imposed on the building, is not part of the calculation, and neither are the cut angles: a 24/12 lower segment sits at 63.4 degrees from the horizontal and the timber changes direction by about 37 degrees at the break with the default pitches. The input ranges stop the lower pitch being shallower than the upper one but not equal to it, so entering 12 for both returns a plain common rafter length for a 45-degree gable roof.
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 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Standard rafter-length trigonometry applied independently to each of the gambrel roof's two slope segments: segment length = segment run × √(1+(segment pitch/12)²)
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Your workspace
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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.
Show the 5 tools this job needsHide tools
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.