Updated August 2026

Gambrel Roof Truss Calculator

A gambrel truss uses four sloping chords instead of two, a steep lower slope for headroom and a shallow upper slope that closes the peak. Set both pitches independently and get the break point, every chord length, both cut angles and the lumber take-off.

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gambrel truss angle calculator
gambrel roof design calculator
gambrel attic truss calculator

The basics

What is a Gambrel Truss?

The gambrel exists for one reason: usable volume. Everything else about it is a trade against that.

For a shed, the gambrel earns its keep purely on storage. A 12 ft by 16 ft gambrel shed gains roughly 60 ft² of loft floor over the same shed with a 6:12 gable roof. That is the whole reason the shape survives in the kit market.

For a barn, the gambrel is doing structural work as well. The break point acts as a natural node for the collar tie (opens in a new tab), and the near-vertical lower chords carry loft floor load down to the walls efficiently. Past roughly a 24 ft span the loft floor becomes a designed floor system with its own load path, and that is the point where a calculator stops being enough.

Loft volume Roughly 40 to 50% of the footprint becomes usable loft with about 6 ft of standing headroom on a 16 ft span.
Steeper lower slope More headroom and a taller loft wall, at the cost of a heavier truss and more sheathing waste on the steep face.
Shallower upper slope Less peak height and less material. Watch shingle warranties, since most require at least a 3:12 pitch.
Higher fabrication cost Around a third above a plain gable of the same span, because there are twice as many chord joints to make.

Loft floor load is a separate question

This calculator sizes the truss for roof loads. If you intend to store anything heavier than light boxes in the loft, the bottom chord and collar tie both need designing for that floor live load before you order.

The maths

Gambrel Truss Formula

Gambrel geometry

Run = Span ÷ 2

Break point X = Run × break fraction (0.5 is standard)

Break height = Break X × (Lower pitch ÷ 12)

Peak height = Break height + (Run - Break X) × (Upper pitch ÷ 12)

Lower chord = √(Break X² + Break height²)

Upper chord = √((Run - Break X)² + (Peak - Break height)²)

Add the heel height to every vertical figure to get the dimension above the wall plate, and multiply the horizontal overhang by the lower slope factor to get the tail length.

Method

How to Calculate a Gambrel Truss

A gambrel is two roofs stacked, so it is calculated in two passes: the lower slope from the wall up to the break point, then the upper slope from the break point to the peak.

1 Measure the span Out-to-out of your bearing walls. Gambrel shed kits usually land on 12 ft or 16 ft; barns run 24 ft to 40 ft.
2 Set both slopes The classic barn gambrel is a 20:12 lower slope with a 7:12 upper slope. Steeper lower slopes buy loft headroom; shallower upper slopes shed the peak faster.
3 Place the break point Half the run is standard. Moving it outward widens the loft floor; moving it inward raises the ceiling but narrows the usable room.
4 Read the cut list The diagram redraws live. Chord lengths, break-point height, peak height and both cut angles update with every keystroke.

Cut angles come straight from the two pitches. A 20:12 lower slope sits at 59.0 degrees, so the lumber is cut 31.0 degrees off square. A 7:12 upper slope sits at 30.3 degrees and is cut at 59.7 degrees.

The joint at the break point is where the two complementary angles meet, which is why gambrel trusses nearly always use a plywood gusset (opens in a new tab) there rather than a plated joint. The horizontal tie between the two break points carries the outward thrust the shape develops, and the calculator includes it in the take-off.

Worked example

A 16 ft gambrel at 20:12 over 7:12 with the break at half the run gives a 4 ft break point, a 6 ft 8 in break height and a 9 ft 0 in peak. That produces a 7 ft 9 in lower chord and a 4 ft 7 in upper chord on each side, before overhang. Confirm any figure you build from against local code and, where required, a licensed engineer.

Typical gambrel geometry using a 20:12 lower slope and a 7:12 upper slope, with the break point at half the run.

Common Gambrel Truss Dimensions
Span	Break point height	Peak height	Lower chord	Upper chord
12 ft	5' 0"	6' 9"	5' 10"	3' 5"
16 ft	6' 8"	9' 0"	7' 9"	4' 7"
20 ft	8' 4"	11' 3"	9' 9"	5' 9"
24 ft	10' 0"	13' 6"	11' 8"	6' 10"
32 ft	13' 4"	18' 0"	15' 7"	9' 2"
40 ft	16' 8"	22' 6"	19' 6"	11' 5"

Note: Chord lengths exclude overhang tails. Add the tail figure the calculator reports for each end.

The tool

Why Choose Our Gambrel Truss Calculator

Most online gambrel tools ask for one pitch and quietly assume the other. This one treats the two slopes as separate inputs, because on a gambrel they are.

Both slopes are yours to set Lower and upper pitch are independent inputs, so you can chase loft headroom without guessing what it does to the peak.
Break point you control Move the break in or out and watch the loft floor width and ceiling height trade against each other in real time.
A drawing, not just numbers The profile redraws to scale as you type, so a wrong pitch is obvious before you cut anything.
Cut list you can take to the saw Every chord, the collar tie and both cut angles, with the overhang tail worked out separately.

More tools

Calculators that pair with the gambrel tool, each running the same geometry engine.

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Answers

Frequently Asked Questions

Common questions about gambrel roof truss calculator.

What angle is a gambrel roof truss?

The traditional barn gambrel uses a 60 degree lower slope and a 30 degree upper slope, which is why the classic layout is drawn from a semicircle divided into four equal arcs. In pitch terms that is roughly 20:12 over 7:12. Modern gambrel sheds often soften this to 18:12 over 6:12 so the loft is easier to sheath.

How do I build a 12 ft gambrel truss?

Set the span to 12 ft and leave the break point at half the run. With a 20:12 over 7:12 layout you get a 5 ft 10 in lower chord and a 3 ft 5 in upper chord per side, a 6 ft 9 in peak and a 5 ft break-point height. Cut all eight chords, lay them out on a flat deck over a template, and gusset every joint on both faces.

How far can a gambrel truss span?

With 2x6 chords a gambrel truss comfortably spans 24 ft. With 2x8 chords and proper gussets you can reach 32 to 40 ft, which covers most pole barns. Beyond 40 ft you are into engineered territory with a sealed drawing and a crane set.

Do gambrel trusses need a collar tie?

Yes. The break points want to push outward under load, so a horizontal tie between them is what keeps the shape stable. The calculator draws this member and includes it in the take-off. In loft designs the tie is often replaced by the loft ceiling joists doing the same job.

Is a gambrel roof more expensive than a gable?

Per truss, yes, by roughly a third. Per square foot of usable space it is often cheaper, because the loft you gain would otherwise need a second storey. Where the roof is purely a roof and nobody will use the space, a gable is the better value.

Can I use this for a gambrel attic conversion?

For layout and material take-off, yes. For an occupied attic room you also need to confirm the bottom chord can carry floor live load, which a standard roof truss cannot. Use the attic truss calculator for room-in-roof geometry and the load calculator to check the floor.

References

Gambrel Truss Sources & Standards

Related calculators on this site, plus the standards and reference works the figures on this page are based on. External links open in a new tab.

Authoritative external references

Codes and standards are revised on their own cycles. Confirm the current edition adopted in your jurisdiction with your local building department before relying on any figure.