🏚️ Gambrel Roof Calculator — Barn Roof Area & Trusses
Free gambrel truss calculator — estimate gambrel roof area, trusses, and materials
Smart Roofing Calculator — Gambrel Roof Calculator — Barn Roof Area & Trusses — smartroofingcalculator.com/barn-roof-calculator/ — printed
Steep lower portion, typically 18:12
Shallow upper portion, typically 4:12
% of half-width for lower slope (67% typical)
24" typical
🛒 Materials for this job
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Calculate above and your quantities fill in here.
🛠️ How This Calculator Works
Enter building dimensions and the two pitch values (steep lower slope, shallow upper slope). Set the break ratio to define where the slope change occurs. The calculator computes each roof plane area separately and sums them for total surface area, squares, and truss count.
📐 Formula
Lower Panel = Length × (Break Width ÷ cos(Lower Angle)) × 2 sides | Upper Panel = Length × (Remaining Width ÷ cos(Upper Angle)) × 2 sides📝 Example Calculation
For a 30 × 50 ft barn with 18:12 lower / 4:12 upper, 67% break: • Half-width: 15 ft • Lower run: 15 × 0.67 = 10.05 ft • Upper run: 15 × 0.33 = 4.95 ft • Total roof area: ~1,850 sq ft • Squares: ~18.5 | Bundles: ~56
💡 Pro Tips from Roofers
- Gambrel trusses are always custom — get engineering drawings before ordering.
- The break point is the most vulnerable area — use extra flashing and sealant there.
- Steeper lower slopes mean more wind uplift risk — use 6-nail pattern below the break.
- Consider a cupola at the ridge for natural ventilation in agricultural buildings.
❓ Frequently Asked Questions
What is a gambrel roof?
A gambrel (barn) roof has two slopes on each side — a steep lower slope and a shallow upper slope. This design maximizes usable attic/loft space while shedding water effectively. Common on barns, Colonial homes, and Dutch Colonial architecture.
How do I calculate gambrel roof area?
Calculate each of the four roof planes separately (two steep lower panels and two shallow upper panels), then sum them. Each panel is a rectangle: length × slope-adjusted width. The break point determines where the slope changes.
What is the break ratio on a gambrel roof?
Break ratio is the percentage of the half-width where the lower (steep) slope transitions to the upper (shallow) slope. 67% is typical — meaning the lower slope covers 2/3 of the distance from eave to ridge, and the upper slope covers the remaining 1/3.
How do I calculate the area of a gambrel roof?
Work out the sloped length of the lower and upper segments separately from their run and pitch, add them to get one side's eave-to-ridge length, add the overhang, then multiply by the roof length and by two for both sides.
What are the standard gambrel roof pitches?
A common barn layout is 18/12 or steeper on the lower slope and 4/12 to 6/12 on the upper slope. The classic equal-segment gambrel is about 29/12 lower and 5/12 upper (67.5 and 22.5 degrees).
How many gambrel trusses do I need?
Divide the building length in inches by the truss spacing and add one. A 50 ft barn at 24 inches on centre needs 26 trusses; at 16 inches it needs 39.
How tall is a gambrel roof?
Add the rise of the two slopes. A 30 ft span at 18/12 and 4/12 with the break at 67% rises 16.7 ft above the wall plate.
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About the Smart Calculator Team
We build free, accurate construction calculators for homeowners and contractors. This article is part of our roofing knowledge base — we also publish guides and tools for other home improvement projects:
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Worked examples updated . Every figure below is computed with the same formulas as the calculator.
Gambrel roof geometry, drawn to scale
A gambrel is two roofs stacked on each side. The lower slope is steep enough to act almost like a wall, which is what opens up the loft; the upper slope is shallow and does most of the shedding. Every quantity on a gambrel — area, rafter length, ridge height — is the sum of the two slopes calculated separately, each with its own pitch.
Gambrel roof calculation, step by step
Worked example: A 30 x 50 ft barn, 18/12 lower, 4/12 upper, 67% break, 12-inch overhang, trusses at 24 in
- Half the span is 15 ft. The lower slope covers 67% of it: 10.05 ft of run. The upper slope covers the rest: 4.95 ft.
- Lower rise = 10.05 x 18/12 = 15.08 ft. Upper rise = 4.95 x 4/12 = 1.65 ft. Ridge height above the wall plate: 16.73 ft.
- Lower slope length = √(10.05² + 15.08²) = 18.12 ft. Upper slope length = √(4.95² + 1.65²) = 5.22 ft.
- One side from eave to ridge: 23.34 ft, plus 1 ft of overhang. Both sides x (50 + 1) ft of length = 2,482 sq ft.
- 24.8 squares, about 75 bundles of shingles before waste, and (50 x 12 ÷ 24) + 1 = 26 trusses.
2,482 sq ft of roof and 26 gambrel trusses. Add 10% waste to the covering, and flash the break line on both sides — it is where gambrels leak.
Gambrel roof area and truss count by barn size
| Barn (span x length) | Roof area | Squares | Ridge height | Trusses |
|---|---|---|---|---|
| 24 x 32 ft | 1,298 sq ft | 13 | 13.4 ft | 17 |
| 30 x 40 ft | 1,996 sq ft | 20 | 16.7 ft | 21 |
| 36 x 48 ft | 2,842 sq ft | 28.4 | 20.1 ft | 25 |
| 40 x 60 ft | 3,918 sq ft | 39.2 | 22.3 ft | 31 |
The classic "semicircle" gambrel, where all four roof segments are the same length, works out to a lower pitch of about 29/12 (67.5°) and an upper pitch of about 5/12 (22.5°), with the break halfway along the half-span. Enter those in the calculator with a 50% break ratio to see its area and height.
Gambrel trusses are engineered to order: the table gives the count and geometry to quote from, not a truss design. For a common gable barn, use the roof truss calculator; for rafter lengths on each slope, the rafter length calculator; and for metal panels on a pole barn, the metal roof calculator.
Gambrel Truss Calculator — How It Works
A gambrel truss is the engineered framing member that gives a barn (or Dutch Colonial) its iconic two-pitch profile. This gambrel truss calculator takes the building width, length, the two pitch values, and the truss spacing — then returns the truss count, ridge height, and total roof surface area for both the steep lower panels and the shallow upper panels.
Standard Gambrel Truss Geometry
- Lower (steep) pitch: typically 18/12 to 24/12 — handles the wall-like lower panel.
- Upper (shallow) pitch: typically 4/12 to 6/12 — forms the roof-like upper panel.
- Break point: usually at 60–70% of the half-width (67% is the classic value).
- Truss spacing: 24" on-center is standard; 16" OC for heavier snow/wind loads.
How many gambrel trusses do I need?
Truss count = (Building Length ÷ Spacing in feet) + 1. For a 50 ft long barn with 24" OC trusses: (50 ÷ 2) + 1 = 26 trusses. Always order 1–2 spares — gambrel trusses are custom-built and lead times can run 2–4 weeks.
Counting fasteners? The metal roof screw calculator lays out the screw pattern for 9 in rib and PBR panels.
Steel roofing over purlins? The purlin spacing calculator gives the rows and boards at 24 in on centre.
Two pitches on all four sides? The mansard roof calculator works out both slopes, the hips, the curb and the bundles.
One long slope and one short? The saltbox roof calculator works each side separately and gives the back wall drop.
Want the geometry behind the result? The gambrel roof guide works a 24 x 40 ft barn by hand: rafter lengths, ridge height and truss count.
🔗 Related Roofing Calculators
- Roof Truss Calculator — Standard gable & common truss counts
- Metal Roof Calculator — Most pole barns use metal panels
- Shingles Calculator — Bundle count for shingled barn roofs
- Roof Pitch Calculator — Run pitch factor for both gambrel slopes
· Reviewed by the Smart Roofing Calculator editorial team