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Roofing & Siding

Gambrel Roof Calculator

Rafter lengths, rise and roof area for a gambrel — the two-slope barn roof — where each side has a steep lower run and a shallow upper one.

Your measurements

The two runs together make up half the span, so setting one fixes the other.

Span and slopes
Where the slope breaks, measured horizontally from the wall. Must be less than half the span.
The steep lower slope. A gambrel typically runs 18:12 to 24:12 here.
Allowance

Your result

Sample values

1,157

Roof area, ft²

12.00

Total rise, ft

10.82

Lower rafter, ft

6.71

Upper rafter, ft

24 ft span, 6 ft lower run at 18:12, upper at 6:12, 30 ft long, 10% waste.

Geometry summary

Measure Value
Half span 12.00 ft
Lower run / rise 6.00 ft / 9.00 ft
Upper run / rise 6.00 ft / 3.00 ft
Lower rafter, per side 10.82 ft
Upper rafter, per side 6.71 ft

How to use this calculator

Enter the full span, then the horizontal run of the lower slope measured from the wall to where the pitch breaks. The upper run is whatever is left of the half span, so you only set one of them.

Give each slope its own pitch. That is the whole point of a gambrel: a steep lower slope buys headroom in the loft, and a shallow upper slope keeps the ridge from getting absurdly high. Treating it as one average pitch gets both the rise and the area wrong.

Formula

Upper run = (Span ÷ 2) − Lower run

Lower rise = Lower run × (Lower pitch ÷ 12)

Upper rise = Upper run × (Upper pitch ÷ 12)

Rafter = √(Run² + Rise²) for each slope

Area = 2 × (Lower rafter + Upper rafter) × Length × (1 + Waste)

Total rise
Lower rise + Upper rise, from wall top to ridge
2 ×
both sides of the roof

Each slope is measured separately. A gambrel is not a pitched roof with an average slope.

Worked example

A 24 ft span, 6 ft lower run at 18:12, upper slope at 6:12, 30 ft long, 10% waste.

  1. Half span: 24 ÷ 2 = 12 ft; upper run 12 − 6 = 6 ft
  2. Lower rise: 6 × (18 ÷ 12) = 9.00 ft
  3. Upper rise: 6 × (6 ÷ 12) = 3.00 ft, total rise 12.00 ft
  4. Lower rafter: √(6² + 9²) = 10.82 ft
  5. Upper rafter: √(6² + 3²) = 6.71 ft
  6. Area: 2 × (10.82 + 6.71) × 30 × 1.10 = 1,157 ft²

1,157 ft² of roof, 12.00 ft total rise

Understanding your results

Compare the result against a single pitched roof over the same span and the reason for the shape becomes obvious. A gambrel puts nearly all its height into the first few feet from the wall, so the loft has usable floor area right out to the edges instead of a triangle you cannot stand in.

The trade is surface area. Two slopes per side mean more roof to cover than a simple gable over the same footprint, so material cost rises even though the ridge sits lower.

Questions

About this calculator

Ask something else

The lower slope is usually between 18:12 and 24:12 and the upper between 4:12 and 8:12. The classic barn proportion puts both break points on a semicircle drawn across the span, which lands close to 24:12 over 6:12 on a 24 ft span.

Because rise and rafter length are not linear in pitch. Averaging 18:12 and 6:12 to 12:12 gives a 12 ft rise by coincidence here but the rafter lengths and the area are both wrong, and on other combinations the rise is wrong too.

It is a design decision, not a fixed value. It sets where the slope breaks and therefore how much full-height loft space you get. Moving it out gives more headroom near the walls and a heavier looking roof.

No. A mansard breaks slope on all four sides, so it has hips and a different area calculation. This assumes a gambrel running the length of the building with gable ends.

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Estimates, not specifications. Results are based on the values you enter and on typical material properties. Densities, coverage rates and product sizes vary between suppliers, so confirm quantities against your supplier's data before ordering. Structural work must be signed off by a qualified professional and meet your local building code.