Gambrel Roof Guide: Angles, Rafter Lengths and Truss Count

Worked Example for a 24 x 40 ft Barn Roof

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A gambrel roof has a steep lower slope and a shallow upper slope on each side. This guide works the geometry for a 24 ft by 40 ft building with an 18/12 lower pitch, a 4/12 upper pitch and a 67% break: 8.04 ft and 3.96 ft of run, a 13.38 ft ridge height, 14.49 ft and 4.17 ft rafters, about 1,493 sq ft of roof and 21 trusses at 24 inches on centre, plus how to flash and ventilate the break.

A gambrel roof has two slopes on each side: a steep lower panel and a shallow upper panel. Every layout comes down to four inputs, the span, the lower pitch, the upper pitch and the break ratio, and the same arithmetic gives the ridge height, the rafter lengths, the roof area and the truss count.

The four numbers that define a gambrel

Span: the wall-to-wall width. Half of it is the run from eave to ridge.
Lower pitch: the steep panel, commonly 12/12 to 21/12. An 18/12 lower slope is 56.3 degrees.
Upper pitch: the shallow panel, commonly 3/12 to 6/12. A 4/12 upper slope is 18.4 degrees.
Break ratio: where the slope changes, as a share of the half-width. At 67% the lower slope covers two thirds of the run.

Worked example: a 24 x 40 ft barn roof with an 18/12 lower pitch, 4/12 upper pitch and 67% break

Gambrel roof geometry for a 24 x 40 ft building, overhang ignored
Step	Working	Result
Half-width	24 / 2	12 ft
Lower run / upper run	12 x 0.67 / 12 - 8.04	8.04 ft / 3.96 ft
Rise below / above the break	8.04 x 18/12 / 3.96 x 4/12	12.06 ft / 1.32 ft
Ridge height above the eave line	12.06 + 1.32	13.38 ft
Lower rafter / upper rafter	8.04 x 1.803 / 3.96 x 1.054	14.49 ft / 4.17 ft
Roof area	(14.49 + 4.17) x 2 x 40	about 1,493 sq ft (14.9 squares)

Counting trusses

At 24 inches on centre a 40 ft building needs 40 x 12 / 24 + 1 = 21 trusses; at 16 inches on centre it needs 31. Gambrel trusses are normally ordered from a truss plant to an engineer's stamped drawing, because the knuckle carries load differently from a plain triangle. Confirm the spacing before comparing quotes.

Where a gambrel roof fails

The break line: water running down the shallow upper panel hits the steep lower panel at speed, so run self-adhered membrane over the break in cold climates.
The steep lower panel: shingles can slip while you work and wind loads the panel hard, so follow the manufacturer's high-wind nailing pattern.
The rakes: long and bent, so plan drip edge joints to land on the break.
Ventilation: a gambrel loft holds a lot of air, so pair ridge ventilation with continuous soffit intake.

Add 10 to 15 percent waste for the break line, ridge and rakes: at 15 percent the example needs about 17.2 squares, or 52 bundles at three bundles per square.

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