Gambrel Roof Calculator
ConstructionCalculate total roof height and upper and lower rafter lengths for a gambrel (barn-style) roof from building width and each slope's pitch.
Total Roof Height (ft)
32.7
Lower Rafter Length (ft)
21.416
Upper Rafter Length (ft)
14.062
What is a Gambrel Roof?
A Gambrel Roof Calculator determines the total roof height and the rafter lengths for both the steep lower slope and shallower upper slope of a barn-style gambrel roof, based on your building's width and each slope's pitch. Unlike a simple gable roof with one continuous pitch, a gambrel roof's two-slope geometry requires calculating each segment separately before summing them for total height.
This distinctive roof shape is common on barns, storage sheds, and some house styles specifically because it maximizes usable interior loft space for a given building footprint. Pair this calculator with the Birdsmouth Cut Calculator for the rafter-to-wall notch dimensions, or the Metal Roof Cost Calculator if you're budgeting roofing material.
How to use this Gambrel Roof calculator
- Enter your Building Width in feet.
- Select the Lower Slope Pitch (rise per 12 in run) for the steep section near the eaves.
- Select the Upper Slope Pitch for the shallower section near the ridge.
- Adjust the Lower Slope Run Fraction to control how much of the building's half-width the steep lower slope covers.
- Review the Total Roof Height, Lower Rafter Length, and Upper Rafter Length results to plan your framing material and confirm the design fits your height requirements.
Formula & Methodology
The calculator splits the building's half-width between two slopes and computes each segment's rise and rafter length: Half Width = Building Width ÷ 2 Lower Run = Half Width × Lower Run Fraction; Upper Run = Half Width − Lower Run Lower Rise = Lower Run × (Lower Slope Pitch ÷ 12); Upper Rise = Upper Run × (Upper Slope Pitch ÷ 12) Total Roof Height = Lower Rise + Upper Rise Lower Rafter Length = √(Lower Run² + Lower Rise²); Upper Rafter Length = √(Upper Run² + Upper Rise²) Worked example: For a 24 ft wide building with a 60/12 lower pitch, 18/12 upper pitch, and 0.35 run fraction: Half Width = 24 ÷ 2 = 12 ft Lower Run = 12 × 0.35 = 4.2 ft; Upper Run = 12 − 4.2 = 7.8 ft Lower Rise = 4.2 × (60 ÷ 12) = 4.2 × 5 = 21 ft; Upper Rise = 7.8 × (18 ÷ 12) = 7.8 × 1.5 = 11.7 ft Total Roof Height = 21 + 11.7 = 32.7 ft Lower Rafter Length = √(4.2² + 21²) ≈ √(17.64 + 441) ≈ 21.4 ft Upper Rafter Length = √(7.8² + 11.7²) ≈ √(60.84 + 136.89) ≈ 14.1 ft
Frequently Asked Questions
A gambrel roof, also known as a barn-style or Dutch barn roof, has two slopes on each side — a steep lower slope near the eaves and a shallower upper slope near the ridge — which maximizes interior loft or attic space compared to a simple gable roof. It's the classic shape seen on traditional American barns and many storage sheds.
The run fraction determines how much of the building's half-width is covered by the steeper lower slope versus the shallower upper slope. A larger fraction (like 0.4-0.5) shifts more of the roof width to the steep lower slope, maximizing interior headroom near the walls, while a smaller fraction shifts more width to the flatter upper slope.
The steep lower slope sheds water and snow efficiently while creating vertical wall space that maximizes usable interior loft area, and the shallower upper slope reduces overall roof height and material compared to continuing the steep pitch all the way to the ridge. This two-pitch design is what gives barns their distinctive silhouette and generous hayloft space.
Lower slopes on gambrel roofs are often quite steep, commonly in the range of 45/12 to 72/12 (well beyond typical house roof pitches), since the steepness is what creates the vertical interior wall space characteristic of barn lofts.
A wider building requires more horizontal run for both slopes to cover, which directly increases the rise (and therefore total roof height) for any given pitch — doubling the building width roughly doubles the total roof height for the same pitch and run fraction.
Yes, gambrel roofs are popular for storage sheds specifically because the barn shape maximizes interior storage headroom for a given footprint — this calculator works the same way regardless of whether you're framing a full barn or a smaller shed.
The Lower Rafter Length and Upper Rafter Length results tell you the minimum stock length needed for each rafter type before accounting for any birdsmouth notch or overhang — see the [Birdsmouth Cut Calculator](/birdsmouth-cut-calculator/) for the wall connection notch dimensions.
No, this calculator computes the core rafter lengths based on the roof's structural geometry; add extra length separately for any eave overhang beyond the wall line, similar to how the [Rafter Length Calculator](/rafter-length-calculator/) handles overhang for simple gable roofs.
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