What is a Gambrel Roof?
A gambrel roof is a classic roofing style divided into two sections per side: a lower section with a steeper pitch and an upper section with a shallower pitch. This distinctive silhouette is instantly recognizable on barns, farmhouses, and Dutch Colonial homes. Compared to a simple gable roof, the gambrel design maximizes the usable attic volume beneath the ridge while keeping the eave line low.
How to Calculate Gambrel Roof Angles and Dimensions — Two-Pitch Method
The two-pitch method is the most common approach for sizing a gambrel roof. You define each section by its own pitch, then calculate where the two rafter planes meet (the knee or break point).
Key Parameters
- Span (W) — full width of the building from eave to eave.
- Half-span (W/2) — the horizontal run available to both roof sections on one side.
- Lower section run (r₁) — horizontal distance from the eave to the knee; expressed here as a percentage of the half-span.
- Upper section run (r₂ = W/2 − r₁) — horizontal distance from the knee to the ridge center.
- Lower pitch — steepness of the lower section (rise per 12 in Imperial, or degrees in Metric).
- Upper pitch — steepness of the upper section (typically much gentler).
Step-by-Step Calculations
- Knee height = r₁ × tan(lower pitch angle)
- Upper rise = r₂ × tan(upper pitch angle)
- Total ridge height = knee height + upper rise
- Lower rafter length = √(r₁² + knee height²)
- Upper rafter length = √(r₂² + upper rise²)
- Total roof area = 2 × building length × (lower rafter + upper rafter)
The Half-Circle Method
The half-circle method is a traditional carpenter's layout technique. A semicircle is drawn with a radius equal to the half-span. By inscribing both rafter lines inside this semicircle, the break point is located geometrically without measuring. The lower rafter is tangent to the horizontal base, and the upper rafter connects the break point to the ridge. This method is especially useful when laying out directly on a framing floor or with a framing square.
How to Use This Gambrel Roof Calculator
- Choose your unit system: Imperial (feet) or Metric (meters).
- Enter the building width (full span, eave to eave) and building length.
- Set the lower section pitch (the steeper angle, e.g., 18 : 12 or 60°).
- Set the upper section pitch (the shallower angle, e.g., 6 : 12 or 27°).
- Adjust the lower section width — what percentage of the half-span the lower section covers (default 60%). A higher value means the knee is positioned further from the eave.
- Optionally enter a material cost per sq ft / sq m and select your currency to get a cost estimate.
- Click Calculate to see ridge height, knee height, rafter lengths, roof areas (with squares for Imperial), gable area, and total material cost.
Frequently Asked Questions
What pitch is typical for a gambrel roof?
A traditional barn gambrel uses a lower pitch of 60° (≈ 20 : 12) and an upper pitch of 30° (≈ 7 : 12). Residential Dutch Colonial styles often use 12 : 12 lower and 4 : 12 upper. The lower section is always steeper than the upper.
What is a "roofing square"?
In the United States, roofing materials are quoted by the square, which equals 100 square feet of roof surface. A 2,400 sq ft roof is 24 squares. The calculator shows the number of squares for Imperial units.
How much does a gambrel roof cost?
Material costs vary widely by region and product. Common roofing materials range from $2–5 per sq ft for asphalt shingles to $10–20 per sq ft for metal roofing. Enter your local price in the material cost field and select your currency to get an instant estimate.
Does the calculator include the gable ends?
Yes. The calculator outputs the total gable area for both gable ends combined. This is useful when estimating siding or sheathing for the triangular/polygonal end walls.
What is the difference between Imperial and Metric mode?
In Imperial mode, dimensions are in feet and pitches are expressed as "rise per 12 run" (e.g., 18 : 12). Results are in sq ft and roofing squares. In Metric mode, dimensions are in meters and pitches are in degrees. Results are in sq m.