Gambrel Roof Calculator

Calculate dimensions, area, and materials for your gambrel roof project

What is a Gambrel Roof?

A gambrel roof is a symmetrical two-sided roof with two slopes on each side. The upper slope is positioned at a shallow angle, while the lower slope is steep. This design provides the advantages of a sloped roof while maximizing headroom inside the building, which is why it’s commonly used for barns, sheds, and some colonial-style houses.

Our free interactive gambrel roof calculator helps you determine key dimensions, calculate roof area, estimate rafter lengths, and plan materials for your construction project. Whether you’re building a barn, storage shed, or unique home, this tool provides accurate calculations based on your specific measurements.

Gambrel Roof Calculation Methods

Two-Pitch Method

This is the most common approach where you specify separate pitch angles for the upper and lower roof sections. The calculator then determines the transition point where these two slopes meet, calculating all dimensions based on your building width and the specified pitches.

When to use: When you have specific pitch requirements or want maximum design flexibility.

Half-Circle Method

This traditional method creates a gambrel roof that follows the arc of a half-circle. The roof’s profile forms a perfect semicircle, with the pitches automatically determined by this geometric constraint. This method often results in a classic barn appearance.

When to use: When you want a traditional barn look with predetermined proportions.

How to Use This Calculator

  1. Select your measurement system: Choose between Imperial (feet, inches) or Metric (meters) units using the toggle.
  2. Enter basic dimensions: Input your building width, length, and overhang measurements.
  3. Set roof pitches: Specify the upper and lower roof pitches as ratios (like 8/12) or angles in degrees.
  4. Review results: The calculator will instantly display all key dimensions, areas, and rafter lengths.
  5. Use advanced options: Switch to Advanced mode for additional calculations like material estimates.
  6. Save or share: Download your results as PDF or share them via a unique link.

Input Parameters

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The total width of your building from outside to outside of the walls
Width must be greater than 0
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The length of your building. Required for area and material calculations.
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The horizontal distance the roof extends beyond the wall at the eaves
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The horizontal distance the roof extends beyond the gable end walls
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The slope of the upper roof section as a ratio (e.g., 8/12) or angle in degrees
Invalid pitch format. Use ratio (8/12) or angle (30°)
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The slope of the lower roof section as a ratio (e.g., 12/12) or angle in degrees
Invalid pitch format. Use ratio (12/12) or angle (45°)

Advanced Options

Calculation Results

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Total Roof Height
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Upper Run
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Lower Run
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Upper Rise
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Lower Rise
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Upper Rafter Length
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Lower Rafter Length
0
Upper Roof Area
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Lower Roof Area
0
Total Roof Area
0
Attic Volume

Roof Diagram

Frequently Asked Questions

What is a gambrel roof?
A gambrel roof is a symmetrical two-sided roof with two slopes on each side. The upper slope is positioned at a shallow angle, while the lower slope is steep. This design provides the advantages of a sloped roof while maximizing headroom inside the building.
What are the advantages of a gambrel roof?
Gambrel roofs offer several advantages: they provide more usable space in the attic or upper level, allow for efficient water drainage, are cost-effective to build compared to other complex roof designs, and have a classic aesthetic appeal particularly suited for barns and colonial-style homes.
How do I measure roof pitch?
Roof pitch is typically expressed as a ratio of vertical rise to horizontal run. For example, a 6/12 pitch means the roof rises 6 inches for every 12 inches of horizontal run. You can measure this using a level and tape measure, or use a smartphone app with an inclinometer function.
What is the difference between the Two-Pitch Method and Half-Circle Method?
The Two-Pitch Method allows you to specify separate pitch angles for the upper and lower roof sections, offering design flexibility. The Half-Circle Method creates a roof that follows the arc of a half-circle, resulting in predetermined proportions that create a classic barn appearance.
How accurate are the calculations?
Our calculator provides precise mathematical calculations based on the inputs you provide. However, actual construction should include appropriate safety factors and professional consultation, especially for structural elements. The results are suitable for planning and estimation purposes.