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Gambrel Roof Calculator

Find ridge height and rafter lengths for a gambrel (barn-style) roof from building width and the lower and upper pitch angles.

Steeper pitch, closer to the wall.

Shallower pitch, near the ridge.

Result

24 ft

A 24 ft wide gambrel roof with an 18/12 lower pitch and 6/12 upper pitch has a ridge height of 24 ft

Quick Answer

A gambrel roof calculator splits the building width in half, then finds rise and rafter length for the lower and upper pitch separately. A 24 ft wide building with an 18/12 lower pitch and 6/12 upper pitch has a lower rafter of 21.63 ft, an upper rafter of 13.42 ft, and a total ridge height of 24 ft.

What A Gambrel Roof Calculator Measures For Barn-Style Framing

Two roof pitches meet at a single ridge in a gambrel design, and finding the ridge height means solving the same right-triangle method twice: once for the steeper lower pitch and once for the shallower upper pitch, each spanning half the building width. Rise for each segment equals segment run times pitch divided by 12, and rafter length is the hypotenuse of run and rise using the Pythagorean theorem. Total ridge height is the sum of both segments' rise. This geometry is standard for gambrel and barn-style roof framing, giving extra headroom in the upper story compared to a single-pitch gable roof of the same width.

How Gambrel Roof Ridge Height And Rafters Are Calculated

Segment run = building width ÷ 2. Rise = segment run × pitch ÷ 12 for each segment. Rafter length = √(run² + rise²). Ridge height = lower rise + upper rise.

  • Segment run = building width ÷ 2.
  • Lower rise = segment run × lower pitch ÷ 12.
  • Upper rise = segment run × upper pitch ÷ 12.
  • Rafter length (each segment) = √(segment run² + segment rise²).
  • Total ridge height = lower rise + upper rise.

Using This Gambrel Roof Calculator In Three Steps

Inputs

  • Building width: total width the roof spans, in feet.
  • Lower pitch: the steeper pitch near the wall, as rise per 12 inches of run.
  • Upper pitch: the shallower pitch near the ridge, as rise per 12 inches of run.

Steps

  1. Enter the total building width.
  2. Enter the lower (steeper) pitch as rise per 12 inches.
  3. Enter the upper (shallower) pitch as rise per 12 inches.
  4. Read ridge height and both rafter lengths.

Gambrel Roof Example With Each Step Shown

24 ft building width, 18/12 lower pitch, 6/12 upper pitch.

  1. Segment run: 24 ft ÷ 2 = 12 ft.
  2. Lower rise: 12 ft × 18 ÷ 12 = 18 ft.
  3. Upper rise: 12 ft × 6 ÷ 12 = 6 ft.
  4. Lower rafter: √(12² + 18²) = √468 = 21.63 ft.
  5. Upper rafter: √(12² + 6²) = √180 = 13.42 ft.
  6. Ridge height: 18 ft + 6 ft = 24 ft.

Ridge height of 24 ft, with a 21.63 ft lower rafter and a 13.42 ft upper rafter.

When A Gambrel Roof Estimate Fits A Barn Or Shed Framing Plan

Use the result for early framing layout on barns, sheds, and gambrel-style additions where extra upper-story headroom is the goal. Confirm final rafter sizing, connections, and load path with a framing plan or engineer before construction.

For nearby tools, the construction calculators hub lists every roofing and framing calculator, and the glossary defines common calculator terms.

Assumptions

  • Each pitch segment spans exactly half the building width.
  • Both roof sides are symmetric (same pitches on each side).
  • Overhang is not included in this base geometry.
  • Rafter length does not include ridge board or birdsmouth notch adjustments.

Limitations

  • Real gambrel designs can use unequal segment widths.
  • Overhang and ridge board thickness will add to real material length.
  • The tool does not check structural load, snow load, or code compliance.
  • Local building codes govern actual construction requirements.

In Practice

A common mistake is applying one pitch across the full building width, which is how a standard single-pitch rafter calculator works. A gambrel roof always has two distinct pitches, each covering half the width, so this tool runs the same rise-and-rafter-length method twice and adds the results for total ridge height.

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Related Guides

Frequently Asked Questions About Gambrel Roof Geometry

Why Does A Gambrel Roof Need Two Pitches Instead Of One?

A gambrel roof uses a steep lower pitch and a shallow upper pitch to maximize usable headroom in the upper story, which is why barns and similar structures use this shape instead of a single-pitch gable roof.

Does Each Pitch Segment Use The Same Run?

In this calculator, yes. Each segment spans exactly half the building width. Some custom gambrel designs use unequal segment widths, which would need a different run value for each pitch.

How Is Ridge Height Different From Rafter Length?

Ridge height is the total vertical rise from wall top to ridge, combining both segments' rise. Rafter length is the actual sloped board length for each segment, found using the Pythagorean theorem.

Does This Include Roof Overhang?

No, this is the base geometry from wall to ridge. Add overhang length separately using the same rafter length method with an extended run, matching a standard rafter calculator approach.

Sources

Last updated: . Reviewed for accuracy against the formula shown above.