Gambrel Roof Calculator

Calculate rafter lengths, ridge height, and roof surface area for a gambrel (barn-style) two-slope roof from building width and slope angles or pitches.

Building Dimensions

Slope Specification

Specify by:
Lower Slope (steeper)
in 12
Upper Slope (shallower)
in 12

How It Works

  1. 1 The half-span is half the building width. Each slope covers half of the half-span horizontally (standard 50/50 gambrel split).
  2. 2 Lower rafter length = horizontal run ÷ cos(lower angle).
  3. 3 Upper rafter length = horizontal run ÷ cos(upper angle).
  4. 4 Ridge height = lower rise + upper rise, where rise = run × tan(angle) for each section.
  5. 5 Roof surface area = 2 sides × building length × (lower rafter length + upper rafter length).

Summary

Calculate rafter lengths, ridge height, and roof surface area for a gambrel (barn-style) two-slope roof from building width and slope angles or pitches.

How it works

  1. Enter the building width (eave to eave) and building length.
  2. Choose whether to specify each slope by pitch (X-in-12) or by angle in degrees.
  3. Enter the lower slope pitch or angle — the steeper, wider bottom section.
  4. Enter the upper slope pitch or angle — the shallower, narrower top section.
  5. Click Calculate to see lower rafter length, upper rafter length, ridge height, and total surface area.
  6. Adjust the inputs to compare different slope combinations.

Use cases

Frequently Asked Questions

Last updated: 2026-05-23 · Reviewed by Nham Vu