Construction

Gambrel Roof Calculator

Find rafter lengths and ridge height for a gambrel roof.

Building span

Lower section run

Lower pitch angle (degrees)

Upper pitch angle (degrees)

Calculated Output

Ridge height 1.443

Lower rafter length 2

Upper rafter length -0.5774

Formula: two pitched sections meeting at a break point

Step-by-Step Breakdown

1. Values used:

2. Building span = 1

3. Lower section run = 1

4. Lower pitch angle = 60 degrees

5. Upper pitch angle = 30 degrees

7. Ridge height = 1.443

8. Lower rafter length = Building span + Lower section run = 1 + 1 = 2

9. Upper rafter length = -0.5774

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The math behind it

Each section is a right triangle: rise = run × tan(angle); rafter = run ÷ cos(angle). Ridge height = sum of the two rises.

24 ft span, 8 ft lower run, 60°/30° → ridge ~16.2 ft.

Why gambrel?

It maximizes usable headroom in the attic/loft.