How do I calculate rafter length?

Rafter length = √(run² + rise²). Use the Pythagorean theorem. For a 12-foot run with a 6/12 pitch: rise = 6 ft, length = √(144+36) = ~13.4 ft.

What size rafters for a 20-foot span?

For a 20-foot span (10-foot run each side) at 16" OC in most snow zones, a 2x10 is typically required. Always check your local span tables.

Rafter Calculator

Calculate the number of rafters needed for your roof based on length and spacing.

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Enter Your Measurements

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How to Use This Calculator

1Measure total roof length in feet.
2Select spacing.
3Click Calculate — result includes rafters for both sides of the roof.

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About This Material

Rafters are the sloped structural members that form the roof frame. Unlike trusses, stick-built rafters allow for an open attic or cathedral ceiling. Common sizes: 2x6, 2x8, 2x10 depending on span and snow load.

Installation Tips

•Cut a test rafter and verify fit before cutting the full set.
•Use a speed square to mark consistent bird's mouth cuts.
•Collar ties or ridge beam required to prevent roof spread.

Common Mistakes to Avoid

Measuring horizontal span instead of rafter run for lumber ordering.
Not accounting for roof overhang in rafter length.
Inconsistent bird's mouth cuts causing uneven eaves.