How do I calculate rafter length?

Rafter length equals the square root of (run squared plus rise squared), using the Pythagorean theorem. For a 12-foot run with a 6/12 pitch, the rise is 6 feet, so the rafter length is the square root of (144 + 36) which equals approximately 13.4 feet. Add the overhang length (typically 12 to 24 inches) to get the total rafter length for ordering lumber.

What size rafters for a 20-foot span?

For a 20-foot total span (10-foot run on each side of the ridge), 2x10 rafters at 16-inch on-center spacing are typically required in most snow load zones. In heavy snow areas (40+ PSF ground snow load), 2x12 may be necessary. Always verify with your local building department's span tables.

How many rafters do I need for my roof?

Divide the roof ridge length by the rafter spacing (16 or 24 inches) and add one, then multiply by two for both sides of the roof. A 40-foot ridge at 16-inch spacing needs ((40x12)/16 + 1) x 2 = 62 rafters. Add 10% extra for waste from cuts and any damaged boards.

What is a bird's mouth cut on a rafter?

A bird's mouth is a notch cut into the bottom edge of each rafter where it sits on the wall top plate. It consists of a horizontal seat cut (bearing on the plate) and a vertical plumb cut (against the outside of the wall). The seat cut should be no more than one-third the depth of the rafter to maintain structural integrity.

Do I need collar ties or ceiling joists with rafters?

Yes. In a traditional rafter roof, opposing rafters push outward on the walls. Collar ties (in the upper third of the rafter span) resist this spreading force. Ceiling joists or rafter ties at the wall plate level are even more effective. Without these horizontal ties, the walls will eventually bow outward. A structural ridge beam eliminates this requirement.

Rafters vs. trusses — which should I choose?

Choose rafters when you want an open attic, cathedral ceilings, or complex roof shapes. Choose trusses when you want faster installation, lower cost, and engineered load ratings. Trusses are 20-30% less expensive for simple gable roofs, but rafters offer more flexibility and do not require crane access for installation.

Rafter Calculator

Rafter calculator: Estimate rafter count by roof length & spacing. Free tool for 400 mm and 600 mm centres — works for gable and hip roofs.

Ad — 728x90 — Leaderboard

Enter Your Measurements

How to Use This Calculator

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

Ad — 728x90 — Mid-Page

About This Material

Rafters are the sloped structural lumber members that form the roof frame in stick-built (site-framed) construction. Unlike factory-built trusses, rafters are individually cut and installed on site, allowing for open attic space, cathedral ceilings, or unique roof geometries. Common rafter sizes are 2x6 (spans up to 10 feet), 2x8 (up to 13 feet), 2x10 (up to 16 feet), and 2x12 (up to 20 feet) at 16-inch on-center spacing, depending on the lumber species, grade, and local snow or wind loads. Each rafter requires a bird's mouth cut where it sits on the wall top plate and a plumb cut at the ridge. Rafters are typically paired on opposite sides of a ridge board and connected with collar ties or ceiling joists to prevent the walls from spreading outward. Rafter lumber costs $8 to $25 per board depending on dimension and length. Stick framing with rafters is more labor-intensive than trusses but offers greater design flexibility.

Next Steps:

Ridge Board Calculator →Roof Sheathing Calculator →

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.

Rafter Calculator

How to Use This Calculator

About This Material

Installation Tips

Common Mistakes to Avoid

Frequently Asked Questions

Related Calculators

Roof Truss Calculator

Ridge Board Calculator

Roof Sheathing Calculator

Shingle Calculator