Roof Rafter Calculator
Enter your building span, roof pitch, and rafter spacing to get the sloped rafter length (including overhang), number of rafters, ridge board length, and lumber cost. Uses the Pythagorean formula adjusted for pitch multiplier.
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Top Pick2×8 Framing Lumber — Home Depotfrom $12.98/board* Affiliate links — we may earn a commission at no cost to you.
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How to Use This Calculator
- 1Enter your measurements in the fields above. Use the units shown next to each field (ft, in, etc.).
- 2Click Calculate to get your material estimate instantly.
- 3Review the highlighted result — it shows the key quantity you need to order.
- 4Use Copy Results to save your estimate, or bookmark the URL — your inputs are encoded in the link.
Pro tip: Always add 10% to your material order to account for waste, cuts, and defects.
Frequently Asked Questions
How long should rafters be for a 28-foot wide house?
A 28-foot span means each rafter covers a 14-foot run. At a 6/12 pitch, the rafter length is √(14² + 7²) ≈ 15.65 ft plus overhang. You'd buy 16-foot boards and cut to length.
What size lumber do I need for roof rafters?
Rafter size depends on span and spacing. For 16" OC spacing: 2×6 up to 13 ft, 2×8 up to 17 ft, 2×10 up to 21 ft, 2×12 up to 25 ft (Douglas fir #2). Always verify against your local span tables.