Calculators / Sheds, pergolas and gazebos
What size rafters do I need?
Put in your snow load, lumber and span. You get the smallest rafter from the building code's span tables, and every size side by side.
Updated · How we check our numbers
Rafter size
2x6 at 24 in
For 30 psf ground snow load, Southern pine #2 rafters at 24 in on center: a 2x6 spans 9' 2", enough for your 8 ft.
- Your span
- 8 fthorizontal
- Rafter
- 2x624 in o.c.
- Load
- 30 psf snow10 psf dead
Estimate for planning only, not professional advice. Check the manufacturer’s instructions, your local code and a licensed professional before you buy, build or install. Use at your own risk. Terms.
Every size
IRC R802.4.1, L/180- 2x4up to 6' 1"Too short
- 2x6up to 9' 2"Smallest that works
- 2x8up to 11' 7"Works
- 2x10up to 13' 9"Works
- 2x12up to 16' 2"Works
The span is measured level, from the support to the ridge (or the other support), not along the slope of the rafter.
Rafters push the walls or beams outward. Without ceiling joists, the IRC needs rafter ties or a structural ridge beam, and the table spans assume one or the other.
A roofed structure usually needs a permit over 200 sq ft, and some towns want one for any size. They will ask for rafter size, spacing, species and grade.
How we work it out
pick the smallest rafter whose table span ≥ your span, measured level
The spans are the International Residential Code's Table R802.4.1, for common species and grades at 12 to 24 in spacing, as published by Greene County, Georgia's building department: roof live load 20 psf, or ground snow load 30, 50 or 70 psf, with 10 or 20 psf of dead load. Open roofs like gazebos use the tables for rafters without a ceiling (deflection L/180).
Above 70 psf of ground snow the IRC's tables don't apply and the roof needs a design professional. Your building department sets the snow load for your address; Minnesota, for example, uses 50 or 60 psf by county.
Where the numbers come from
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