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Rafter Span Calculator

Builders Supply · Project Calculators

Rafter Span Calculator

Tell us how far your rafters need to span and the load they’ll carry. We’ll show the smallest lumber size that does the job in each grade we stock — so you buy right the first time.

For reference & budgeting only — not an engineered design.
Final sizes must be confirmed by a licensed engineer before you build or pull a permit. Builders Supply can get your design stamped and supply the material — just ask.

Spans computed from American Wood Council span equations using NDS® 2018 design values.

Sizing based on AWC (American Wood Council) published span tables and NDS design values.

This sizes

  • Sloped roof rafters in 2×8, 2×10, 2×12
  • A single straight run — supported at both ends (wall to ridge, or wall to wall)
  • Rafters spaced 24″ on center or less
  • Snow + roof weight, dry/covered conditions
  • Doug Fir-Larch #2, #1, and MSR — the grades we stock

Talk to us about

  • The ridge beam or any beam holding up the rafters — that’s a different size (ask about glulam / LVL)
  • Floor or ceiling joists — different limits, ask at the counter
  • Valleys, hips, dormers, or rafters with a midpoint support
  • Anything that needs a permit stamp — we’ll route it to our engineer
  • Spans no 2×12 can reach — we’ll spec an engineered beam
Not sure if your roof fits? That’s what we’re here for — bring your plans to any location or request a stamped design below, and we’ll get you the right material.

Your roof

ft
in
Measured flat (horizontal projection), support to support.
rafter board SPAN = wall to ridge
Span The flat, level distance your rafter crosses — what you enter here. Always less than the angled board.
Spacing How far apart the rafters sit, center to center (12″–24″).
psf
Design roof snow (not ground).
Sets the deflection limit (L/240 vs L/180).

Need it engineered or stamped?

For permitted builds, complex roofs, or anything outside these tables, Builders Supply will get your design sealed by a licensed engineer and supply the material — one stop. Contact us to start.

Request a stamp

Spans are computed from American Wood Council span equations using NDS 2018 design values, for simple-span rafters spaced 24″ o.c. or less, fully sheathed, dry service, snow load duration. Results are a material-selection guide and assume the listed conditions. Final sizing for permitted construction should be confirmed against your local building code and conditions, or by a licensed engineer. Lengths over 20 ft may need special-order lumber — check with us on availability.

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