Built-up beams from dimensional lumber show up everywhere in residential framing — under floors, over carrying walls, across deck rims. The question is always the same: how far can I span this thing? Here is the practical version of the math, with the disclaimers that have to come with it.

What "simple span" means

A simple-span beam is supported at two ends only, with a uniformly distributed load between them. That covers most residential floor and roof beams. If your beam has a column or post in the middle, or carries a point load (a stack of walls coming down on one spot), the math is different and you need an engineer.

The two limits that govern span

A wood beam has to pass two checks before you can call its span allowable:

  • Bending strength. The beam must not crack under load. Required section modulus comes from M = wL²/8, with allowable bending stress (Fb) for the species and grade. For #2 SPF, Fb is 875 psi adjusted by a size factor (Cf) that gets bigger for shallow sections.
  • Deflection (sag).Even if it doesn't break, a beam that bows too much cracks finishes above and below it. Floor beams are limited to L/360 of live- load deflection — that is, span divided by 360. A 12-ft beam can sag no more than 144/360 = 0.4 inch under live load.

The beam span calculator computes both and reports whichever governs.

Tributary width — the load on the beam

Tributary width is the half-distance from the beam to the next parallel support. A beam down the middle of a 24-ft- wide room with joists hanging off both sides has a 12-ft tributary width. Multiply that by your floor load (40 psf live + 10 psf dead = 50 psf in residential) to get the uniform load in pounds per linear foot. A 12-ft tributary at 50 psf is 600 plf.

Member, ply, and species

The calculator covers four sizes (2x6, 2x8, 2x10, 2x12) at one to three plies, fastened together. Three plies are not automatically three times as strong — they must be properly nailed (typically 16d at 12" o.c., staggered top and bottom) or bolted to act as a unit. Spruce-Pine-Fir #2 is the assumed species; Douglas Fir is stiffer and stronger and would let you span farther, while Hemlock or Southern Yellow Pine vary in either direction.

For wide openings or heavy loads, switch to engineered lumber: LVL (laminated veneer lumber), PSL (parallel strand lumber), or glulam. They span 30%+ farther than the same depth in built-up dimensional, with predictable manufacturer-published values.

How to use the result

The number the calculator returns is the maximum span where the beam stays inside both bending and deflection limits. In practice you want at least 10–15% margin, because field conditions never match assumptions perfectly: lumber grade varies, point loads sneak in, contractor changes happen. If the calculator shows a 10.5-ft max span and you need 11 ft, step up a size or add a ply rather than trust the margin.

What this calculator does not do

It does not consider:

  • Shear at the supports (rarely controlling for typical residential floor beams, but real)
  • Bearing length at the supports — your beam needs at least 1.5–3 inches of bearing on a stud pack or column, sized for the reaction
  • Connection design — how the joists hang off the beam, how the beam transfers to columns below
  • Snow drift, seismic, or wind uplift loads on roof beams
  • Notches and holes drilled in the beam (which can reduce capacity dramatically)

For deck beams specifically, IRC R507.5 has prescriptive tables that cover most residential decks; use those before this calculator if your project fits.

Quick FAQ

How far can a triple 2x10 span? Under a typical 12-ft tributary width and standard residential load, around 11–12 feet for #2 SPF. A double 2x12 will go slightly farther. For longer spans, engineered LVL is almost always the right answer.

Why is the deflection limit L/360 instead of L/240? L/240 is for roof rafters with no finished ceiling below; the visible sag is hidden by the slope. Floors under finished ceilings need L/360 because drywall cracks around 0.5 inch of midspan deflection.

Can I use this for a deck beam?Yes for planning, but always cross-check against IRC R507.5 deck- beam span tables. Decks have specific outdoor-exposure and bearing rules the calculator doesn't enforce.

Estimate only. Built-up beams in real structures must be sized by a licensed structural engineer or selected from a code-stamped span table. The beam span calculator is for planning. ProjectCalc is not responsible for structural failures resulting from use.