Joist & Beam Spans

Deck joist spans come from IRC Table R507.5, and the table is written around four assumptions that all have to hold: 40 psf live load plus 10 psf dead, No. 2 grade lumber, wet service conditions, and no cantilever.

Break one and the table no longer applies. A hot tub, a heavy planter wall, a roof over the deck or a high-snow jurisdiction all put you outside it, and outside it the answer comes from an engineer rather than a chart.

Species is worth about a foot

A 2×10 at 16″ on center spans 14′ 0″ in Southern Pine and 13′ 0″ in redwood or cedar. Same size, same spacing, same code — a foot of difference that nobody mentions when you buy whatever the yard has in stock.

Douglas Fir-Larch, Hem-Fir and SPF share a column in the table and sit between the two.

Spacing is the expensive way to buy span

Halving the spacing from 24″ to 12″ doubles the number of joists and buys you a couple of feet. Going up one size — a 2×8 to a 2×10 — usually buys more for less money and less labor.

Work out which you need before you order, because changing your mind after the lumber is delivered is expensive in both directions.

Cantilever is a quarter of the actual span

Not a quarter of the maximum. If the table allows 16 ft and your joists run 10, the overhang is 2½ ft, not 4. This is the single most misread line in the deck section of the code.

The ledger is what actually fails

Span tables get the attention; ledger attachment kills people. Most deck collapses are ledger failures — nailed instead of bolted, or fastened through rotten sheathing into nothing. IRC R507.9 specifies the fastener pattern, and flashing above the ledger is what stops the rot that undoes it.

The inspector decides

Local codes amend the IRC and some jurisdictions use higher live loads. Pull the permit before you buy lumber, not after.