Deck Joist Span Chart

Southern pine joist spans (No. 2 grade)

Spans below are joist length between supports — the ledger and the beam, or two beams — measured center to center of bearing. Every figure assumes No. 2 grade, ground-contact pressure-treated lumber, a 40 psf live load, and a 10 psf dead load, which is the standard residential deck loading in IRC R507.

Douglas fir-larch and hem-fir spans

Species matters. If your lumberyard stocks Douglas fir-larch or hem-fir rather than southern pine, reduce the span accordingly — using a southern pine number on hem-fir lumber is one of the most common framing errors inspectors catch.

Cantilever limits

Joists may extend past the beam, which is how most decks get their outer few feet without a second beam line. The IRC caps that overhang at one quarter of the backspan, and the total joist length still has to satisfy the span table.

How to read the chart correctly

Measure the span, not the deck. If your deck projects 14 feet from the house and the beam sits 2 feet in from the outer edge, your joist span is 12 feet with a 2-foot cantilever — a 2×10 at 16 inches handles it easily. Three details commonly get missed: spans are to the center of bearing, not the face of the ledger; hot tubs, planters, and built-in benches add dead load and put you outside the table entirely; and ledger attachment is a separate calculation from joist span.

Deflection, bounce, and going one size up

Span tables are limited by strength and by an L/360 deflection limit — meaning a 15-foot joist may legally sag half an inch under full load. That passes code but feels springy. If you want a deck that feels like a floor, either drop one size in span or go up one size in joist depth. Upgrading from 2×8 to 2×10 across a 12x16 deck costs about $120 in lumber and is the best money you'll spend on the whole build.

Blocking and bridging

Joists deeper than 2×10, or any joist spanning more than about 12 feet, benefit from mid-span blocking to stop rotation. The IRC requires blocking over the beam at interior supports, and most inspectors want solid blocking wherever a butt joint in the decking lands. Blocking also stiffens the deck noticeably, which is the cheapest fix for a bouncy feel.

Once you've picked a joist size, size the beam with the deck beam span chart, count pieces with the joist calculator, and price the whole frame with the deck material calculator.