What size joist for a deck

Updated 2026-09-04 7 min read

Pencil sketch of three joists of different depths on end — Three joist depths stood on end. 2×8 · 2×10 · 2×12

Start with the span, not the size

The question is almost always asked backwards. You do not choose a joist and see how far it goes; you measure how far it has to go and read off the smallest joist that gets there. Everything else — stiffness, hardware, price — is a tiebreak between the size the table gives you and the one above it.

Measure the clear span: ledger face to beam face. On a 12-foot-deep deck with the beam set inboard for a cantilever, that is nearer 10'-6" than 12 feet, and it can drop you a whole size. Run it through the span calculator before you price anything.

Pencil sketch of a joist end with a crown mark on the top edge — The crown marked on the top edge before the joist is hung — depth is only half the choice.

What each size is for

Deck joist sizes, span at 16 inch centres in southern pine, and typical use
Size	16" o.c.	Where it belongs
2x6	9'-0"	Landings, small platforms, ground-level squares under 9 feet. Rarely worth it on a real deck — the hanger and rim stock cost the same and you have boxed yourself in.
2x8	11'-10"	The workhorse. Most attached decks 10 to 12 feet deep frame in 2×8 and never need more.
2x10	14'-0"	Buys you a beam moved outboard, or a stiffer floor at the same span. Where most builders end up when the deck is more than 12 feet deep.
2x12	16'-6"	Long spans and heavy point loads. Also the size you cut stringers from, so the offcuts get used.

The four things span does not tell you

Bounce. The table's deflection limit is a code minimum, not a comfort standard. A joist at 99% of its allowed span passes inspection and still moves under a party. If the deck will hold a table of people, take the next size up or tighten the centres — it is the cheapest upgrade on the whole build.

What sits on it. A hot tub, a stone planter, an outdoor kitchen: point loads the table never saw. Those want a designer, not a bigger joist chosen by feel.

Hardware depth. Hangers, ledger fasteners and rim connections are all sized to the joist. Changing depth late means changing the hanger schedule and often the ledger fastener pattern with it.

Where the deck sits. Deeper joists lift the frame. On a deck that has to meet a door threshold within a set drop, going from 2×8 to 2×10 can push the whole structure two inches out of position.

Sizing up the joist does not fix a beam or footing that was sized for the old load path. Re-check the beam and the footings after any change.

Buying it

Buy for the species you are actually getting, not the one on the span chart you memorised. The same nominal 2×10 reaches 14'-0" in southern pine and 13'-0" in western cedar. Read the grade stamp on the pile before you load it.

Order joists long and cut to a chalk line on site. Trimming a rim square once beats fighting four crooked ends for a day, and the offcuts become blocking. Prices move week to week, so get your yard's number rather than trusting one you read somewhere — the board-foot tool turns their quote into your package price.

Code requirement

Your jurisdiction may amend the model code or adopt a different edition. Confirm with your building department before you cut.