Every other number on a deck follows from whether the joist actually spans. This checks that against species and spacing before it counts a single board.
Projection is ledger to outer edge. If you cantilever past the beam, the joist span that matters is the back span — ledger to beam — not the full projection.
Beam span sets post spacing, not deck size. 6 to 9 ft is typical for a residential beam; the exact figure comes from the beam table for your size, species and tributary load.
Deck joist spans at 40 psf live plus 10 psf dead, No. 2 grade, dry service. Span is measured clear between supports — ledger to beam.
| Species | 2×6 | 2×8 | 2×10 | 2×12 |
|---|---|---|---|---|
| Southern pine — 16″ o.c. | 9'-0″ | 11'-10″ | 14'-0″ | 16'-6″ |
| Douglas fir-larch | 8'-8″ | 11'-1″ | 13'-7″ | 15'-9″ |
| Hem-fir | 8'-6″ | 10'-9″ | 13'-0″ | 15'-1″ |
| Redwood / cedar | 8'-0″ | 10'-7″ | 13'-0″ | 15'-1″ |
Tightening from 16″ to 12″ on centre buys roughly a foot of span. Opening to 24″ costs about two. That trade is usually cheaper than going up a joist size, but it means more joists, more hangers and more labour.
Joists may cantilever past the beam up to one quarter of the back span. A 12 ft back span allows 3 ft of overhang. The cantilever is not additional span — it is a lever, and it reduces what the back span can carry, which the table already accounts for.
The other half of that rule is what happens at the ledger end: a cantilever tries to lift the far end, so the connection at the house has to resist uplift, not just shear.
Most deck collapses are ledger failures, and almost none of them are the joists breaking. The usual causes are nails instead of through-bolts or structural screws, attachment to rim board that is not backed by anything, and water getting behind the ledger and rotting the band joist over a decade.
Flashing over the ledger is not optional and neither is a proper fastener schedule. This is the one part of a deck where an inspection is genuinely worth the fee.
Board coverage is width plus gap. A 5/4×6 measures 5½″, and with a ¼″ gap covers 5.75″ — so a 12 ft deep deck takes 26 rows regardless of how wide it is.
Gap depends on material and moisture. Wet-treated lumber shrinks and can be laid tight; kiln-dried and composite need the gap designed in. Follow the composite manufacturer, since expansion is temperature-driven and the gap is part of the warranty.
For a short projection, yes — around 9 ft at 16″ o.c. in southern pine. Most decks are deeper than that, which is why 2×8 and 2×10 are the common sizes.
Below the local frost line, onto undisturbed soil, and sized for the load each post carries. Frost depth is jurisdiction-specific — it is inches in the south and several feet in the north.
Almost always, for anything attached to a house or above a modest height. It is also the mechanism that gets the ledger looked at by someone other than you.