What the joists hand the beam

Open the deck beam table and the columns are not beam spans, or loads, or sizes. They're joist spans. The table is asking a question about the other members — because what sizes a beam is not its own length, it's how much deck the joists hand it.

The beam picks up half the field

A joist resting on a ledger at one end and a beam at the other splits its load between them. Half the deck between ledger and beam ends up on that beam — so a longer joist run means a wider strip of deck per foot of beam, and the beam has to get deeper or grow a ply.

That's the intuition. The code turns it into a lookup with one extra move: before you enter the table, you convert your actual joist span into an effective one.

The credit you get for not cantilevering

$\text{effective joist span} = \text{actual span} \times \text{factor}$

A deck with no overhang gets its joist span multiplied by 0.66 — the beam is genuinely carrying less than the geometry suggests, because the joist ends simply sit down on it and stop.

Start hanging deck past the beam and that stops being true. The overhang doesn't just add area — it levers. Load out on the tip pries the joist over the beam like a see-saw, pushing down harder at the fulcrum, which is the beam. So the credit shrinks: 0.72, 0.80, 0.84, 0.90 — and at a cantilever one quarter of the backspan the credit is gone entirely and your actual span counts in full.

What that costs, in posts

Take 12 ft joists, southern pine, 40 psf:

• No cantilever: 12 × 0.66 = 7.9 ft → you read the 8 ft column. A triple 2×10 beam spans 11 ft 2 in between posts.
• Two feet of overhang (C/J = 1/6): 12 × 0.90 = 10.8 ft → you read the 12 ft column. The same triple 2×10 now spans 9 ft 2 in.

Two feet of pleasant, cheap-looking overhang cost you two full table columns and two feet of post spacing. On a 30 ft deck that's an extra footing, dug, poured and inspected — a real number that never appears on the sketch where the overhang was drawn.

Both directions are legitimate. Cantilevers are good engineering: they shorten the backspan, keep posts out of the sightline, and let the deck edge float. Just buy them knowingly.

Two rules that don't bend

Round the column up, don't split it. If your effective span lands between two columns, the honest read is the next column up. The table's own footnote permits interpolating; the conservative read costs you a little span and never costs you a beam.

The cantilever has its own limit. Past C/J = 1/4 you're outside what the joist table permits in the first place — at that point the beam factor is no longer your problem, the joist is.

Run it

Open the deck beam sizer with the no-cantilever case loaded — 12 ft joists, southern pine:

1. Read the effective span, the column it lands in, and the smallest beam that carries your post spacing.
2. Now add the two foot overhang and watch the factor jump to 0.90 and the column move.
3. Keep the overhang and get back the span you lost — add a ply, or go up a size. Then decide which one your budget and your headroom actually prefer.