spec my yardFigure your mulch

Calculators / Structures

What joists does your deck need?

Measure the span from the house to the beam and pick your lumber. You'll get the smallest joist that carries it, and when you need a beam instead.

These are example numbers. Put in yours.

Your span

ft

Ledger to beam, clear distance. 13 ft 6 in is 13.5.

ft

Leave blank if the joists stop at the beam.

Wood and spacing

Lumber

The stamp on the board says the species. Most treated lumber in the South and East is southern pine.

Joist spacing

These joists carry it

2x10 at 16 in o.c.

2x10 is the smallest southern pine joist that spans 12 ft at 16 in. Or drop to 2x8 at 12 in.

Span
12 ft12 ft
Spacing
16 in o.c.
Wood
southern pine
Joist
2x10

span = 12 ft = 144 in

2x6 at 16 in: max 108 in < 144 in

2x8 at 16 in: max 142 in < 144 in

2x10 at 16 in: max 168 in ≥ 144 in

2x12 at 16 in: max 198 in ≥ 144 in

design load = 40 psf live + 10 psf dead, IRC Table R507.6

Joists at 16 in on center

southern pine
  • 2x6 at 16 in o.c.Max span 9 ft. 12 in: 9 ft 11 in, 24 in: 7 ft 7 in
    3 ft short
  • 2x8 at 16 in o.c.Max span 11 ft 10 in. 12 in: 13 ft 1 in, 24 in: 9 ft 8 in
    2 in short
  • 2x10 at 16 in o.c.Max span 14 ft. 12 in: 16 ft 2 in, 24 in: 11 ft 5 in
    Smallest that works
  • 2x12 at 16 in o.c.Max span 16 ft 6 in. 12 in: 18 ft, 24 in: 13 ft 6 in
    Carries it

No. 2 grade lumber, wet service. Spans from IRC 2021 Table R507.6.

Overhang

If the joists run past the beam, keep the overhang to 1/4 of the back span. For 12 ft that's 3 ft. Put it in above and we'll recheck.

Measuring

Span is the clear distance from the ledger or rim to the face of the beam. Not the deck depth.

Snow

These spans are for 40 psf. If your ground snow load is over 40 psf, your town will want the snow columns of the table.

Hot tub

Don't plan a hot tub on these numbers. A full tub runs 100 psf or more. That deck needs an engineer.

Hangers

Hang each joist at the ledger in a galvanized or stainless joist hanger sized for the lumber. Fill every nail hole.

See 2x10 joist hangers on Amazon

As an Amazon Associate we earn from qualifying purchases.

How we work it out

We check your span against IRC Table R507.6, the deck joist table most towns enforce. It assumes No. 2 treated lumber, 40 psf of people and furniture, and 10 psf for the deck itself.

span in inches = feet × 12

passes when table max span ≥ your span

We start at 2x6 and go up until one carries the span at your spacing. If none do, we try tighter spacing. Past 18 ft, nothing in the table works. Add a beam in the middle and size each half.

If the joists run past the beam, the overhang can be 1/4 of the back span at most. Joists that overhang also get shorter allowed back spans, and we use those.

Where the numbers come from

More calculators