Laying out a deck frame
A prescriptive deck frame has four parts: a ledger bolted to the house band joist, joists hung from the ledger, a beam on posts near the outer edge, and a rim joist across the joist ends. AWC's DCA 6 guide sets the rules for this layout at 40 psf live plus 10 psf dead load[1]:
- Ledger: at least 2x8, at least as deep as the joists and no deeper than the house band; never on brick, stone or masonry veneer[1][2].
- Joists: span from the ledger to the beam; their size comes from the joist span table at your spacing[1].
- Cantilever: joists may run past the beam by the lesser of the table value or a quarter of the back span[1].
- Beam: may overhang its end posts by up to a quarter of the beam span (IRC R507.5)[2].
- Posts: 6x6 minimum in DCA 6; 8x8 for taller posts[1].
Deck joist spacing
Deck joist spacing is set by the decking: 16 in for 5/4 (1-1/4 in) boards laid square, 12 in for 5/4 boards on the diagonal, and up to 24 in for 2x decking, for boards that span three or more joists (IRC Table R507.7)[2]. A board resting on only two joists needs closer spacing: 12 in square or 8 in diagonal for 5/4. Closer joists also allow longer spans. As a deck joist calculator, this tool counts the joists for your spacing: deck width ÷ spacing, rounded up, plus one.
| Deck width | 12 in centres | 16 in centres | 24 in centres |
|---|---|---|---|
| 10 ft | 11 | 9 | 6 |
| 12 ft | 13 | 10 | 7 |
| 16 ft | 17 | 13 | 9 |
| 20 ft | 21 | 16 | 11 |
| 24 ft | 25 | 19 | 13 |
Joist span and cantilever check
The back span is measured from the ledger face to the beam face. The calculator subtracts the cantilever, the ledger and the rim from the deck depth, and flags a cantilever longer than a quarter of the back span. Example back spans for a 12 ft deep deck:
| Cantilever | Back span | Quarter of back span | OK? |
|---|---|---|---|
| 12″ | 10′ 9″ | 32-1/4″ | yes |
| 18″ | 10′ 3″ | 30-3/4″ | yes |
| 24″ | 9′ 9″ | 29-1/4″ | yes |
| 30″ | 9′ 3″ | 27-3/4″ | no |
| 36″ | 8′ 9″ | 26-1/4″ | no |
Check the back span against the joist span calculator with Wet service ticked, or against the joist span table in DCA 6.
Posts, hangers and blocking
Posts sit under the beam at no more than the beam's allowable span, which DCA 6 tabulates by beam size and joist span[1]. Each joist gets a hanger at the ledger; blocking between joists over the beam keeps them upright. The beam, rim and ledger each run the full deck width.
From frame to finished deck
Decking, screws and railing come from the deck calculator; wood or composite boards from the deck board calculator. Joist and rafter span tables from the American Wood Council cover other loads and species[3].
Frequently asked questions
How does a deck joist calculator count joists?
Deck width ÷ joist spacing, rounded up, plus one. A 20 ft deck at 16 in centres takes 16 joists, each with a hanger at the ledger.
How far can deck joists cantilever past the beam?
Under DCA 6, no more than the tabulated overhang or a quarter of the joist back span, whichever is less.
How far apart should deck posts be?
No farther than the beam can span, from the beam span table for your beam size and joist span. Many decks land between 6 and 8 ft.
What is the standard deck joist spacing?
16 in on centre for 5/4 deck boards laid square to the joists and 12 in for diagonal 5/4 boards, when each board crosses three or more joists; up to 24 in for 2x boards (IRC Table R507.7).
How many joist hangers do I need?
One per joist at the ledger, plus one at each end of any joist hung from a beam or header.
Can a deck ledger go on brick veneer?
No. DCA 6 and the IRC prohibit attaching a deck ledger to stone or masonry veneer; build a freestanding deck instead.