12x20 deck materials: boards, joists & footings
A board-by-board takeoff for a deck that projects 12 ft from the house and runs 20 ft along it. The numbers are computed, not estimated by area.
For a 12×20 deck, buy 41 composite boards (8 × 20 ft + 10 × 16 ft + 23 × 12 ft) or 51 pressure-treated 5/4×6 boards (9 × 16 ft + 24 × 12 ft + 17 × 10 ft + 1 × 8 ft).
26 rows of 5½″ boards, including 5% spare. Framing: 19 × 2x10 joists at 16″ o.c., a (2) 2x10 beam on 3 posts, and 3 footings using 13 bags of concrete.
Customize this deck in the planner →Decking layout
compositeFraming plan
IRC R507Deck boards by material
| Decking | Layout | Buy | Boards | Waste | Screws / clips |
|---|---|---|---|---|---|
| Composite, 12/16/20 ft | Staggered seams (3 seam joists) | 8 × 20 ft + 10 × 16 ft + 23 × 12 ft | 41 | 8.5% | 448 clips |
| 5/4×6 PT, 8–16 ft | Staggered seams (3 seam joists) | 9 × 16 ft + 24 × 12 ft + 17 × 10 ft + 1 × 8 ft | 51 | 8.8% | 929 screws |
| Composite, boards along the 12 ft side | Fewest seams | 46 × 16 ft | 46 | 24.2% | 441 clips |
5½″ boards, 3/16″ gap (composite) or 1/8″ (wood), ¾″ overhang on three sides, joists 16″ o.c., 1″ trim allowance per piece, 5% spare boards. “Waste” is the offcut share of the board length bought. Compare with other sizes in our deck board waste study, or see how to stagger the seams.
Framing, footings & hardware
| Joists | 19 × 2x10 @ 16″ o.c. |
| Joist span (ledger to beam) | 11′ 10 1/2″ of 14′ allowed |
| Beam | (2) 2x10, posts @ 9′ |
| Posts / footings | 3 × 6x6 / 3 × Ø19″ |
| Concrete (36″ deep, 10″ tubes) | 7.7 ft³ = 13 × 80 lb |
| Ledger fasteners | 17 × ½″ lags @ 15″ |
| Joist hangers / ties | 19 / 19 |
Assumes a deck attached with a ledger, southern pine framing, a dropped beam at the outer edge, 1,500 psf soil and a 36″ deck height. Change any of these in the planner, for example Douglas fir framing, a freestanding deck or a cantilever, and the whole list updates.
Joists that carry butt seams are doubled, and the joist count includes them. Spans come from IRC Tables R507.5 and R507.6 at a 40 psf live load. They are not a substitute for your local code or an engineered design. See the methodology.