Enter your deck's size and boards. You get the boards to buy for each stock length, joists at 12, 16 or 24 in centres, rim joists, screws or clips, and a plan to check it against.
Your materials
16 ft × 12 ft deck, 5/4 wood boards 5 1/2 in wide with 1/8 in gaps, laid parallel to the house; joists at 16 in centres; 10% waste allowance.
Deck area 192 ft² · 26 rows of boards · 338 board-to-joist crossings
| Board length | Buy (incl. 10%) | Before waste | Offcuts | Total length |
|---|---|---|---|---|
| 8 ft | 58 | 52 | 0% | 464 ft |
| 10 ft | 58 | 52 | 20% | 580 ft |
| 12 ft | 43 | 39 | 11% | 516 ft |
| 14 ft | 36 | 32 | 7% | 504 ft |
| 16 ft least wood | 29 | 26 | 0% | 464 ft |
| 20 ft | 29 | 26 | 20% | 580 ft |
| Part | Count | Length each | Buy |
|---|---|---|---|
| Joists | 13 | 12 ft | 13 × 12 ft |
| Ledger board | 1 | 16 ft | 1 × 16 ft |
| Rim joists | 1 | 16 ft | 1 × 16 ft |
| Centres | Joists | Total joist length | For these boards |
|---|---|---|---|
| 12 in | 17 | 204 ft | within IRC limit |
| 16 in chosen | 13 | 156 ft | within IRC limit |
| 24 in | 9 | 108 ft | too wide |
IRC R507.7 and DCA 6: two 8d threaded nails or two #8 screws per board at every joist. Composite boards: use the maker's screws and pattern.
Joist size check (US lumber, DCA 6)
For a joist span of 12 ft at 16 in centres, the smallest sizes in the table are: Southern pine 2x10; DF-larch, hem-fir, SPF 2x10; Redwood, cedars, pond. & red pine 2x10.
| Size | Southern pine | DF-larch, hem-fir, SPF | Redwood, cedars, pond. & red pine |
|---|---|---|---|
| 2x6 | 9′ 0″ | 8′ 4″ | 8′ 0″ |
| 2x8 | 11′ 10″ | 11′ 1″ | 10′ 7″ |
| 2x10 | 14′ 0″ | 13′ 7″ | 13′ 0″ |
| 2x12 | 16′ 6″ | 15′ 9″ | 15′ 1″ |
Boards to buy come from cutting every row out of the lengths you ticked, with joints landing on joists and every piece bearing on at least three joists, as DCA 6 requires. The waste allowance covers split ends, bad boards and staggering the joints; diagonal decks usually need 15% or more.
Joists are counted from one side of the frame to the other at your spacing, both outer joists included. The rim joist caps the joist ends; the ledger is the board bolted to the house. Boards running out from the house need joists parallel to it, carried on beams.
Fasteners follow IRC R507.7: two screws or nails per board at every joist. Hidden clips are counted as one per board per joist.
A 16 × 12 ft deck with 5 1/2 in boards and 1/8 in gaps needs 26 rows: 26 boards plus 25 gaps make 146 in, just over the 144 in depth, so the last board is ripped. With 16 ft boards that is 26 boards, 29 with 10% waste. At 16 in centres, 192 in ÷ 16 = 12 spaces, so 13 joists, and 26 × 13 × 2 = 676 screws.
The span table in the results is AWC's DCA 6 Table 2, the prescriptive guide most US building departments accept for simple single-level decks. It assumes a 40 psf live load and 10 psf dead load, No. 2 lumber in wet service and an L/360 deflection limit, and it measures the span from the face of one support to the face of the other, without the overhang. Its Southern pine figures match the 40 psf column of 2021 IRC Table R507.6. Hot tubs, heavy snow, second-storey decks and anything outside its tables need a design.
Joist spacing also depends on the decking: the 2021 IRC (Table R507.7) allows 16 in centres for 1 1/4 in wood boards laid square to the joists but only 12 in at 45°, and 24 in for 2 in boards. Composite and PVC boards follow the maker's installation guide.
These tables are a starting point for planning, not an approval. Your area may use another code edition, local amendments or its own frost depth, and outside the US the timber grades and rules are different. The building department and the inspector decide what gets built.
Finish the deck, then the patio, the fence and the footings under it all.
Litres or gallons for the floor, railings and stairs, by surface condition, and the cans to buy.
Pavers for any mix of shapes, with joints and pattern waste, plus gravel base and bedding sand.
Posts, panels or pickets, rails, gates and the concrete for every post hole.
Volume and pre-mixed bags for tube or square footings, with DCA 6 footing sizes.
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Try WebTraceCheckWith 5 1/2 in boards and 1/8 in gaps, 26 rows cover 12 ft, so 26 boards of 12 ft, or 29 with a 10% waste allowance. At 16 in centres the frame has 10 joists. Wider boards or bigger gaps mean fewer rows; enter yours above to check.
Most do, because the joists can then run out from the house and hang from a ledger. Boards running out from the house need joists parallel to it, carried on beams. Diagonal boards look good but take more cutting and closer joists: the 2021 IRC limits 5/4 wood boards laid at 45° to 12 in centres.
The 2021 IRC (Table R507.7) allows joists at up to 16 in centres under 1 1/4 in wood boards laid square to them, 12 in when laid diagonally, and 24 in under 2 in boards either way, as long as each board crosses at least three joists. Composite and PVC boards follow the maker's installation guide.
No. It shows what DCA 6 allows for common US lumber at a 40 psf live load, which helps you plan and price. Your building department decides which code applies, along with local snow loads, frost depth and amendments, and the inspector signs off the work.
Every piece of decking has to reach across at least three joists, with its ends on joists. If a stock length cannot do that at your spacing, especially on a diagonal deck where joists are further apart along the board, the calculator leaves it out.