A 16 × 12 ft deck attached to the house takes 29 composite boards 16 ft long: 26 rows of 5½ in boards with 3/16 in gaps, plus 10 % waste. Under them go 13 joists at 16 in on center, a two-ply beam on three posts, a 16 ft ledger with 13 hangers, and about 370 hidden clips. A railing on the three open sides, less a 4 ft stair opening, needs 81 balusters. The calculator above builds this list for any rectangular deck.
How the calculator works
Enter the deck length along the house, its width out from the house and its height above grade, choose the decking, the gap and the board length. Then the framing — joist spacing, size and species, post spacing — and whether the deck has a railing or a roof. The result counts boards, joists, beam, posts, ledger, hangers, ties, clips or screws and railing parts, and checks the joist span.
Boards
Boards run along the deck. Each row is a board wide plus the gap: 5½ + 3/16 = 5 11/16 in, so 12 ft of width takes 26 rows. The board run is rows × length plus waste — 10 % for straight runs, 15 % for diagonal boards or picture-frame borders — and the board count is that run divided by the board length.
Joists and their span
Joists run out from the house, one every spacing plus one: 16 ft at 16 in is 13 joists. Each spans from the ledger to the beam. The calculator checks that span by the AWC method for 40 psf live and 10 psf dead load at L/360, the loads of the deck provisions in IRC R507, with the wet-service factors for lumber used outdoors. On that basis:
| Southern pine No. 2 at 16 in | Maximum span |
|---|---|
| 2x6 | about 9 ft |
| 2x8 | 11 ft 10 in |
| 2x10 | 14 ft 0 in |
| 2x12 | 16 ft 7 in |
These are on the safe side of IRC Table R507.6; where your jurisdiction has adopted that table, it governs. Composite boards usually need joists at 16 in — 12 in when laid diagonally.
Beam, posts and ledger
The joists sit on a beam at the outer edge, carried by posts at the spacing you choose — 8 ft gives three posts on a 16 ft deck. An attached deck hangs its joists from a ledger bolted to the house; a freestanding one gets a second beam and row of posts instead. Size the beam and footings by IRC R507.4 and R507.5.
Railing
A guard is required where the deck is more than 30 in above grade (IRC R312.1.1), at least 36 in high, with openings that a 4 in sphere cannot pass (R312.1.3). With 1½ in balusters that means one every 5⅜ in: 36 ft of railing takes 81 balusters, with posts no more than 6 ft apart.
Common mistakes
- Boards from the area alone. Count rows and lengths; the gap changes the count.
- Joists parallel to the boards. Boards must cross the joists.
- Composite on 24 in joists. Most composite decking sags between joists that far apart.
For the lumber order, use the Board Foot Calculator; for the concrete in the post footings, the Concrete Calculator.