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Materials for a 12x20 deck
12x20 ft deck (240 ft²) · 43 rows of 12 ft boards, 5.5 in wide, 10% waste
12x20 deck materials
| Item | Quantity | Notes |
|---|---|---|
| Deck boards (5.5 in) | 48 | 43 rows x 1 board(s), 12 ft |
| Joists (16 in centers) | 10 | 20 ft long |
| Deck screws | 903 | ~3 boxes of 350 |
| Joist hangers (if ledger-attached) | 18 | Two per joist, one at each end where used |
Surface material and joists only - beams, posts, footings, ledger flashing and railings are extra. Check local code and span tables.
Boards by board width
| Board | Rows | Boards, no waste | With 10% waste |
|---|---|---|---|
| 5/4 x 6 or 2 x 6 (5.5 in) | 43 | 43 | 48 |
| 2 x 4 (3.5 in) | 67 | 67 | 74 |
| 2 x 8 (7.25 in) | 33 | 33 | 37 |
FAQ
How many deck boards do I need for a 12x20 deck?
With 5.5 inch wide boards and a 1/8 inch gap, a 12x20 deck has 43 rows. Using 12 ft boards that is 43 boards, or about 48 with a 10% waste allowance.
How many joists for a 12x20 deck?
With joists spanning 20 ft at 16 inch centers you need 10 joists (plus a rim joist at each end and doubled joists where your design needs them). At 12 inch centers: 13.
How many screws for a 12x20 deck?
About 903 deck screws (two per board-joist crossing plus 5%), or roughly 3 boxes of 350.
Other sizes: 8x8 · 10x10 · 10x12 · 12x12 · 12x14 · 12x16 · 14x14 · 14x16 · 16x16 · 16x20 · 20x20 · 10x16 · 8x12 · deck calculator
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