How many deck boards do I need for a 20×20 deck?
A 20 × 20 ft deck is 400 square feet and takes 93 deck boards of 16 ft (5/4 × 6, with 10 percent waste), or 93 boards of 12 ft. Framing it at 16 in on center takes 16 joists, and fastening it takes about 1,412 lb of deck screws or 706 hidden clips.
Deck boards for a 20×20 deck by board length
| Board length | Boards to buy (10% waste) | Linear feet bought |
|---|---|---|
| 8 ft | 139 | 1,112 |
| 12 ft | 93 | 1,116 |
| 16 ft | 93 | 1,488 |
| 20 ft | 47 | 940 |
For 5/4 × 6 boards (5.5 in wide) with 1/4 in gaps laid across the 20 ft dimension: 42 rows and 1,488 linear feet before waste. Pick the length that divides your deck with the least cutoff.
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Lumber list for a 20×20 deck
- Decking: 93 boards of 5/4 × 6 × 16 ft, which is 1,488 linear feet plus waste.
- Joists: 16 pieces of 2 × 8 (spans to 12 ft) or 2 × 10 (spans to 15 ft), each 20 ft long, at 16 in on center. Composite decking usually requires 12 in spacing; check the brand.
- Rim joists and beam: two rim joists of 20 ft, plus a doubled 2 × 10 beam of 20 ft on 8 posts of 4 × 4 or 6 × 6 at 8 ft spacing, each on a concrete footing. The concrete calculator sizes the footings.
- Ledger: a 20 ft 2 × 10 lagged to the house every 16 in, with flashing, if the deck attaches to the house.
- Hardware: 32 joist hangers, 1,412 lb of 2-1/2 in deck screws, and a box of structural screws or 1/2 in lags for the ledger and beam.
- Stairs: figure them separately with the stair calculator.
What a 20×20 deck costs
Pressure-treated 5/4 × 6 decking runs about $1.50 to $2.50 per linear foot, so the boards alone are $2,232 to $3,720. Framing lumber, hardware, and footings add roughly $2,400 to $4,000, putting the whole deck at $4,800 to $8,000 in materials, or $12 to $20 per square foot. Composite decking at $4 to $7 per linear foot raises the board cost to $5,952 to $10,416. A contractor-built pressure-treated deck of this size typically prices at $12,000 to $20,000; composite runs $18,000 to $32,000.
Common questions
How many 16 foot deck boards for a 20x20 deck?
93 boards of 5/4 × 6 with 10 percent waste. In 12 ft boards it is 93, in 20 ft boards 47, and in 8 ft boards 139. Boards run across the 20 ft dimension in 42 rows.
How many joists do I need for a 20x20 deck?
16 joists at 16 in on center across the 20 ft length, plus two rim joists. For composite decking at 12 in on center, add about a third more.
How many screws for a 20x20 deck?
About 1,412 lb of 2-1/2 in deck screws for 400 square feet, at two screws per board per joist. With hidden fasteners you need about 706 clips, usually sold in boxes rated by square footage.