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A 12x20 deck covers 240 square feet and costs about $4,224 – $6,600 installed in pressure-treated pine, up to $9,216 – $14,400 in composite. It takes about 29 decking boards at 20 ft, 16 joists, 4 footings and 44 linear feet of railing.
| Material | Installed cost |
|---|---|
| Pressure-Treated Pine | $4,224 – $6,600 |
| Cedar | $6,144 – $9,600 |
| Redwood | $7,296 – $11,400 |
| Composite (Trex/TimberTech) | $9,216 – $14,400 |
| PVC / Cellular | $11,136 – $17,400 |
| Ipe / Tropical Hardwood | $12,480 – $19,500 |
A 12x20 deck is 240 square feet. In pressure-treated pine, expect roughly $4,224 – $6,600 installed. Cedar runs about $6,144 – $9,600 and composite about $9,216 – $14,400. Building it yourself typically removes 40 to 55 percent of those figures, because labour is around half of a finished deck price.
Laying boards across the 12 ft direction, you need 26 rows — that is 520 linear feet of decking, or about 29 boards at 20 ft with a waste allowance. A 20 ft board crosses the deck in one piece, so there are no butt joints.
About 4, one under each post, with posts at no more than 8 feet along the beam. That is roughly 0.49 cubic yards of concrete for 12 inch tubes poured 42 inches below grade — around 23 eighty-pound bags. Your own frost depth sets the real number and is not negotiable.