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A 20x20 deck covers 400 square feet and costs about $7,040 – $11,000 installed in pressure-treated pine, up to $15,360 – $24,000 in composite. It takes about 47 decking boards at 20 ft, 16 joists, 4 footings and 60 linear feet of railing.
| Material | Installed cost |
|---|---|
| Pressure-Treated Pine | $7,040 – $11,000 |
| Cedar | $10,240 – $16,000 |
| Redwood | $12,160 – $19,000 |
| Composite (Trex/TimberTech) | $15,360 – $24,000 |
| PVC / Cellular | $18,560 – $29,000 |
| Ipe / Tropical Hardwood | $20,800 – $32,500 |
A 20x20 deck is 400 square feet. In pressure-treated pine, expect roughly $7,040 – $11,000 installed. Cedar runs about $10,240 – $16,000 and composite about $15,360 – $24,000. 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 20 ft direction, you need 43 rows — that is 860 linear feet of decking, or about 47 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.