16×20 Deck Cost

A 16×20 is 320 sq ft. The decking surface runs about $8,000–$12,800 installed in pressure-treated and $16,000–$25,600 in mid-range composite at 2026-Q1 national rates. The complete project at the calculator's default spec — railing, four steps, permit, 10% contingency — comes to roughly $34,800, or $109 per square foot. That is the lowest rate of any size with a page on this site. Fixed costs are down to about 21% of the budget, and the framing is still ordinary prescriptive construction that any deck crew builds without an engineered drawing. If budget efficiency is the goal and the yard allows it, this is the size to build.

Two beam lines, 8 footings and 80 linear feet of beam. Joists run the 16-foot depth, which is past every practical single span — 2×10 SPF at 16 inches on centre stops at 13 feet 0 inches in the IRC 2021 R507.6 table — so the run splits into two 8-foot bays over an intermediate beam, bringing the joists back onto 2×8 at 16 inches on centre. That works out at 16 joists and roughly 42 sixty-pound bags of concrete in the footings, which is the point at which most crews bring an auger or a mini-excavator rather than digging by hand. Railing is 52 linear feet, and it is a code requirement rather than a choice once the surface sits more than 30 inches above grade under IRC 2021 R312.

Yes, and by a clear margin. Running the same calculator spec on both, a 16×16 comes to about $28,900 or $113 per square foot, while the 64 sq ft that a 16×20 adds costs roughly $5,900 — about $92 per added square foot. The extra length rides on framing you were already committed to: the second beam line already exists, the permit is filed once, the stair flight is built once, and the railing grows along only two of the four sides. This is the general shape of deck economics, and it is the reason quotes should be compared as project totals rather than per-square-foot rates, which flatter large decks and penalise small ones.

Thirty-five rows of 20-foot boards — 49 boards and roughly 768 linear feet with waste. Twenty feet is the longest decking length sold, in wood and composite alike, so every row is a single piece: no butt seams, no offcut, and no sistered joist at a seam station. At this quantity the order behaves differently, though. It is large enough that a supplier will quote a delivery rather than expect a pickup, and large enough that a mixed batch matters: check colour lot numbers on composite before the crew starts, because a lot change partway across the deck is visible for the life of the deck and cannot be fixed without replacing boards.

A two- or three-person crew typically spends five to seven working days on 320 sq ft: a day of layout, a day or more on eight footings, two days framing two beam lines and 16 joists, a day and a half decking, and a day on railing and stairs. Project time is longer. The permit has to be issued before work starts, footings must be inspected before concrete is poured, and framing has to pass before decking covers it — realistically three to five weeks from application to finished deck in most jurisdictions. A weekend DIY build on this footprint is a season, not a project, and the two beam lines are where hiring help pays for itself.