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MaterialCalc — 16x20 Deck Material List & Cost

Estimate prepared 09/01/2026. Estimates only; verify against your plans before ordering.

16x20 Deck Material List & Cost

The dimension the joists span.

Over 30 in and the code wants guardrails.

%

Result

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16x20 is 320 square feet — two-thirds again the size of a standard 12x16, and the point where the framing stops being simple. Joists spanning 16 ft need real depth, the beam needs an extra post, and the material list is long enough that missing a line item costs hundreds. This is the full list, priced across three decking materials.

Assumptions used

These are the numbers behind the result. Where one of them is soft, the cost range widens rather than pretending to a precision the data does not have.

Lumber waste allowance
0.1

Cut-offs, crooked stock and the boards you set aside.

Standard industry overage — Industry standard, varies by supplier, checked 09/01/2026

Concrete waste allowance
0.1

Covers spillage, over-excavated holes and forms that bulge. Footings dug by hand run higher than forms.

Standard industry overage — Industry standard, varies by supplier, checked 09/01/2026

Deck board coverage width
5.5 in

A nominal 5/4x6 or 2x6 board covers 5.5 in. Gaps come out of the waste allowance.

Home Depot / Lowe's published product data — Published product data, checked 09/01/2026

Gap between deck boards
0.1875 in

Drainage and expansion. Composite wants a wider gap than wood — check the maker.

Trex and TimberTech installation guides — Industry standard, varies by supplier, checked 09/01/2026

Deck joist spacing
16 in o.c.

Composite decking laid diagonally usually requires 12 in instead.

International Residential Code (2021) — Published product data, checked 09/01/2026

Deck beam post spacing
8 ft

IRC deck provisions (DCA 6) — Industry standard, varies by supplier, checked 09/01/2026

Deck screws per square foot
3.5 screws/ft²

Fastener manufacturer coverage tables — Industry standard, varies by supplier, checked 09/01/2026

Prices verified 09/01/2026 (US national average, big-box retail shelf price and regional ready-mix quotes). Regional adjustment: Blended from RSMeans city cost index bands and BLS construction wage differentials.

The formula

320 ft² of surface. Boards: 16 ft of width ÷ 5.6875 in = 34 rows, each needing a 20 ft run — so two lengths per row with a staggered butt joint. Joists: 20 ft ÷ 16 in + 1 = 16, each spanning 16 ft. Posts at 8 ft centers. Railing on three sides, 52 linear feet.

Worked examples

16x20 composite, 36 in high

Deck length: 20 Deck width: 16 Height above grade: 36 Decking material: composite Waste: 10

About 75 boards and 18 joists on 4 posts. Composite at this size is a five-figure material order — worth pricing the alternatives properly.

16x20 pressure-treated

Deck length: 20 Deck width: 16 Height above grade: 36 Decking material: pt Waste: 10

Same frame, roughly a third of the decking cost. On a deck this size the material choice is a several-thousand-dollar decision.

16x20 in cedar

Deck length: 20 Deck width: 16 Height above grade: 36 Decking material: cedar Waste: 10

Between the two on price and the best-looking of the three, at the cost of sealing it every year. Cedar that goes unsealed greys within a season.

Choosing materials

  • A 16 ft joist span needs 2x10 at 16 in on center in southern pine, and 2x12 in weaker species. This is the single biggest cost jump between a 12x16 and a 16x20 deck.
  • At 20 ft long, decking will not span in one board. Every row gets a butt joint, every joint lands on a doubled joist, and the joints should be staggered so they do not form a visible seam.
  • Consider a mid-span beam instead of deep joists. Splitting a 16 ft span into two 8 ft spans lets you drop back to 2x8 joists, and the extra beam and posts often cost less than the deeper lumber.
  • Fifty-two feet of railing is a serious line item — $1,600 to $3,100 installed depending on material. Price it before you commit to a railing style.
  • At this size, most jurisdictions require engineered lateral load connections to the house. Budget for the hardware and expect the inspector to look for it specifically.

Common questions

How much does a 16x20 deck cost?

Materials typically run $4,800 to $7,800 pressure-treated and $9,500 to $14,000 composite, including railing. Contractor-built, expect roughly $13,000 to $19,000 pressure-treated and $19,000 to $29,000 composite.

What size joists for a 16 ft span?

2x10 at 16 in on center in southern pine, or 2x12 in Douglas fir and hem-fir. Alternatively, add a mid-span beam and drop to 2x8 — that is frequently cheaper and always stiffer underfoot.

How many boards for a 16x20 deck?

Thirty-four rows to cover 16 ft of width, each needing two board lengths to reach 20 ft — about 68 boards, or 75 with waste. Stagger every butt joint so they do not line up across the deck.

Does a 320 square foot deck need engineering?

Prescriptive code paths in the IRC deck provisions cover most decks this size, so full engineering is usually not required. But lateral load connections to the house are required, and some jurisdictions want a stamped drawing above 200 square feet.

Should I build one large deck or two levels?

Two levels cost more in framing and railing but break up the span, often letting you use lighter joists on each. They also make a large deck feel like usable rooms rather than one open platform. It is a design decision with a real cost attached.

How many footings for a 16x20 deck?

Four along the outer beam at 8 ft centers if the house side carries a ledger. Freestanding, you need beams and posts on both long sides — eight footings. Each one goes below the frost line and gets inspected.

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