20x20 Deck Cost and Material List
A 20x20 deck covers 400 square feet. Below is what it costs in each common material, the full material list down to the screw count, and the framing it needs — all worked from the real board width and a 16 inch joist spacing rather than from square footage alone.
The short answer
A 20x20 deck (400 sq ft) costs roughly $7,040 – $11,000 installed in pressure-treated pine and $15,360 – $24,000 in composite. You will need about 47 decking boards at 20 ft, 16 joists, 4 footings and 60 linear feet of railing on three sides.
20x20 deck cost by material
Installed cost, including framing, fasteners and labour. Ranges are national averages and move with local lumber prices and how busy your area's builders are.
| Material | Installed cost | Per sq ft | DIY materials only |
|---|---|---|---|
| Pressure-Treated Pine | $7,040 – $11,000 | $18 – $28 | $3,168 – $6,600 |
| Cedar | $10,240 – $16,000 | $26 – $40 | $4,608 – $9,600 |
| Redwood | $12,160 – $19,000 | $30 – $48 | $5,472 – $11,400 |
| Composite (Trex/TimberTech) | $15,360 – $24,000 | $38 – $60 | $6,912 – $14,400 |
| PVC / Cellular | $18,560 – $29,000 | $46 – $73 | $8,352 – $17,400 |
| Ipe / Tropical Hardwood | $20,800 – $32,500 | $52 – $81 | $9,360 – $19,500 |
Want a figure for your own material, height and region? The deck cost calculator takes all of it into account, and the labour calculator splits out what a contractor charges to build versus what the materials cost.
Full material list for a 20x20 deck
| Item | Quantity | How it was worked out |
|---|---|---|
| Decking boards | 47 × 20 ft | 43 rows at 5½" plus a ⅛" gap, 1 per row, 860 linear ft, plus waste |
| Joists | 16 × 20 ft | 16" on centre across 20 ft, plus the closing joist |
| Ledger and rim | 60 linear ft | Ledger on the house side, rim on the other three |
| Beam | 40 linear ft | Doubled, running the full 20 ft outer edge |
| Posts | 4 | 8 ft maximum spacing along the beam |
| Footings | 4 | One per post, about 0.49 cu yd of concrete at 12" × 42" |
| Joist hangers | 16 | One per joist end at the ledger |
| Deck screws | ≈ 1,514 | Two per board at every joist crossing, plus 10% |
| Railing | 60 linear ft | Three sides; the fourth meets the house |
Check the counts against your own layout with the deck board calculator, framing calculator and concrete calculator.
Is a 20x20 deck big enough?
400 square feet seats roughly 26 people at a table, or holds around 40 standing. Those are planning rules of thumb — about 15 square feet per seated diner once chairs pull back from the table, and 10 per standing guest. They are not code figures, and they assume the deck is otherwise empty.
The things that eat the space are predictable. A six-seat dining set with room to walk behind the chairs wants a clear 12 by 12 area on its own. A grill needs a metre of clearance from the house and from anything combustible. A standard six-person hot tub takes about 49 square feet and, on a 20x20 deck, would leave 351 square feet for everything else — before you account for the framing it needs underneath it.
If the 20x20 footprint is fixed by the yard or the budget, the lever that matters most is shape: a deck that is 20 by 20 feet works far better when the traffic route from the door to the steps runs along one edge rather than through the middle of the seating.
What drives the price up or down on this size
- Height. Anything more than about three feet off the ground means longer posts, deeper footings, guard rail everywhere and staging. It can add 20 to 40 percent to a deck of this size without changing a single board.
- Stairs. Not included above. A flight down from a three foot deck adds roughly $700 to $2,000 depending on width and material — see the stair calculator.
- Railing material. On 60 linear feet, the difference between wood at roughly $1,680–$2,880 and cable rail at $4,800–$9,000 is one of the largest single choices on a deck this size.
- Shape. Every angle, curve or level change costs framing time. A plain rectangle is always the cheapest 400 square feet you can build.
- Ledger versus freestanding. Freestanding needs a second beam and another row of footings, but it avoids the house connection entirely — which is sometimes the only option and often the easier inspection.
Do you need a permit for a 20x20 deck?
Very likely. At 400 square feet a 20x20 deck is over the area threshold used by many jurisdictions, and attaching it to the house removes the freestanding exemption anyway. The usual conditions for skipping a permit are all three of: freestanding, under about 30 inches above grade, and under a set area — commonly 200 square feet. Check your own building department rather than a general figure, because the numbers vary. The deck permit guide walks through what the application normally asks for.
Questions people ask about a 20x20 deck
How much does a 20x20 deck cost?
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.
How many deck boards do I need for a 20x20 deck?
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. The count uses the real 5½ inch board width plus a ⅛ inch gap — estimating from square footage alone comes up short by roughly 8 percent.
How many footings does a 20x20 deck need?
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.
Is a 20x20 deck big enough?
At 400 square feet, a 20x20 deck comfortably seats about 26 people at a table or holds around 40 standing. Those are planning rules of thumb — roughly 15 square feet per seated diner once chairs pull back, 10 per standing guest. Add a grill, a pathway around the table, or a hot tub and the usable figure drops quickly.
Other deck sizes
See every size side by side on the deck size guide, or read the full breakdown for a 12x16 deck.
Materials for a 20x20 deck
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