12x20 Deck Material List: Boards, Joists, Screws, Footings
A 12x20 deck (240 square feet) with the boards running the 20 ft way needs 29 deck boards of 20 ft, 16 joists of 12 ft and about 832 screws. With the boards running the 12 ft way instead, the same deck takes 48 boards of 12 ft, 10 joists of 20 ft and 860 screws. Both lists are below, and the deck materials calculator opens on this deck.
Part of the project planner: Planning and building a deck.
A 12 × 20 ft deck is long and narrow, the shape that runs along the back of a house with room for a dining end and a sitting end. At 240 square feet it needs 2 gallons of stain for one coat, where a 12 × 16 deck needs 1.
12x20 deck material list, boards running the 20 ft way
This is the usual layout: the 20 ft side along the house, the boards parallel to it, and the joists spanning the 12 ft out from the house.
| What to buy | How much | How we worked it out |
|---|---|---|
| Deck boards, 20 ft, 5.5 in wide | 29 | 26 rows across the 12 ft width, with 10% waste |
| Joists | 16 | 12 ft long, at 16 in on center along the 20 ft side |
| Deck screws | 832 | Two per board at each joist |
| Joist hangers, up to | 32 | One per joist end hung from a ledger or rim |
| Joist tape | 192 ft | On the top of every joist |
| 80 lb bags | 26 | Footing concrete, for example 6 footings, 12 in across, 3 ft deep. Your plans set the number |
| Stain or sealer | 2 gal | One coat at about 200 sq ft a gallon (3 gal for two coats) |
Beams, posts, the ledger board and railings come from your plans and local code.
How the numbers are worked out
- Rows: the 12 ft width is 144 in. Each row takes 5.625 in (a 5.5 in board plus a 1/8 in gap), and 144 ÷ 5.625 = 25.6, so 26 rows.
- Boards: 26 rows plus 10% waste is 28.6, rounded up to 29 boards of 20 ft.
- Joists: the 20 ft side is 240 in, and 240 in ÷ 16 in = 15 spaces, which needs 16 joists.
- Screws: 26 rows × 16 joists × 2 = 832.
The same deck with the boards running the 12 ft way
Turned the other way the joists would be 20 ft long, longer than a single span of ordinary deck lumber usually allows, so that layout needs beams part-way and is rarely the simpler build.
| Board direction | Rows | Boards to buy | Joists | Screws |
|---|---|---|---|---|
| Boards run the 20 ft way | 26 | 29 of 20 ft | 16 of 12 ft | 832 |
| Boards run the 12 ft way | 43 | 48 of 12 ft | 10 of 20 ft | 860 |
In the calculator the first box, deck length, is always the way the boards run, so swap the two numbers to see the other layout.
Which board length to buy
Twenty-foot boards cover each row in one piece (29 boards), but they are heavy, awkward to move and not every yard stocks them. In 16 ft boards the deck takes 36 with a joint in every row. In 10 ft boards it takes 58, two to a row, and then every joint falls on the same line across the middle of the deck, which needs a doubled joist under it.
| Board length (boards run the 20 ft way) | Boards to buy | Joints |
|---|---|---|
| 8 ft boards | 72 | A joint in every row |
| 10 ft boards | 58 | A joint in every row |
| 12 ft boards | 48 | A joint in every row |
| 16 ft boards | 36 | A joint in every row |
| 20 ft boards | 29 | None: one board is one row |
With narrower 2x4 boards (3.5 in) this deck takes 44 boards of 20 ft, and with wide 2x8 boards (7.25 in) it takes 22.
Joists and screws by spacing
The frame has 16 joists of 12 ft, 3 more than a 12 × 16 deck, because the extra 4 ft of length adds 3 more 16 in spaces.
| Joist spacing | Joists, boards the 20 ft way | Screws | Joists, boards the 12 ft way | Screws |
|---|---|---|---|---|
| 12 in | 21 | 1,092 | 13 | 1,118 |
| 16 in | 16 | 832 | 10 | 860 |
| 24 in | 11 | 572 | 7 | 602 |
Footing concrete by number of footings
The number of footings comes from your plans, so the list above uses 6 only as an example. For footings 12 in across and 3 ft deep, with 10% extra:
| Footings in your plans | 80 lb bags of concrete |
|---|---|
| 4 footings | 18 |
| 6 footings | 26 |
| 8 footings | 35 |
| 9 footings | 39 |
| 12 footings | 52 |
Checks before you start
- Ask the yard whether it stocks 20 ft decking before you plan around it, and what delivery of that length costs.
- Get a permit and find out the frost depth for your footings.
- Size the joists for the 12 ft span from the span tables for your lumber.
- Use hot-dip galvanized or stainless fixings for treated lumber.
Other sizes: 8x8 deck, 10x12 deck, 12x12 deck, 12x16 deck, 14x20 deck, 16x16 deck. See also the materials list for a deck and how many deck boards you need. The 12x20 deck boards page counts the boards running the 12 ft way. These are estimates for planning.
Frequently asked questions
How many deck boards for a 12x20 deck?
29 boards of 20 ft when the boards run the 20 ft way (26 rows across the 12 ft width, with 10% waste). If the boards run the 12 ft way it is 48 boards of 12 ft in 43 rows.
How many joists for a 12x20 deck?
16 joists of 12 ft at 16 inches on center when the boards run the 20 ft way, or 10 joists of 20 ft when the boards run the 12 ft way. The span table for your lumber decides the joist size and whether a beam is needed.
How many screws for a 12x20 deck?
About 832: 26 rows × 16 joists × 2 screws. With the boards the other way it is about 860.
Can I build a 12x20 deck with 16 ft boards?
Yes. It takes about 36 boards of 16 ft instead of 29 of 20 ft, with one joint in every row. Each joint must land on a joist, and the joints should be staggered from row to row rather than lined up.
How much concrete for the footings of a 12x20 deck?
It depends on the number of footings in your plans. As an example, 6 footings 12 inches across and 3 feet deep take about 26 bags of 80 lb mix. Your frost depth and plans set the real size and number.
More guides
Tools & materials for this project
Affiliate links As an Amazon Associate we earn from qualifying purchases. These links do not change what you pay.
Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.
Open the calculator →