Materials list for a 12x20 deck
This list is for a deck 20 feet long against the house and 12 feet deep, with 5.5 inch wide deck boards, a 1/8 inch gap, joists 16 inches on center and 10 percent waste. Change any of those in the calculator above.
| Item | Quantity |
|---|---|
| Deck boards, 5.5 in wide | 29 boards, 20 ft long |
| Joists | 16 boards, 12 ft long |
| Ledger board | 1 board, 20 ft long |
| Rim board | 1 board, 20 ft long |
| Beam boards | 2 boards, 20 ft long |
| Posts and footings | 4 |
| Joist hangers | 16 |
| Deck screws | About 916 |
Prices swing with the lumber market and your region, so take this list to your supplier for a quote.
Framing a 12x20 deck
Joists run the 12 foot direction, out from the house. At 16 inches on center you need 16 of them, each 12 feet long. Each joist spans about 12 feet from the house to the beam. In Southern Pine at 16 inches on center, the American Wood Council span table allows a 2x10 joist to span up to 14 ft. Other species span less, and an overhang past the beam changes the numbers, so check the table for your lumber.
The ledger board bolts to the house and the rim board caps the outer ends of the joists. Each is 20 feet long. The beam is usually two boards nailed together, so it takes 2 boards 20 feet long. It sits on 4 posts spaced no more than 8 feet apart.
Railing and stairs
The International Residential Code requires a railing when the deck surface is more than 30 inches above the ground. Railing on the three open sides of this deck comes to 44 feet. With railing posts no more than 6 feet apart, that is about 9 railing posts and 96 balusters 1.5 inches wide. An opening for stairs lowers those numbers. Use the baluster spacing calculator for each section and the deck stair calculator for the steps.
Footings and concrete
This deck needs 4 footings. With holes 12 inches wide and 36 inches deep, that takes 18 bags of 80 lb concrete mix. Your frost line may call for a different depth, so check with your building department and run your own numbers in the concrete footing calculator.
Other deck sizes
8x1010x1010x1212x1212x1412x1612x2014x1614x2016x1616x2020x20
Common questions
How many deck boards do I need for a 12x20 deck?
29 boards 20 feet long, using 5.5 inch wide boards with a 1/8 inch gap and 10 percent waste. That is 26 rows across the 12 foot depth.
How many joists do I need for a 12x20 deck?
16 joists 12 feet long at 16 inches on center, counting the two end joists.
How many screws do I need for a 12x20 deck?
About 916. That is two screws wherever a board crosses a joist, plus 10 percent extra.
How many posts and footings does a 12x20 deck need?
4, set no more than 8 feet apart under the beam. Your building department sets the footing size and depth.
How many square feet is a 12x20 deck?
240 square feet.
Sources
- International Residential Code 2021, Chapter 5: decks (R507)
- American Wood Council, DCA 6: Prescriptive Residential Wood Deck Construction Guide
- International Residential Code 2021, Chapter 3: stairs (R311.7) and guards (R312)
Facts on this page were last checked against these sources in October 2026.