Materials list for a 16x20 deck
This list is for a deck 20 feet long against the house and 16 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 | 39 boards, 20 ft long |
| Joists | 16 boards, 16 ft long |
| Ledger board | 1 board, 20 ft long |
| Rim board | 1 board, 20 ft long |
| Beam boards | 4 boards, 20 ft long |
| Posts and footings | 8 |
| Joist hangers | 16 |
| Deck screws | About 1,232 |
Prices swing with the lumber market and your region, so take this list to your supplier for a quote.
Framing a 16x20 deck
Joists run the 16 foot direction, out from the house. At 16 inches on center you need 16 of them, each 16 feet long. With two beams, each joist spans about 8 feet between supports. In Southern Pine at 16 inches on center, the American Wood Council span table allows a 2x8 joist to span up to 11 ft 10 in. 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. A deck this deep usually needs two beams, one near the middle and one near the outer edge. Each is two boards 20 feet long, and each sits on 4 posts, for 8 posts in all.
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 52 feet. With railing posts no more than 6 feet apart, that is about 11 railing posts and 116 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 8 footings. With holes 12 inches wide and 36 inches deep, that takes 35 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 16x20 deck?
39 boards 20 feet long, using 5.5 inch wide boards with a 1/8 inch gap and 10 percent waste. That is 35 rows across the 16 foot depth.
How many joists do I need for a 16x20 deck?
16 joists 16 feet long at 16 inches on center, counting the two end joists.
How many screws do I need for a 16x20 deck?
About 1,232. That is two screws wherever a board crosses a joist, plus 10 percent extra.
How many posts and footings does a 16x20 deck need?
8, set no more than 8 feet apart under each of the two beams. Your building department sets the footing size and depth.
How many square feet is a 16x20 deck?
320 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.