BuildWellHow a deck is laid out
Most attached decks are framed the same way. A ledger board is fastened to the house. Joists run out from the ledger to a beam near the outer edge, spaced evenly, usually 16 inches on center. The beam sits on posts, and the posts sit on footings. The decking runs across the joists, which means along the house.
This calculator counts the parts that scale with the deck’s size: the decking, the joists, the joist hangers at the ledger and the screws. Beams, posts, footings, stairs and railings depend on the design and the local code, and they’re priced as their own lines.
Two measurements describe a rectangular deck: its length along the house and its projection, how far it comes out. A 16 × 12 deck runs 16 feet along the house and comes out 12 feet.
The deck formulas
- Rows of boards = projection × 12 ÷ (board width + gap), rounded up.
- Decking = rows × deck length, in linear feet.
- Boards = decking × (1 + waste) ÷ board length, rounded up.
- Joists = deck length × 12 ÷ spacing, rounded up, plus one. The plus one is the joist at the far end.
- Screws = rows × joists × 2, for two face screws where every board crosses a joist.
Use the board’s actual width, not its name. A 5/4 × 6 deck board and a 2 × 6 are both about 5½ inches wide. A 4 × 4 post is about 3½ inches square.
Decking, joists and screws by deck size
| Deck | Rows of boards | Decking with 10% waste | Joists at 16 in | Screws |
|---|---|---|---|---|
| 10 × 10 ft | 22 | 242 lf | 9 × 10 ft | 396 |
| 12 × 12 ft | 26 | 344 lf | 10 × 12 ft | 520 |
| 16 × 12 ft | 26 | 458 lf | 13 × 12 ft | 676 |
| 20 × 12 ft | 26 | 572 lf | 16 × 12 ft | 832 |
| 20 × 16 ft | 34 | 748 lf | 16 × 16 ft | 1088 |
| 24 × 16 ft | 34 | 898 lf | 19 × 16 ft | 1292 |
Worked examples
A 16 × 12 deck in pressure-treated boards
Boards are 5½ inches wide with a 3/16 inch gap, so each row takes 5.6875 inches. The 12 foot projection is 144 inches: 144 ÷ 5.6875 = 25.3, so 26 rows. Each row runs the 16 foot length, for 416 linear feet. With 10% waste in 16 foot boards, 457.6 ÷ 16 = 28.6: 29 boards.
Joists at 16 inches on center: 192 inches ÷ 16 = 12 spaces, so 13 joists, each 12 feet. That’s 13 hangers at the ledger and 26 × 13 × 2 = 676 screws.
A 20 × 10 deck at 12 inches on center
With a 1/8 inch gap, each row takes 5.625 inches: 120 ÷ 5.625 = 21.3, so 22 rows and 440 linear feet. The boards come in 12 foot lengths, which don’t divide the 20 foot run, so ends have to be joined over joists and staggered. With 10% waste, 484 ÷ 12 = 40.3: 41 boards. Joists at 12 inches: 240 ÷ 12 = 20 spaces, 21 joists, and 924 screws.
A length that doesn’t divide evenly
A 15 foot deck at 16 inches on center: 180 ÷ 16 = 11.25 spaces. That rounds up to 12, and the last space is narrower than the rest. Plus one for the end: 13 joists, the same as a 16 foot deck.
Spacing, gaps and board direction
Joist spacing. 16 inches on center is the usual spacing for decking that runs square to the joists. Many composite and some wood decking products call for closer spacing, often 12 inches, when boards run on a diagonal. The decking maker’s install guide sets the limit.
Closer spacing adds joists, hangers and screws. For a 16 × 12 deck:
| Joist spacing | Joists | Joist lumber | Hangers | Deck screws |
|---|---|---|---|---|
| 12 in on center | 17 | 204 lf | 17 | 884 |
| 16 in on center | 13 | 156 lf | 13 | 676 |
| 24 in on center | 9 | 108 lf | 9 | 468 |
Joist size and span. How far a joist can span depends on its size, species, grade and spacing and the load it carries. Use the span tables your local code adopts; many US jurisdictions base their deck rules on the International Residential Code.
Gaps. Pressure-treated boards are often wet when installed and shrink as they dry, so many installers lay them tight or nearly tight. Kiln-dried lumber and composite boards need the gap the maker specifies, for drainage and for movement.
Butt joints. When boards are shorter than the deck, their ends have to meet over a joist. Stagger the joints so they don’t line up across the deck, and expect a little more waste.
Fasteners and hardware. Pressure-treated lumber is corrosive to plain steel, so connector makers call for hot-dip galvanized or stainless steel hangers, screws and nails. Near salt water, stainless is the usual choice. Check the lumber tag and the hardware maker’s guidance before you order.
Common mistakes
Using nominal sizes. A "6 inch" board is 5½ inches wide. Count rows with the real width.
Forgetting the end joist. Twelve spaces need thirteen joists.
No flashing at the ledger. Water getting behind the ledger rots the rim of the house. Flash it, and tape the tops of joists if the decking maker recommends it.
Guessing at spans. Undersized joists bounce and can fail. Check the span table for your lumber and spacing.
Skipping the footing check. Footings go below the frost line where the ground freezes, and the size depends on the load and the soil. Your building department will tell you what it requires.
Buying every board the same length. Mixing lengths so the butt joints land on joists and stagger across the deck often wastes less. Lay out the rows before you order.
From quantities to an estimate
A deck estimate also covers demolition of an old deck, footings and posts, the ledger with its flashing, beams, joists, decking, fascia, stairs, railings, and any skirting, lighting and finish.
Closer joist spacing means more joists, more hangers and more time. The ledger, beams, posts, footings, stairs and railings are separate lines, priced by the linear foot or the each, and a typical deck needs all of them, plus a permit and inspections where your building department requires them.
When you estimate a deck in BuildWell, describe it by voice, text or photos and it plans those as lines with labor hours. Lumber and hardware are priced at stores near the job, checked within 30 days, and labor at your metro’s carpenter wage plus your overhead and markup. You can change any quantity, hour or price before you send it.
