Building a deck involves four separate categories of lumber, and each one gets calculated differently. The decking boards that make the surface are easy. The framing underneath is where most people undercount, because the joist and beam layout depends on span tables and load requirements, not just the deck footprint.
Here is how to estimate each category accurately.
The four lumber categories
1. Decking boards (the walking surface) 2. Joists (the interior framing that supports the deck boards) 3. Beams (the larger members that support the joists) 4. Posts (the vertical supports from ground to beam)
Estimate each separately. Buying all four from the same supplier in one order usually gets you a better price per board foot.
1. Estimating decking boards
Deck boards run perpendicular to the joists. The most common sizes are 5/4 x 6 and 2 x 6 pressure treated. A 5/4 x 6 board has a nominal face width of 5.5 inches but installs at 5 to 5.25 inches with a 1/4-inch gap for drainage.
Number of rows = deck length / (board face width + gap in feet)
For a 16-foot-long deck using 5/4 x 6 boards with 1/4-inch gaps:
- Board width + gap = 5.5 in + 0.25 in = 5.75 in = 0.479 ft
- Rows = 16 / 0.479 = 33.4, so 34 rows
Board length: Choose a length that covers the deck width with minimal waste. For a 12-foot-wide deck, 12-foot boards work perfectly. For a 14-foot-wide deck, you need 16-foot boards (the next standard length up) and cut 2 feet of waste from each.
Add 15 percent waste to account for board ends that need trimming, knots, and the occasional split board. So 34 rows x 1.15 = 40 boards for a 12-foot deck.
2. Estimating joists
Joist spacing depends on the deck board thickness. Standard guidance:
- 5/4 x 6 deck boards: 16-inch joist spacing
- 2 x 6 deck boards: 24-inch joist spacing
Number of joists = (deck width in inches / spacing in inches) + 1
For a 12-foot-wide deck (144 inches) at 16-inch spacing:
- Joists = (144 / 16) + 1 = 9 + 1 = 10 joists
Joist length equals the deck depth minus the thickness of the rim joists on each end. For a 16-foot-deep deck: joists run 16 ft minus 3 inches on each side = 15 ft 6 in. Use 16-foot lumber and cut to fit.
Add 2 rim joists that run the full width of the deck on each end.
3. Estimating beams
Beams span parallel to the deck boards, perpendicular to the joists. The number of beams depends on the joist span. Typical 2x10 joists can span up to 12 feet. For a 16-foot-deep deck, you need an interior beam at 8 feet, creating two joist spans of 8 feet each, plus the ledger board attached to the house.
Beam size depends on the tributary area each beam carries. For residential decks, 3-ply 2x10 beams are common for most spans. A structural engineer or your local building department can confirm if your load or span requires something heavier.
4. Estimating posts
Posts go from the footing piers to the underside of the beams. Count the number of beam-to-post connections on your plan, then measure from grade to the underside of the beam at each location. Posts are typically 4x4 for decks under 8 feet high and 6x6 for taller decks or heavier loads.
A complete worked example
Deck dimensions: 12 feet wide by 16 feet deep. Ground-level deck, 5/4 x 6 decking, 16-inch joist spacing.
| Category | Quantity | Size |
|---|---|---|
| Decking boards | 40 pieces | 5/4 x 6 x 12 ft |
| Interior joists | 8 pieces | 2 x 10 x 16 ft |
| Rim joists | 2 pieces | 2 x 10 x 12 ft |
| Ledger board | 1 piece | 2 x 10 x 12 ft |
| Interior beam (3-ply) | 3 pieces | 2 x 10 x 12 ft |
| Posts (24 in above grade) | 4 pieces | 4 x 4 x 4 ft |
Pressure treatment: what grade to specify
All lumber in contact with the ground or within 6 inches of grade requires ground-contact pressure treated lumber, rated UC4A or UC4B depending on your region. Framing that is above ground but covered by decking, like the joists, uses above-ground rated lumber at UC3B. Decking boards exposed to the weather use UC3A or composite alternatives.
Do not use interior-rated or untreated lumber anywhere in a deck. It will rot within 5 to 10 years and compromise the structure.
Use the lumber calculator to calculate board feet for any deck dimension and lumber size. For full board-foot pricing, the board foot calculator converts any lumber dimension to board feet so you can compare quotes from different suppliers accurately.
Frequently Asked Questions
What lumber size do I need for deck joists?
2x8 joists at 16-inch spacing work for spans up to 12 feet. For spans of 12 to 16 feet, use 2x10 joists at 16-inch spacing. For spans over 16 feet, 2x12 joists or reducing spacing to 12 inches is necessary. Always check a span table for your species and grade of lumber, since different wood species have different structural ratings.
How far apart should deck joists be spaced?
Sixteen inches on center is standard for most deck boards. If you use 5/4 x 6 decking, 16-inch spacing is the maximum; 12-inch spacing gives a stiffer surface. For 2 x 6 decking (actual 1.5 inches thick), 24-inch spacing is possible but 16 inches is better. Composite decking often allows up to 16 inches, but check the manufacturer’s guidelines.
Do deck boards need to be spaced apart?
Yes. Leave a 1/4 inch gap between boards for drainage and to allow for expansion. Treated lumber is wet when installed and will shrink as it dries; boards butted tight together will push against each other and cup or bow. Use a 16d nail as a spacer while fastening. Composite decking has tighter spacing requirements specified by the manufacturer.
What is the best wood for deck boards?
Pressure treated pine is the most common and affordable choice. Cedar and redwood are naturally rot-resistant, lighter, and easier to work with, but cost two to three times more than treated pine. Composite decking costs the most upfront but requires almost no maintenance. In coastal or high-humidity areas, naturally rot-resistant species or composite are worth the premium.
How many screws do I need per deck board?
Two screws per joist crossing per board. On a 12-foot-wide deck with joists at 16-inch spacing, each board crosses 9 joists, requiring 18 screws per board. For a 40-board deck, that is about 720 screws. Buy fasteners by the pound or the 1-pound box: 3-inch deck screws run about 90 to 100 screws per pound, so 8 pounds covers the deck boards on most standard-sized decks.
Do I need a permit to build a deck?
In most jurisdictions, yes. Any deck attached to the house or elevated above a certain height (commonly 30 inches) requires a permit. Freestanding ground-level decks may be exempt in some areas. The permit process requires a structural plan showing post, beam, and joist sizes. Unpermitted decks create problems when selling the house, so pull the permit even if it feels like extra work.