Deck Joist Spacing and Composite vs Wood Decking Explained
Two choices shape most of a deck's cost, strength and upkeep: how you frame it, and what you put on top. This guide explains joist spacing, how it affects your materials list, and how wood and composite decking compare.
What joists do
Joists are the horizontal boards that sit under the decking and carry its weight. Decking boards run across the joists, so the spacing of the joists decides how much support each board gets. Spacing, joist size and how far each joist spans all affect how firm the deck feels, so check the span tables or your local building department for your lumber.
Common joist spacing
| Spacing | Where it is used |
|---|---|
| 16 in on center | The common choice for most decks |
| 12 in on center | Diagonal board patterns, and some composite products |
| 24 in on center | Only for products made for it |
The decking manufacturer's instructions come first. Composite and PVC boards can be less stiff than wood, so many need closer spacing.
How spacing changes the joist count
The number of joists is the deck length in inches divided by the spacing, plus one. For a 12×12 foot deck, 16-inch spacing needs 10 joists. Switching to 12-inch spacing raises that to 13. Each extra joist is 12 feet of lumber, so tighter spacing adds up. Our deck calculator shows joist count and linear feet for any size and spacing.
Wood vs composite decking
| Material | Upfront cost | Upkeep | Things to know |
|---|---|---|---|
| Pressure-treated wood | Lower | Needs regular sealing or staining | Can warp, split or fade |
| Cedar or redwood | Higher than treated | Needs sealing to keep its color | Naturally resistant to rot |
| Composite | Higher | Washing, no staining | Can get hot in direct sun |
| PVC | Highest | Very low | Light, with a plastic look |
Wood costs less to buy but costs more of your time over the years. Composite costs more up front and saves on maintenance. Prices vary by region and brand, so compare quotes for the same deck size.
Other things to plan for
- Use screws rated for treated lumber, since the chemicals in it can corrode ordinary fasteners
- Leave the board gap the manufacturer recommends for drainage
- Check whether your deck needs a permit, especially if it attaches to your house
- Make sure footings go below the frost line where required
Pick your decking first, because it sets the joist spacing. Then work out your joist count, add a waste allowance of about 10 percent for boards, and get written quotes before you commit.
Enter your deck size to estimate the decking boards and joists you need and what those materials cost.
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