How Far Apart Should Deck Joists Be?
Space joists 16 in on center for composite decking laid straight, and 12 in for diagonal decking, according to Trex. Check your decking’s instructions and local code.
Spacing by Layout
Trex gives this guidance for its composite decking:
| Decking layout | Joist spacing |
|---|---|
| Straight (perpendicular to joists) | 16 in on center |
| Diagonal | 12 in on center |
| Stairs, heavy loads, or intricate patterns | 12 in or less |
Wood and other composite brands publish their own span ratings, so check the instructions for the boards you’re buying. Joist size, span, and the beams and footings under them are set by your local building code.
How Many Joists?
Joists = deck length in inches ÷ spacing + 1 (round the division down)
| Deck length | Joists at 16 in | Joists at 12 in |
|---|---|---|
| 8 ft | 7 | 9 |
| 10 ft | 8 | 11 |
| 12 ft | 10 | 13 |
| 16 ft | 13 | 17 |
| 20 ft | 16 | 21 |
Rim joists, blocking, and doubled joists at board seams add to the count, depending on your design.
Board Gaps
Trex specifies these gaps for composite boards: 3/16 in side to side (up to 3/8 in under trees), 1/8 in end to end above 40°F or 3/16 in below, and 1/4–1/2 in where boards meet the house.
The deck calculator counts boards, joists, and screws from your deck size and spacing.
Frequently Asked Questions
Can Deck Joists Be 24 in Apart?
Some wood decking can span wider spacing, but composite makers like Trex call for 16 in on center for straight decking. Follow the spacing your decking’s maker and local code allow.Why Do Diagonal Decks Need Closer Joists?
A board laid at an angle spans farther between joists than one laid straight across, so the joists move closer together to support it.What Does "On Center" Mean?
It means spacing is measured from the center of one joist to the center of the next.Sources
Prices are national ballparks checked October 2026. Local prices vary.