How Many Deck Joists Do I Need?
Quick answer: divide the deck width by spacing and add one — at 16-inch centers, a 16-foot-wide deck needs about 13 joists. Add double rim joists on the two outer edges.
- The count formula
- Joist spacing rules
- Sizing by span
- Joists by deck width
- Rim joists and heavy loads
- Worked example: 12×16 deck
- FAQ
The count formula
Joists run across the deck width, so count = width ÷ spacing + 1. At 16-inch (1.33 ft) centers, a 16-foot-wide deck: 16 ÷ 1.33 = 12, plus one = 13 joists. Round up to the next whole number.
Joist spacing rules
16 inches on center is standard for residential decks with composite or 2×6 decking. Use 12 inches for diagonal decking or when heavy stone/pavers sit on top; 24 inches only with engineered decking rated for it. Check the decking manufacturer's spec before widening spacing.
Sizing by span
The joist size depends on the span: 2×8 spans about 11–12 feet at 16-inch spacing, 2×10 spans about 13–14 feet. For a 10-foot span, 2×8 is fine; go 2×10 for spans over ~12 feet or heavy loads.
| Deck width (joists across) | Joists @ 16 in | Joists @ 12 in |
|---|---|---|
| 10 ft | 9 | 11 |
| 12 ft | 10 | 13 |
| 16 ft | 13 | 17 |
| 20 ft | 16 | 21 |
Rim joists and heavy loads
Add double rim joists on the two outer edges — they tie the joists together and carry the decking ends. Heavy concentrated loads like hot tubs and outdoor kitchens need doubled or sistered joists directly beneath them, often on a shorter span. Never guess on a hot tub.
Worked example: a 12×16 deck
- Joists run across the 16-ft width
- 16 ÷ 1.33 = 12 + 1 = 13 joists
- Span = 12 ft → 2×8 joists
- + 4 rim joists (doubled, both ends) = 17 pieces
FAQ
Divide the deck width by the spacing and add one. At 16-inch centers, a 16-foot-wide deck needs about 13 joists; add double rim joists on the two outer edges.
16 inches on center is standard. 12-inch spacing for diagonal decking or heavy tops; 24-inch only with engineered decking rated for it.
A 2×8 at 16-inch spacing spans about 11–12 feet. For a 10-foot span, 2×8 is sufficient; go 2×10 for spans over about 12 feet.
About 13 joists at 16-inch spacing across the 16-foot width, plus 4 rim joists (doubled). The 12-foot dimension is the joist span — 2×8 handles it.
Yes — hot tubs, outdoor kitchens and large planters need doubled joists directly beneath, and often a shorter span. Check the manufacturer's load spec.