For most residential decks with standard decking laid straight across the joists, 16” on center is the usual spacing. Diagonal layouts typically need 12”. Spacing of 24” is only allowed with thicker 2x wood decking. The real answer for your deck depends on two separate limits: what your deck boards can span between joists, and what your joists can span between supports at that spacing. Local codes can modify both.
This guide explains how each limit works, where the numbers come from, and how to count joists for a real deck. It doesn’t replace your local code or a plan reviewer. Treat the figures here as a guide to reading the official tables, not as a design.
Two limits, one spacing
Joist spacing has to satisfy two checks at once.
- The decking check. Deck boards bend between joists. Thinner and softer boards need joists closer together. This check is about the boards.
- The joist span check. Joists bend between the ledger and the beam, or between beams. The closer together they are, the less load each one carries, so the farther each can span. This check is about the joists.
You pick a spacing that passes the decking check, then pick a joist size that can span your distance at that spacing. Sometimes the second check pushes you to a tighter spacing than the decking alone would need.
The decking check: IRC Table R507.7
For wood decking, the International Residential Code sets maximum joist spacing in Table R507.7. As reproduced in Anne Arundel County’s 2021 IRC deck guide:
| Wood decking | Perpendicular, single span | Perpendicular, multiple span | Diagonal, single span | Diagonal, multiple span |
|---|---|---|---|---|
| 1-1/4” thick (5/4) | 12” | 16” | 8” | 12” |
| 2” thick (2x) | 24” | 24” | 18” | 24” |
“Single span” means a board resting on only two joists. “Multiple span” means it crosses three or more. This is where homeowners most often get caught out. A 5/4 board is allowed at 16” only when it runs continuously over at least three joists. Short pieces that span just one bay, such as a filler strip at a picture-frame border or a small offcut, fall under the 12” limit.
The table also explains the common rule of thumb: 5/4 decking at 16”, 2x decking at up to 24”. (5/4 means “five-quarter”, nominally 1-1/4” thick but actually about 1”.) The deck board sizes guide explains the difference between nominal and actual sizes.
Composite and PVC boards follow the manufacturer
The IRC doesn’t give one spacing for plastic composite decking. R507.7 says “other approved decking or fastener systems shall be installed in accordance with the manufacturer’s installation requirements.” Composite boards, or their packaging, must carry a label showing compliance with ASTM D7032 and the maximum allowable span (IRC R507 text, 2018 New Jersey edition, via Lakewood Township).
The major brands publish similar rules:
- TimberTech: “Joist spacing should never exceed 16” on center,” and all angled or commercial installations need 12” on center at most. Its span table for PRO, EDGE and ReliaBoard lines gives 16” at 90°, 14” at 30° and 12” at 45° (TimberTech installation guide).
- Trex: 16” on center for residential and 12” for commercial, with diagonal layouts at 12” (Trex Academy joist guide).
- Fiberon: 16” on center for residential perpendicular installs. For diagonal installs up to 45°, reduce the spacing by 4”. It also specifies 2x8 or larger joists (Fiberon installation instructions).
Stair treads usually have their own, tighter stringer spacing. TimberTech, for example, lists 9” or 10” for several of its lines (TimberTech guide). Always check the guide for the exact product you are buying, since lines within one brand differ.
The joist span check: IRC Table R507.6
Once you know your spacing, the joist has to span the distance from ledger to beam. The 2021 IRC joist span table, reproduced by Anne Arundel County for southern pine at a 40 psf live load (Anne Arundel County deck guide):
| Joist size | 12” o.c. | 16” o.c. | 24” o.c. |
|---|---|---|---|
| 2x6 | 9’-11” | 9’-0” | 7’-7” |
| 2x8 | 13’-1” | 11’-10” | 9’-8” |
| 2x10 | 16’-2” | 14’-0” | 11’-5” |
| 2x12 | 18’-0” | 16’-6” | 13’-6” |
A few cautions about reading this table:
- These values are for southern pine at 40 psf. The full code table has different spans for other species, and higher snow loads or local amendments can reduce them. Use the version your building department enforces.
- The table gives the maximum. Plenty of builders size up a step to get a stiffer, less bouncy deck.
- Cantilevers past the beam have their own limits. The joist cantilever is capped at one-fourth of the joist span or the value in the cantilever table, whichever is less (IRC R507.6 text, 2018 New Jersey edition).
Worked example: a 20 × 12 ft deck
Take a 20 × 12 ft deck attached to the house along the 20 ft side. The joists run 12 ft from the ledger to a beam at the outer edge. You plan to use composite boards laid straight, so the manufacturer allows 16” on center.
Step 1: count joists. Joists sit along the 20 ft ledger. 20 ft = 240”. Divided by 16”, that gives 15 spaces, which means 16 joists counting the two end joists. The general formula is: joists = (length in inches ÷ spacing), rounded up, + 1.
Step 2: check span. The span from ledger to beam is about 12 ft. In the table above, a 2x8 at 16” spans 11’-10”, which is just short. That leaves two options:
- 2x10 joists at 16” (14’-0” allowed), or
- 2x8 joists at 12” (13’-1” allowed). This needs 21 joists instead of 16.
Step 3: compare. Five extra 2x8s plus their hangers, against 16 slightly larger 2x10s. The 12” option also adds about a third more deck fasteners, because every board crosses more joists. In most cases the 2x10 at 16” is simpler. Price both for your area. The deck framing calculator runs this comparison quickly.
Step 4: add the extras. Butt joints in the decking usually need a doubled joist or blocking under the seam. TimberTech requires doubled joists at butt joints (TimberTech guide). Picture-frame borders and breaker boards add more doubled joists and blocking, so plan them before you order framing lumber.
Same deck at other spacings
| Spacing | Joists along 20 ft | Notes |
|---|---|---|
| 24” o.c. | 11 | Only with 2x wood decking; 2x10 at 24” spans 11’-5”, short of 12 ft |
| 16” o.c. | 16 | Standard for 5/4 wood and most composites laid straight |
| 12” o.c. | 21 | Diagonal layouts, commercial use, or to shorten joist size |
The 24” row shows how the two checks interact. Even if your decking allows 24”, your joists still have to span the distance at that spacing.
When you need tighter spacing
- Diagonal or herringbone layouts. Boards crossing joists at an angle span farther between supports, so spacing drops, typically to 12”. The R507.7 table drops 5/4 wood to 12” for diagonal multiple-span and 8” for diagonal single-span.
- Hot tubs, planters and heavy loads. The prescriptive tables assume ordinary deck loads. Concentrated loads like a hot tub need an engineered design. Some jurisdictions say this outright. Anne Arundel’s guide states that its decks “are not approved for future hot tub installations” (Anne Arundel County deck guide).
- A stiffer feel. A deck can meet code and still feel bouncy at maximum span. Tighter spacing or a deeper joist reduces that.
- Hidden fastener systems may list their own spacing requirements. Check the clip instructions as well as the board guide. The hidden fasteners guide covers clip counts.
How spacing affects your board order
Joist spacing doesn’t change how many square feet of decking you need. It does affect the details. Butt joints must land on a joist, so your board lengths and offcuts depend on where the joist lines fall. With joists at 16”, seams can only land at 16” increments. A run that needs 1 ft more than a 16 ft board wastes material back to the nearest joist line. Plan board lengths and joist layout together. The deck board calculator handles that, and choosing deck board lengths explains the strategy.
Before you build
Spacing tables tell you what the code allows for a typical deck. They don’t account for your soil, snow load, species of lumber or local amendments. Get a permit, follow the tables your jurisdiction actually uses, and talk to the plan reviewer or an engineer for anything unusual. That includes long spans, heights, hot tubs, or attaching to an older house. The how to plan a deck guide covers the permit process.
Frequently asked questions
Is 16” on center always fine for composite decking?
It’s the most common maximum for residential composite laid perpendicular to the joists, but it isn’t universal. Some product lines, angled layouts and stair treads need tighter spacing. The rule that applies is the one in the installation guide for your exact board.
Can I use 24” joist spacing to save lumber?
Only with decking rated for it, which in the IRC table means 2” thick wood. Even then, joists at 24” span less than joists at 16”, so you may need a larger joist size or an extra beam. That can wipe out the savings.
How do I count joists for my deck?
Divide the length the joists are spread along, in inches, by the spacing. Round up and add one. Then add doubled joists at butt seams, breaker boards and picture-frame borders, plus the rim joists. A 20 ft run at 16” on center needs 16 joists before those extras.
Does joist spacing affect the number of deck screws?
Yes. Every board needs fasteners at every joist it crosses, so tighter spacing means more fasteners. Going from 16” to 12” on center adds about a third more screws or clips for the same deck area.