These tables are for understanding and budgeting a common deck. Your building department reviews the actual plans, and some areas change the numbers. Heavy loads are a separate matter, as explained below.
What "on center" means
Joist spacing is measured from the middle of one joist to the middle of the next, not across the gap. A joist is 1½ inches thick, so 16 inches on center leaves a clear gap of 14½ inches. If you're measuring an old deck from underneath, that's why the tape reads short.
To count the joists, divide the length of the ledger in inches by the spacing, round down and add one. A deck 16 feet along the house is 192 inches. At 16 inches on center that's 12 spaces, so 13 joists. It's the same count our deck cost calculator gives.
The boards set the spacing
How far apart the joists can go depends on what's lying on top of them. Thin boards sag between joists that are too far apart, and a board on a diagonal has farther to travel from one joist to the next. Here's the model code's table for wood decking, the 2021 version of Table R507.7:
| Wood decking | Straight across, over 3+ joists | Straight across, over 2 joists | Diagonal, over 3+ joists | Diagonal, over 2 joists |
|---|---|---|---|---|
| 1¼-inch boards (sold as 5/4) | 16 in | 12 in | 12 in | 8 in |
| 2-inch boards (sold as 2×6) | 24 in | 24 in | 24 in | 18 in |
Composite and PVC aren't in that table. Each maker sets its own limit for each board, and it's part of the warranty, so go by the installation guide for the exact product you're buying. Many need 16 inches or less, and closer still on a diagonal. Stairs have rules of their own too, so don't assume the same spacing works for treads.
How far a joist can reach
Spacing is half the question. The other half is the span, the distance a joist runs without support. This table is for No. 2 southern pine, the most common treated framing in much of the country, from the 2021 version of Table R507.6:
| Joist size | 12 in on center | 16 in on center | 24 in on center |
|---|---|---|---|
| 2×6 | 9 ft 11 in | 9 ft 0 in | 7 ft 7 in |
| 2×8 | 13 ft 1 in | 11 ft 10 in | 9 ft 8 in |
| 2×10 | 16 ft 2 in | 14 ft 0 in | 11 ft 5 in |
| 2×12 | 18 ft 0 in | 16 ft 6 in | 13 ft 6 in |
Those are maximums for a 40-pound live load and a 10-pound dead load per square foot. The code lists other species separately, so if your yard stocks something else, look up its own row.
A 12-foot deck, worked through
Say the deck runs 12 feet out from the house and the joists run the same way. There are three common ways to frame it:
- 2×8s at 16 inches, beam at the far end. This doesn't work. They reach 11 feet 10 inches, just short of the span.
- 2×10s at 16 inches. These reach 14 feet, with room to spare, and a 16-foot ledger takes 13 of them.
- 2×8s at 12 inches. These reach 13 feet 1 inch, but the same ledger now takes 17 joists.
There's a fourth, and it's the neatest: move the beam in. Set it 2 feet back from the outer edge and the 2×8s at 16 inches now span only about 10 feet, well within the table, with a 2-foot overhang past the beam. Which is cheapest depends on your local lumber prices, so price them before you decide.
How far joists can overhang
A joist can run past its beam, up to the shorter of two limits: a quarter of its span, or the overhang listed in the code's table. With a 10-foot span, 2×8s can overhang up to 2 feet 6 inches. A 2×10 with a span of 12 feet 6 inches gets a quarter of that, about 3 feet 1 inch.
The ends of the joists also need holding upright so they can't roll over. The model code calls for blocking or a rim at each end and where joists cross a beam.
When these tables don't apply
- Hot tubs. A tub full of water and people is far more than 40 pounds a square foot. Anne Arundel County's deck guide, for one, says outright that its decks aren't approved for a future hot tub. That framing has to be designed for the load.
- Heavy snow. The basic tables don't cover areas with ground snow loads above 40 pounds a square foot. Your building department will know if that's you.
- Heavy decking. Stone, tile or concrete on top changes the dead load.
Old decks and new boards
Older decks were sometimes framed at 24 inches for 2×6 boards, which the table allows. Put thinner boards or composite on that frame and the joists are now too far apart, so you'll need to add more between them first. Check before you order. The deck resurfacing calculator covers how to tell if an old frame is worth keeping.
Fair questions
Can I go to 24 inches to save lumber?
With 2-inch wood boards, the table allows it. With 5/4 boards it doesn't, and with composite it depends on the maker. Wider spacing also shortens how far the joists can reach, so you may need bigger joists anyway.
How many more joists does 12-inch spacing take?
On a deck 16 feet along the house, 17 instead of 13. You'll also use a few more hangers and fasteners.
Does a diagonal pattern really need more joists?
For 5/4 wood boards, yes: 12 inches on center instead of 16. So a diagonal deck costs more in framing as well as in cut boards.
Can I build from these tables?
Treat them as a way to understand your plans. The American Wood Council's free guide, DCA 6, and your local deck guide cover the rest: beams, posts, footings and connections. Our guide on deck permits explains what the building department will ask to see.
Sources
- Anne Arundel County, Maryland, Deck Construction Guide: a county guide to the 2021 IRC, with its joist span, overhang and decking tables for southern pine.
- Georgia Department of Community Affairs, Prescriptive deck details: joist spans and the assumptions behind them: No. 2 southern pine, 40 psf live load, 10 psf dead load.
- Fine Homebuilding, Get deck joists right: how span, spacing, overhangs and blocking work under the IRC and DCA 6, and where the tables stop applying.
- American Wood Council, DCA 6: Prescriptive Residential Wood Deck Construction Guide: ledger attachment, joist spans, guards and stairs for wood decks.
- International Residential Code: sections R312 (guards), R311.7.8 (handrails) and R507 (decks). Your city or state adopts and amends it, so check the local version.
- How we calculate: every suggested price behind this calculator, with the date it was last reviewed.
