Joist spacing changes everything
Three identical 8-foot deck framings — same lumber, same load, different joist spacing. Watch the decking dip under load and see how much firmer 12″ O.C. is compared to 24″. Switch the load preset to see what a hot tub does to your bounce.
- Relative deflection at 12″ / 16″ / 24″ under same load
- IRC R502.3.1 max-span compliance for each spacing
- Where a hot tub (100 psf) breaks 24″ O.C. compliance
- Why 16″ O.C. is the residential default
- Visual deflection is exaggerated 8× for clarity
- Numbers assume 2×10 SPF #2 lumber
- For southern pine or DF, deduct ~15% from max spans
- Diagonal decking requires 12″ O.C. (most manufacturers)
Calculate the exact numbers for your deck
Joist spacing FAQ
What does 16″ O.C. mean for deck joists?
On-center (O.C.) is the distance from the center of one joist to the center of the next. 16″ O.C. means each joist's center sits 16 inches from its neighbor's center. Because joist lumber is 1.5 inches thick (actual, not nominal), the clear gap between joists at 16″ O.C. is 14.5 inches. The 16″ standard exists because it divides cleanly into common decking lengths (8, 12, 16 ft) and matches structural sheathing module sizes.
Can I use 24″ O.C. for a deck?
Yes, but with caveats. 24″ O.C. is code-legal under IRC R502.3.1 for 2×10 joists spanning up to 11′-6″ (40 psf live, SPF #2). The trade-offs: noticeable bounce, faster deck-board wear, and most composite manufacturers void warranty above 16″ O.C. for diagonal patterns. Use 24″ O.C. only for ground-level utility decks where bounce isn't a comfort concern and you're using 5/4 boards rated for 24″ spans.
Why does 12″ O.C. cost more than 16″?
More joists, more joist hangers, more bolts, more labor. A typical 16×16 deck adds 4-6 extra joists going from 16″ to 12″ — about $85-130 in lumber + hangers + $90-140 in install labor. The reverse — going 24″ instead of 16″ — saves about $65-95 lumber and $50-80 labor on the same deck. Most contractors include 16″ O.C. in standard quotes; you must specify if you want tighter or looser.
What's the difference between L/360 and L/240 deflection limits?
L/360 means total mid-span deflection cannot exceed span (in inches) divided by 360. For an 8 ft span: 96/360 = 0.267″ max sag. L/240 is the older, more permissive standard: 96/240 = 0.4″ max sag. IRC R502.3.1 requires L/360 for living spaces where joist sag would crack flooring above; some jurisdictions allow L/240 for exterior decks (no finished ceiling below). DeckMath defaults to the stricter L/360 in all span tables to match best practice.
Does hot tub weight require special joist spacing?
Yes. A typical 6-person hot tub with water weighs 4,500-6,500 lbs concentrated over 50 sqft = 90-130 psf — over 2× residential live load. Spec calls for 12″ O.C. directly under the tub footprint plus doubled joists at the tub perimeter. The animation's 'Hot tub' preset shows what 100 psf does to a standard 16″ O.C. deck: deflection more than doubles, pushing 24″ O.C. construction outside L/360 entirely. Plan ahead — re-framing later costs 4-6× the up-front upgrade.
Where does the 2×10 lumber assumption come from?
IRC R502.3.1 publishes span tables for 2×6, 2×8, 2×10, and 2×12 dimensional lumber in southern pine, Douglas fir, and SPF (spruce-pine-fir). 2×10 SPF #2 is the most common choice for residential decks because it hits the 16 ft span max at 16″ O.C. — covering 95% of suburban deck geometries. Larger 2×12 lumber extends spans by ~22% but costs ~30% more per board.
Two decks can both pass IRC R502.3.1 and feel completely different underfoot. Bounce — the dynamic deflection when you walk fast — scales roughly with static deflection. A deck at its L/360 max will feel springy at year 1 and worse at year 10 as lumber moisture cycles. If you can afford one upgrade, tighten the joist spacing before going to a bigger lumber size.
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