Built a deck five years ago and my joists bounced. Not dangerous bouncing, just noticeable when you walked. Turns out I pushed the span too far. Should have checked the charts first.
What the Chart Tells You
A joist span chart shows how far a board can stretch between supports before it sags or feels springy. The bigger the lumber, the farther it spans. The closer you space joists, the farther they can each go.
It’s all about load – how much weight the deck holds, divided among the joists. Math works out differently for different lumber sizes.
Factors That Matter
Lumber species and grade – Not all 2x10s are equal. Pressure-treated southern pine is stiffer than some other species. Higher grades have fewer knots and defects.
Joist size – 2×6, 2×8, 2×10, 2×12. Each step up allows longer spans. Depth is what matters for stiffness.
Joist spacing – 12, 16, or 24 inches on center. Closer spacing means each joist carries less load, so they can span farther individually.
Load requirements – Code typically wants 50 psf total load (40 live, 10 dead). Hot tubs and heavy planters change the math.
Quick Reference Numbers
| Joist Size | 16″ Spacing | 24″ Spacing |
|---|---|---|
| 2×6 | 8 feet | 7 feet |
| 2×8 | 11.5 feet | 10 feet |
| 2×10 | 14.5 feet | 12.5 feet |
| 2×12 | 17 feet | 15 feet |
These are approximate for common pressure-treated lumber. Your local code might differ. Check before building.
How to Use This
Measure from your ledger to your beam, or between beams if you have multiple. That’s your span. Find a joist size and spacing that covers it.
Usually you work backwards – figure out where you want beams (or where posts can go), then select joists to match.
If the span is too long for standard lumber, add a beam in the middle. Breaking one long span into two shorter ones makes everything easier.
Why It Matters
Undersized joists bounce. People notice. It feels wrong even when it’s structurally fine.
Really undersized joists sag permanently. Then they crack. Then decking fasteners pop out. Then you’re rebuilding.
Oversized joists waste money but cause no problems. When in doubt, go up a size.
Common Mistakes
Assuming all lumber is the same. Some boards have more knots or grain runout. The chart assumes standard grades.
Ignoring the bouncy feeling because “it’s strong enough.” Technically maybe. Practically annoying.
Not accounting for heavy stuff. Hot tubs, planters full of soil, that grill you never move. Concentrated loads stress joists differently than distributed loads.
My Advice
Stay conservative. If the chart says 11 feet max, don’t go 10 feet 11 inches. Build in some margin.
Use joist hangers everywhere joists meet beams or ledgers. Metal connectors are cheap insurance.
Check the chart. Then check local code. They don’t always match, and code wins.
