Q. Will “up-sizing” deck joists so that they exceed code requirements result in a safer or stiffer deck?
A. Mike Guertin, a builder and remodeler in East Greenwich, R.I., and frequent presenter at DeckExpo and JLC Live, responds: Some deck builders oversize their deck framing by sizing joists greater than code requires or limiting their joist and cantilever spans to less than code allows. A common reason they cite for exceeding the code is that the joist tables are a “minimum standard,” and they’re concerned about the deck feeling bouncy. Other deck builders use up-sizing the framing members as part of their marketing and sales effort.
But there is no structural benefit to beefing up the framing for safety or stiffness. The tables in the IRC are conservative, with a substantial safety factor already built in and an L/360 deflection limit. That deflection translates into about 3/8 inch on a 12‑foot joist span when a deck is fully loaded at 40-psf live load—something that rarely happens unless you have a heavy snowfall. And the same would be true for higher loads if joists were sized per the new joist sizing tables in the IRC that provide for 50-, 60-, and 70-psf snow loads (see “Right-Sizing Deck Joists,” Mar/23).
But even the L/360 limit is a bit conservative, because the IRC tables are based on #2-grade treated lumber, while some lumberyards stock mixed #1- and #2-grade treated lumber and others stock all #1 grade. When you use better than #2-grade lumber, the allowable span based on the American Wood Council’s joist sizing tables is greater than the span in the tables published in the IRC. For example, the span difference between #2- and #1-grade 2x8 southern pine treated lumber used for deck joists is 7 inches. So your decks are already stiffer with less deflection when you purchase better-grade treated lumber and size the joists based on the code table. If you want to take advantage of the even greater spans allowed when using #1 treated lumber for deck joists, you can size your joists using the AWC’s free joist- and rafter-span calculator (see screenshot below).
I usually right-size deck joists to the maximum span that can be eked out of the table. I haven’t noticed—nor have my clients—any bounciness or excessive deflection. I won’t fault any deck builder for building stronger decks, but rather than overbuilding the frame, I like to right-size the footings, beams, and joists, and spend more time and money on safety elements that the building code doesn’t have prescriptive requirements for, like guardrails and stairs. And I take steps that make a deck last longer, like treating field cuts with a topical wood preservative (a frequently overlooked code requirement east of the Rockies) and capping joists and beams to help shed water. To me, these measures offer better value to my clients than oversized deck framing, which I think is a waste of money and materials.