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).
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