You’d think most people who own houses know that a “two-by-ten” is actualy only about an inch and a half thick. Nope. When you buy it from the yard, all they tell you is the nominal dimension; that’s their handy little bit of marketing lingo. In reality, the stuff you hold in your hand is one and a half inches wide by nine and twenty-five sixteenths deep. So you’re losing close to three quarters of an inch when it gets planed off.
Yet you don’t get credit for it when figuring out its structural strength. Those fractional inches really do matter. They determine how much weight the beam can absorbs before it sags under you. The table above explain the max span allowed for each actual dimension. That’s why all these tables for rough-cut lumber aren’t going to help you out here: You need to use the engineered spec for dressed lumber to be on the safe side.
How to Choose Wood for Strong Floors and Decks
Five primary considerations determines what span is allowed. Bending strength and stiffness are determined by lumber type (species) and quality (grade). Next is spacing; fewer pieces means more load for each one to handle. Then there are dead loads, drywall, subfloor, and live loads such as furniture and human bodies. Again, verify your figures with today’s building codes because the local edition may be wildly different than. A house that meets code in one county may not make it past inspection in next, due to differences in snow load requirements of just a few pounds per square foot.
The standard assumption for live load in houses is that there will be about forty pounds per square foot. You know, bouncing around all over the place. For something like a hot tub deck or ceramic tile floors, you’ll want them spaced closer together so they don’t bounce. The tightest you can space the floor joists and still have any strength is twelve inches on center, which means twenty-five percent more lumber than the usual sixteen inches apart. But most builders goes with sixteen because they are only a few dollars more per sheet and it is perfect for using 3/4″ T&G plywood. Space them up to twenty-four inches and you save some lumber but you get that springy feeling that you really don’t want under tile. So if you ever see tile over that configuration, run!
It’s a budget vs. It is a comfort trade-off everyone has to face as a builder. Maximum Spans Based on Actual Dimensions Moisture exposure makes deck joists an even tougher environment. Always go with naturaly resilient species or pressure-treating with ACQ. Deck span tables resembles floor-joist ones, though fasteners are more important. Moddern treated wood will quickly ruin ordinary steel hangers. Galvanized or stainless steel connectors rated for those chemicals will be necessary.
And here’s where the rules for cantilevers come into play: Yes, you may project out past the end of a deck beam, but not by more than one quarter the length of the supporting span (and never longer than two feet). This protects against loading on the structure causing it all to topple forward. And then there are ceiling joists, which carry almost nothing but their own weight and some plaster or drywall. But if you’re planning on storing boxes up in the attic, now you has to rate the ceiling joists as though they were floor joist of greater strength. That’s why a dusty crawl space is different than an accessible attic storage space; it can mean several feet of additional span allowed.
There’s another wrinkle with roof rafters: snow load (and here in the heaviest snow areas, it’s not small). Without ceiling joists acting as tension ties to prevent the walls from spreading outward, you’ll also need a structural ridge beam. This is how people mess things up when converting an old barn into loft living.
But here’s the actual secret: deflection limits. Under live load, floors can deflect up to one three-hundred-sixtieth of their span. That means a fifteen-foot span can deflect a half-inch before being structurally inadequate. But you’ll notice that bounce in your feet long before it fractures the wood. Engineers restrict this to keep tile and plaster from cracking. It’s a little thing, but it makes more difference to day-to-day comfort than pure strength ever could of.
Don’t put safes or heavy waterbeds in the middle of a span unless you check first to make sure they aren’t on top of a bearing wall. A floor might support distributed weight just fine, but concentrated loads will crush it. Numbers aren’t as important as they might seem. They’re there to help, but it’s the physics of load and span and spacing that teaches you how to build with two-by-tens. The math is a guide.
To walk across a floor without bouncing is to stand on something more than lumber: It’s to stand on the result of thoughtful consideration of each fraction of an inch lost.
