At the hardware store, you grab a two-by-six; it’s strong enough to frame a house wall or support a deck, yet light enough to throw across your shoulder. In other words, it’s doing a lot of work in construction without demanding any notice. How far can you estimate a board will span before it starts to sag? Well, you can’t just eye it off. Enter span charts. Span charts tells you exactly where your lumber stops, and where structural failure occur.
Two-by-six: We’re conditioned to thinking they’re all alike, but there’s a big difference between grades and species. At the high end in terms of strength are Douglas fir-larch. Not surprisingly, it can stand a bit farther apart than those that includes hemlock and spruce-pine-fir. Southern pine is close behind. Why? Its density is off the charts. High density mean it can carry a serious load. When purchasing locally, look for number stamped on the edge. Number one gives you a bit more wiggle room; number two is your go-to workhorse. If you plan to stretch the limits of spacing, use number one.
How to Use Two-by-Six Lumber
In the real world, however, it all comes down to the spacing of supports. Sixteen-inch on center is normal framing. Why? Because that’s the standard thickness for a typical sheet good (OSB or plywood). So it hits the sweet spot: high enough strength, low enough cost. And as the chart reveals, stepping up to 24-inches can save almost two feet from your maximum floor span. Initially it saves you lumber, but it costs you by requiring thicker subflooring. After all, you don’t want it bouncing when somebody trots across the room. Then there’s 12-inches, which is just plain luxurius. Either you’re dealing with serious loads, or you just want solid-as-a-rock footing beneath your feet.
Floor joists bear all this weight and more, moving people and furnitures around. Ceilings don’t need to hold nearly as much. They’re designed for just ten pounds per square foot of live load. At sixteen-inches on center, you can span a two-by-six more than thirteen feet. That’s why it’s often the sweet spot for homeowners looking to create an open ceiling without spending big bucks on beefy steel beams. If you intend on stashing old boxes in your attic, however, you’ll want to rethink your plans. Your holiday decorations aren’t meant for those joists. Those are for drywall support, only.
And then there are decks and roofs to complicate things with added wind and moisture. That complicates the math even more. For instance, pressure treated lumber is recommended for deck joists. The treatment actualy has a slight adverse effect on structural integrity of the lumber. To compensate, the chart includes a wet-service factor that reduces the allowable span. So you won’t be able to apply dry-interior spans to your outdoor projects. Roof rafters bear snow loads in accordance with where you live and what the pitch is. Always measure by the horizontal projection, not the sloping length itself.
Sometimes it’s about span requirements vs. Budget, so if you’re on a budget, go with the two-by-six. But sometimes it’s just about which one will be more manageable overhead, and the two-by-six is lighter and easier to handle overhead. It’s much less expensive by the linear foot as well. For floor joist applications, the two-by-six works very nicely at spans up to ten feet. That covers most of today’s typical room size. If you have a longer run, such as over 12 or even 14-feet, then that’s when the two-by-eight makes more sense. The two-by-eight doesn’t bend as easily. You can try to make a two-by-six work on a long span but you’ll end up with sagging problems that really frustrate home buyers during inspections. Always check with your local building codes prior to cutting a single board!
Tables is designed using certain assumed loads, which may vary by your location’s code. Wind or earthquake requirements may be stricter then what is shown here. Bridging your joists at midspan will keep the floor system rigid over time and prevent twisting. It is an old school trick but it works wonders for how long it lasts.
Bottom line: Treated within its limitations, a two-by-six is a mighty handy thing. Let it carry out its purpose and you’ve got something that’ll support your house. And what’s that? It spans decent lengths from a proper structure. If you’re unsure, go big instead of going creaky-creaky-ceiling or saggy-saggy-floor. Your timeline won’t mind; your comfort zone will thank you years from now.
