When you walk around you can feel a bounce in the floor. That’s what a lot of folks consider wasted money, and annoying.
It turns out that several things affect whether or not your floor will be solid or bouncy, including span, joist depth, wood species, spacing, and weight of your finishes. A lot of builders concentrates on strength over stiffness. You might have a floor that could support a grand piano without falling through, yet feels like walking on boat deck because the deflection hasn’t been controlled.
How to Stop Your Floor From Bouncing
This chart plots allowed spans for various grades and species of lumber. You can see where a two-by-ten is allowed to bend and when it will be in violation of the deflection limit. Deflection limits are tighter then strength limits. And the truth about a two-by-ten: it’s really only a one and a half inches by nine and a quarter inches piece of wood, not two by ten. That makes a big difference in terms of bending capability. If you try this at home and do the math, its nominal size again. Nominal sizing gets you every time. But the chart is calculated based off dressed dimensions, so no need to pull your hair out on that front.
Common Softwood: Douglas Fir-Larch does well. The longest spans exceeds twenty-one feet for Select Structural. Not many people buy Select Structural because of the cost. Most purchase what is called the standard utility grade Number Two. Every piece of lumber should of have a grade stamp that needs to be read. Lower grade lumber can be mixed in but reduces the allowed span. To account for this difference, each row on the chart provide allowances for different grades.
Southern Pine is tough and dense, often equaling or outperforming Douglas Fir. A bit back are Hem-Fir and Spruce-Pine-Fir. Changing species without changing layout will reduce span by a couple inches.
The wood is important but so is spacing. Twelve inches on center gives you the stiffest floor. Sixteen inches is what industry standard uses, but it’s also more expensive. In many situations tightening up your spacing can be less costly than moving up to a deeper beam. You’ll see from the chart that if you go from 24 inch spacing to 12 inch spacing you’re getting several feet of additional span for no cost at all. That’s free strength from having more joist. But there comes a point where spacing doesn’t help, and that point is when you reach amount of load expected.
The game-changer is heavy finish. Gypsum underlayment, tile, stone, all this add a good bit of dead load. To account for this, there’s an entire section on heavier floor assemblies in the chart that will shrink your allowable span. If you’re putting down marble in the bath, don’t expect the same spans as you would in a living room. More dead load = more work by joist to keep it up = less capacity for you to walk around. This is a common mistake. People design based on the naked building and disregard weight of the finish.
These are the bedrooms. The code permits a lower live load in bedroom spaces, which means you can span just a bit further. Living room/kitchen spaces are not given this privilege. Those spaces must be rated at more stringent forty PSF live load requirement. Always design for the worst-case situation on highly trafficked floors.
Mid-span blocking also keeps the joist from twisting. It provides lateral support, particularly if your span exceeds eight feet. It is a simple step that makes the overall feel of the floor better.
Install the crown up. That means the bowed side of the lumber goes up and gravity will pull it flat as time passes. Install it upside down and gravity will work against you by making the bow worse. It is a small thing, but it matters. You want the board to flatten out from the load, not get bent under the load some more.
And then check the code in your area. The chart represents minimal standards nationwide, but local amendments may be tighter. Play it safe: Engineer it when in doubt.
A stiff floor makes for a happy floor, and getting the span correct assures just that.
