A two-by-six, in your imagination, is typically thought off as a real two-by-six-inch board of lumber. Actualy it’s one-and-a-half by five-and-a-half inches. And that difference mean floors sag. Nominal size misapprehension are the gap between what we think a name stands for and what its meaning is as a building material. Those half inches makes all the difference when you frame a floor.
How Far Can Joists Span Before Gravity Wins? This chart categorize by species and grade, with the latter being the primary consideration when purchasing lumber. I’ll bet you thought all softwoods was created equal. Nope. Douglas Fir-Larch sits at the top of the strength hierarchy for common framing, but Spruce-Pine-Fir is what you usually see in big-box stores.
How to Build Strong Floors with Wood
If the grade is Select Structural, it will span as much as 12 feet and six inches on twelve-inch centers. If the grade is Select Structural, it will span as much as 12 feet and six inches on twelve-inch centers. The more common variety found in big-box stores are Spruce-Pine-Fir, which reaches a maximum height of only about 11 feet and seven inches under same conditions. That gap doesn’t seem like much until you’ve already cut your joists and discovered that you’re four inches shy. Now you has to go back to the yard for a deeper piece of lumber or redesign entire beam layout.
And then there are all the loads. These fall into two categories: heavier assemblies and normal residential loads. For normal living spaces we take as our starting point 40 pounds per square foot (the “live” load) with an additional ten pounds for “dead” weight. Adding substantial amounts of dead load from laying down heavy stuff like gypcrete or ceramic tile increases your dead load. This have the effect of lowering the allowable span, as you can see in the chart.
Increasing the dead load by ten pounds per square foot lowers the span by a couple of inches (a small penalty, typically), but it moves a slight span over the line. This is why engineers distinguish between these two types of loads, there’s a good reason for the deflection limits; nobody wants their floor bouncing when someone walks through it.
And then there’s spacing. The industry standard are 16 inches on center. That strikes a good balance between structure performance and materials cost. To get maximum span, you could space joists at 12 inches on center… But that costs more in labor and lumber. To save money, you might space joists at 24 inches apart. This shortens the span and typically call for thicker subflooring to keep it from sagging between joist. Typically the best bet for most homeowners is right down the middle.
The crown is also an easy trick when installing. Lumber never runs completely straight, it has a slight curve to it. So you always want the curve up on your joists. Eventually as it gets loaded up, the curve will get flattened out and you have a flat floor. But if you run it crown down, gravity will exaggerate the curve and you’ll be left with a sagging floor in the middle of the room… A bowl if you will. It is a little thing but it matters.
Before cutting your first board: check the grade stamp. That’s how they tell you what kind of wood it is (species) and what the strength grade is. Don’t assume it’s No.1 grade if it doesn’t say so. The grades does not match the span tables. For example, 3 lumber are dangerous. Match the stamp to the table.
Want bigger spans than a two-by-six can provide? Consider manufactured wood such as I-joists or LVL. It costs more money up front, but it saves you from needing to frame a big central beam. Respecting the limits of the materials you’re carrying around is floor framing 101. Read the stamp and measure twice. Remember a two-by-six is as strong as its weakest point.
