How many times have you gone to do a project around the house and thought, I’m pretty sure this is what I should be doing. But then after you go grab the two by eight, you wonder if that’s enough to hold a floor, or if it will just sag over time? It doesn’t mean you’re bad at it. It means that you don’t have any hard numbers on how far apart boards should be, or what type of wood you should use.
That’s why we created the chart: so you can make educated guesses on how far to space and which wood to use for your particular use. Span has nothing to do with brute force as much as stiffness; meaning a floor is a dud if it snaps under your feet, or worse yet, sags like a trampoline. Even though it won’t break, a two by eight are popular for its depth which will flex instead of bend. It also doesn’t weigh so much you has to call in a team to carry it around.
How to Use Two by Eights for Floors
That means this size work nicely on a home project where span and load is moderate. And the lesson here from this visual reference is understanding how different woods and spacings changes these distances. Many homeowners do not know that wood species also matter. For example, a number two grade Douglas Fir Larch will be stronger board than a number two grade Southern Pine or Hemlock Spruce Fir. Because the Douglas Fir has a higher modulus of elasticity (it’s stiffer out of the box), it can spans further then the other woods as indicated in the chart below. More stiffness translate into less bounce when walking across floor.
A lower-priced wood may cost less at first. However, you might need to buy thicker subflooring or space the joists closer together to make up for the lack of stiffness, which could mean spending more overall. Structurally, this all depend on the spacing. Sixteen inches on center is typical (it works with standard plywood sheet sizes, and is a good balance of performance versus cost). Going to 24-inch spacing reduces the max span considerably, since each board has to support more weight before reaching a support. On the other hand, going down to a 12-inch space will result in a much stiffer floor, at the expense of about half again as much lumber. Most people build at sixteen inch centers, since they don’t want to waste money buying extra lumber, but want the floor to be comfortabley.
How far can one piece of wood span? It depends on the job the wood has to do. If you are spanning a beam in your home used solely to support drywall, it will go a lot further than if you’re supporting a floor with people, pets, and furnitures. This is where the infographic gets interesting. It separates out how each type of beam is used, and how that affects the length of the span. For example, a living room floor (which needs to hold things) drop to barely thirteen feet. Don’t mix up these lengths. You don’t want to assume that you can use your floor the same way as you would your ceiling, as this could lead to structural collapse when you put weight on it.
Remember: Span Ratings drop dramatically with moisture. Span ratings in tables is based off dry conditions. However, outside on a deck or other exterior situation, the humidity will soften the wood fibers and affect span. You’re supposed to cut your calculated span by roughly fifteen percent if it’s a wet service condition. This means you could of get away with twelve-foot spans inside, but you may not be able to reach ten feet on an outdoor deck. That makes all the difference. More overhang is dangerous, so always account for where boards will go.
Before committing to a build, check your bearing length and how you’re installing it. It won’t matter how good a board is if you don’t support it well at both ends. You must have enough bearing on the rim joist or beam for support without crushing the wood. Use the right hangers, nail things down tight, and remember: Lumber bows as it dries. Face the crown of the board upward. This way, gravity fights its bow to make sure your floor begins life flat. This keeps the overall structure more intact in the long term.
You don’t need an engineering degree to build with two by eights; you just have to respect the numbers. Base everything on the chart, then tweak according to the type of use and how much moisture is present. The math will be your friend. Listen to what it says instead of relying only on gut feel. This approach will result in a structure that lasts through use and time if you respect its limits.
