Now you’re left with a hole where a load bearing wall existed, and you want to find out how much wood can support it? No good intuition here so we hand it off to the engineers. Now enter the wood beam span chart.
Gravity doesn’t give a fuck what you think; if you do the math incorrect, the house collapse or sags until it cracks through the drywall. And that’s why you don’t choose the largest available piece of lumber without looking at the chart (which shows how different beam sizes handles certain loads). You choose balance.
Why You Need Wood Beam Span Charts
Take a 14-foot-long beam of Douglas Fir. Standard load? Sure. Swap out the fir for Southern Pine and you’ve got another inch or two to work with. Doesn’t sound like much, but in framing, every inch count, meaning that a support column can be placed where it fit into an odd space.
And the data compares what’s called dead load (the weight of permanent loads such as floors and walls) and live loads (people walking around). For most residential structures, floor design assume a live load of forty pounds per square foot. Now multiply that by a television set, a sofa, and the dinner guests you’re hosting, and then think about how much weight that is. This is something most folks don’t think off.
Strength in beams isn’t necessarily stiffness. You can have a beam that’s strong enough to support the load with no issues; however, it may bounce like a trampoline as someone walk over it. The deflection limits on chart use ratios, which are typicaly L/360 for most floors. This ratio ensures the beam doesn’t sag more than its span divided by three hundred and sixty. That way the beam doesn’t sag more then its span divided by three hundred and sixty.
If you’re putting down heavy ceramic tile, you’ll want an even stricter deflection limit such as L/480 since tile don’t like to move. It’s a little thing but it makes a difference in how comfortable your guests feel. No one wants their guests thinking they’re standing on a deck when it’s supposed to be a solid floor.
But there’s a whole other side of engineered lumber: LVL beams. As you can see in the tables, an LVL beam with a width of just 1-3/4 inches will carry the load across more than twenty-five feet; no way would you achieve that with conventional sawn lumber. Because they’re fabricated to be uniform (no knots or other irregularities in the wood), LVL are stronger. More costly, sure. But at longer spans, it frees you from creating a giant wooden box in your house, one that impede air flow and sunlight. You’re trading money for space.
For short runs, sawn lumber is perfectly adequate, particularly when time or budget is limited and you want some off cuts from the local yard. It is easy to notch onsite and widely available. But for huge loft or when you’re attempting to open up a vaulted ceiling, mother nature has some practical limitations on wood.
Builders make common errors failing to account for the bearing points. Even with the right sized beam you can crush them and bring down the house if they’re sitting off three inches from the wall plate or the supporting post. That’s not a material mistake, that’s a structural one.
And never substitute species unless you check the grade stamp. Not all soft woods is created equal. Douglas Fir is stronger than Hem-Fir, so you could of knock out fifteen percent of your allowable span. This is a dangerous difference.
If you’re calling an engineer for your complicated remodel project or if you’re eyeballing a simple span table for an approximate figure, it doesn’t realy matter how you get there. What matters is getting there and creating a house where you walk on something solid, not something that sways and squeaks. The foundation won’t improve with age. Do it once and do it right.
