How much weight can a 2×4 hold? There’s a meme going around social media showing the differences in 2x4s over the years, decrying how old houses were stronger. But appearances can be deceiving. Modern 2x4s are probably stronger than you think.
Under optimal conditions, a single 2×4 can support about 1,000 pounds, or half a ton vertically. And when we build a house, we don’t ask it to support much more than that horizontally.
How much weight can a 2×4 stud hold vertically?
Any time someone points out in a model train forum that a 2×4 can hold 1,000 pounds of weight, the doubters swarm. But consider trees. A 50-foot pine tree with a 12-inch diameter weighs 2,000 pounds. A literal ton. If a pine tree couldn’t hold its own weight, it couldn’t survive as a species.
One reason people doubt the strength is because wood isn’t equally strong in all directions. We know wood is pretty strong, but we’ve seen humans break it. We’ve seen martial artists break boards, and we’ve seen baseball players break bats. Bo Jackson was famous for breaking his bat over his own leg after striking out, without injuring himself in the process.
Wood is weakest along the grain, a fact martial artists take advantage of. When they break a board, they break it in the same direction as the grain. It’s strongest on the end grain, the rough edge on each end of the board. That part is difficult to glue and has poor side-to-side strength when you drive nails or screws into it, but it supported the weight of the tree it was part of, so a series of them can support your house.
Your house weighs about 200 pounds per square foot. That’s why your house can stand with four exterior walls and a relatively small number of load-bearing walls in the middle. It’s also why the studs in your wall are probably placed 16 inches apart. That spacing is far more than enough to support the weight. The extras provide enough bracing to deal with factors like wind.
How much weight can a 2×4 stud hold horizontally?
A 2×4 is much less strong horizontally. An 8-foot length of 2×4 that can support 1,000 pounds vertically will sag unacceptably under 200 pounds vertically without any support in the middle. Technically it can hold the weight, but it will sag three inches toward the middle, which is much more than acceptable. An acceptable amount of weight for a 2×4 to hold horizontally is more like 20 pounds.
What about that old 2×4 in the photo that was made of oak and was a full 2 inches by 4 inches in dimension? That idealized board will sag 3/4 of an inch in the middle. That’s better, but still not an acceptable amount without bracing. It can hold about 70 pounds without bracing.
The species of wood matters much more horizontally than it does vertically. But when we build a house, we make up for the difference with bracing. We’re not asking the 2x4s in our walls to span 8 feet, we’re asking them to span 16 inches. And a 16-inch span of cheap pine can hold 3,250 pounds without sagging. So when we brace every foot and a half or so, like we do when we build houses, the extra strength of hardwood is wasted anyway. Maybe it’s 3.5 times as strong, but we don’t need the strength.
Are the 2x4s in old houses better?
A 2×4 from the 1920s was built of old growth wood, and probably whatever trees were growing in the area, so it probably wasn’t all pine. The boards were also thicker, because people measured them before planing them smooth. In that regard at least, the old houses are stronger, assuming all else is equal. Having gone over a dozen houses with a building inspector, I’ve learned all else wasn’t always equal. Work crews took shortcuts 100 years ago just as they do now, and even though the materials were marginally better, the inspection wasn’t.
Why modern 2x4s are skimpier than old ones
There’s a simple reason why modern lumber is skimpier than older lumber. People don’t want to pay for it. We have a better understanding today of the minimum required when it comes to material strength and size. Framing walls with hardwood is overkill, and old growth hardwood is scarce because we did a poor job of conserving it over the past 400 years. So we frame our walls with new growth softwoods like pine, fir, and spruce. It’s good enough for the application and much cheaper. Hardwood then goes into making things like furniture, where its quality is better appreciated.
I don’t know about you, but when I look around, I don’t see a rash of houses falling down, at least under normal circumstances. We don’t build houses to withstand the direct impact of a tornado because almost no one could afford them. It’s more economical to build them the way we do, then have all of us buy insurance to spread out the risk of tornado damage. When a house falls over, it’s news. And typically when a house does fall over, at least in St. Louis where I live, it’s an old, neglected house that’s been abandoned for years. And in the end, the quality of the lumber that went into it isn’t enough to save it when there’s no one around to take care of it.
And in the end, how much weight a 2×4 from 1920 can hold compared to how much weight a 2×4 from 2020 can hold doesn’t matter as much as it seems. The softwood 2×4 of today is almost as good and costs less than half as much to produce. If it supports 25% less weight, we can simply add 25% more boards to make up the difference and come out ahead. This meme laments a problem that doesn’t really exist.