How can they build houses in the rain?

I live in Montana, and home building slows down during the winter as you might imagine, but it still goes on.

As the snow turns to sleet, and the sleet turns to rain, they continue construction. Sometimes they will throw tarps over the structure to try to keep it dry, but for the most part they don’t seem to care. It seems to me building a house in bad weather is always a bad idea. What am I missing here?

My specific question are:

  1. If you are framing a house in rain doesn’t the lumber get soaked, and after it stops raining and the lumber dries out won’t it warp? Perhaps it doesn’t warp for some reason, or it doesn’t warp enough to matter, but then why do they sometimes cover up the house as if they are trying to keep it dry?

  2. How can you frame a house if the poured concrete foundation is not 100% dry? Doesn’t concrete have to cure before it is strong enough to support framing?

Also in MT, formerly in CO. Short answer to #1 is yes to a certain extent, but materials are getting more water resistant. It used to be that man-made materials like OSB and plywood tended to swell when wetted, but they are much better at this point. Dimensional lumber can warp, but when in an assembly not that much more than it would by natural drying, if at all. Working in the rain kind of sucks, and the danger for a shock or a slip and fall increase. In CO we might roll it up for rain (not for snow), but if you’re building in Seattle you keep going. You can rig protection like tarps and such, but have to beware of trapping moisture once the storm clears. EZ-up tents are getting popular for cut stations and the like.

Concrete cures great when wet. Better, in fact. It will cure just fine under water.

And wood getting wet is mostly a problem when it’s completely submerged, not just had water sprinkled on it. Most of the surface area of the framing pieces will be vertical, and so will dry fairly quickly, before much soaks in, once the rain stops.

At the architecture form where I work, our specs require that OSB not be used on any horizontal surfaces, and since OSB is basically wood chips glued together, it absorbs water and swells more than plywood. AdvanTech subflooring is OSB with better resins and manufacturing so it performs a lot better in that regard. For vertical surfaces and framing that doesn’t collect water, it’s not such a big deal.

In fact, contractors and engineers tend to prefer OSB for wall sheathing since you can get it in 4’x10’ sheets (with custom sizes up to 24’ long) instead of the usual 4’x8’, so it’s easier to build shear walls and less edge nailing is required. Anyway, it’s unlikely that it will be raining constantly, and once the house is under roof then the moisture can equalize while the rest of the construction goes on. There can be months between the roof going on and drywall being installed.

As for concrete, like Tride said, it cures rather than dries. It’s a chemical reaction that depends on water as one of the inputs. 28 days is the usual design-time for cured concrete, but that’s more applicable for tightly designed structures like bridges and high rises where they’re trying to minimize weight and maintain the most efficient structure possible. It generally reaches more than half its design strength after a week, and it continues curing and strengthening indefinitely, though for all practical purposes it’s done after about two to three months.

Slabs can be built on after just a day, and even foundation walls can have their forms stripped off after this time, but generally it’s best to leave them for a week before framing. Even at half strength, it’s still going to take more than the three weeks required to get to its design strength before it’s fully loaded. While they could finish the framing in that time, you’re not going to see drywall, tile, hardwood, or other finishes going in that quickly. Plus with the design safety factor, they could probably drop a finished modular house on top of a foundation that’s only cured for a week and a half.

Never thought about warping - what I’ve always wondered about is mold growth on damp wood.

Lumber doesn’t warp because it gets rained on. It warps because it was poorly stickered (stacked for curing), but not from rain.

Concrete is improved in strength the longer it stays damp. It will harden under water. You don’t want it to freeze, though, while curing.

A house can be framed in a few days and once the roof is sheathed it will be dry enough for the generality of weather.

Finish materials like sheetrock, flooring, etc. – those are a different story entirely. That’s why the house has to be made safely dry inside before finish work can commence.

I don’t think I’ve ever seen a house framed and then tarped so it doesn’t get rained on. It can sometimes be weeks from framing till the roof get’s put on, and even then the shingles may not be added for some time.

If residential home or multifamily home builders were limited to only building when there wasn’t going to be rain, then hardly nothing would get built.

I framed lots of houses back in the day. Rain doesn’t hurt anything already built, usually, because everything is nailed together which keeps it straight for the most part. Sometimes after a lot of rain interior studs will twist, good builders will fix those before drywall. Trusses and rafters can twist as well, but those can be worked with. We were more concerned about lumber in unbanded bundles, if those loose boards get soaking wet a lot of them will be useless when they dry.

The only time I saw something ruined was a subfloor after an extended period of rainy days. The top layers delaminated. Glad I wasn’t doing flooring in that place.

If the rain doesn’t stop the builders, then pretty quickly the structure will be complete - plywood roof keeping most of the rain out of the interior. So what if it leaks a bit? A few decent days and it dries up. Once the finishing is applied -shingles, siding, the wrap underneath all that - there is no more moisture getting in and the rest will dry quickly. Mold is a problem with persistent moisture, not a one-time soaking that then dries up. And as mentioned, except fo a flat floor, water will drain off everything else so no soaking in standing puddles - we hope. For the DIY builders who may build a piece at a time at their won pace and leave the wood exposed for weeks or months it may be a different story.

Pressure-treated lumber kills almost everything that might try to grow on it. Never burn pressure-treated lumber—the stuff that kills mold isn’t good for your lungs, either.

Thanks everyone. Ignorance fought.

Who’s building houses with pressure treated lumber? The only time I see anything other than doug fir used around here for framing is engineered stuff like glulam beams.

We always used it for the sill plate that gets bolted down tight to the concrete foundation. Beyond that narrow use. I can’t think of any need for treated lumber when framing a house.

I think I inadvertently implied something I didn’t mean to say. I was responding narrowly to the concern about mold growth on damp wood. My thought was, “Concerned about mold on damp wood? Pressure-treated lumber solves that problem!”

You know, like, for a deck.

But the whole thread is explicitly about house framing, and, well, I kind of forgot. :slight_smile:

I think that pressure-treating and chemical-treating are actually two different things, though often used together. Wood that’s only pressure-treated wouldn’t be any more dangerous to burn than normal wood. Chemical treating is also generally avoided for wooden playground equipment, if you can find that any more.

I’ve never heard of pressure treating without chemical treating. It’s almost all done with copper arsenic compounds and you wouldn’t want to inhale smoke from those. Most are considered safe for contact with proper finishing. Wood that is only chemical treated has a higher concentration of chemicals, and much more on the surface, so it’s not safe for contact and thus not allowed for playground equipment, and now many other uses based on the chemicals used.

To build a house in the rain in Spain, abstain from the plain.