Why Is There Water in My Crawl Space?

Why is there water in my crawl space? Something is feeding it, and in Central Texas it is almost always rain getting under the house from downspouts, grading or a crawl floor that sits lower than the yard, a leaking supply or drain line, or clay that stayed soaked after a long wet stretch. When the water shows up tells you which of those you have. Fix the source outside first, because plastic sheeting and pumps only manage water that is already under the floor.

We are a foundation company. We level pier and beam houses, replace sills and shims, install steel and hybrid piers, and correct the drainage that keeps water away from all of it. Some crawl space water belongs to other trades, and this guide says so where it does.

What the timing of the water tells you

Write down when you found the water and what the weather had been doing that week.

Water that appears within hours of a hard rain, deepest along one wall, is surface water. It came off the roof or across the yard and ran under the skirt or the perimeter beam on that side. Look straight up the wall from the wettest spot and you will usually find a downspout, a gutter corner that overflows, or a flower bed edged higher than the soil around it.

Water that shows up days after a long wet spell, spread through the low spots across the whole crawl space, is water sitting on the clay. The ground around the house is full, and the crawl floor is the lowest place on the lot. The house is simply the bottom of the bowl.

Water in dry weather is plumbing until proven otherwise. A wet spot that stays wet through a dry week, in the same place, under a bathroom, the kitchen or the water heater, is a leak. Water that appears right after a hard freeze is a split pipe until a plumber tells you otherwise. And a steady drip under the air handler or the ductwork on a muggy August afternoon is condensation or a condensate drain, which is a call for your HVAC contractor.

Why winter rain soaks Central Texas crawl spaces

Wet crawl spaces here tend to show up in late fall and winter, and the reason is the soil. Much of the clay around San Antonio belongs to the Houston Black series, which the USDA describes as very slowly permeable with high shrink-swell potential. You can look up the soil under your own address in the USDA Web Soil Survey. In a dry summer this clay shrinks and cracks. The USDA series description puts those cracks at half an inch to four inches wide, running a foot or more deep, and open for 90 to 150 days in most years. Texas A&M AgriLife Extension describes the same family of clays across the Blackland Prairies, rich in a mineral that swells when wet and shrinks when dry.

Then the first fronts of the season arrive. The same USDA description notes that water enters this soil rapidly while it is dry and cracked, and very slowly once it is moist. So the first soaking rain after a drought pours straight down those cracks, and cracks along the house can carry it under the perimeter beam into the crawl space soil. A few weeks later the clay has swollen shut and turned close to watertight. The next rain sits on top and runs to the lowest place it can find, and on a lot of older raised houses that place is the crawl floor.

Winter keeps it there. Short days and cool air mean very little evaporates, and water that would dry in a week in June can sit for a month in January. Our winters are mild and hard freezes are rare, but they do come. Winter Storm Uri in February 2021 split pipes under raised floors all over the region and left slab houses with leaks that kept running long after the thaw. Frost depth here is shallow, and plumbing in this part of Texas was rarely built with deep cold in mind. Under a raised floor the pipes hang in open air near the vents, and our guide on how to keep pipes from freezing in a crawl space covers the nights that matter.

The outside sources to check, starting at the wall

Walk the outside of the house during a steady rain, as long as there is no lightning around. Ten minutes under an umbrella teaches more than an afternoon under the floor.

Start with the downspouts. A downspout that ends at the wall, or on a splash block that cracked or slid away years ago, drops the water from that whole section of roof at the foundation. On a raised house it does not stop at the wall. It runs along the top of the clay and under the skirt. We want every downspout discharging at least six feet out, and our guide on how far downspouts should drain from the foundation explains why that distance works on clay.

Check the grade next. Section R401.3 of the International Residential Code asks for the ground to fall at least 6 inches within the first 10 feet away from the foundation. Years of mulch, sod and flower bed edging tend to build the yard up against the skirt until it slopes toward the house instead. A level on a straight board tells you which way yours runs. Holes and washouts along the wall, whether from armadillos or old runoff, act as small funnels of their own, and filling holes around a foundation properly takes compacted clayey soil, not sand.

Then look at the crawl floor itself. On some older raised houses the crawl floor sits below the yard, either dug out when the house was built or left behind as the yard built up around it. Where surface water does not readily drain from the site, Section R408.6 of the same code wants the ground under the house at least as high as the outside grade, unless an approved drainage system is provided. A crawl floor a foot below the yard is a basin, and every wet season fills it.

Last, look at what else drains there. An AC condensate line, a water heater relief line or a sprinkler head aimed at the skirt can each feed one wet spot all year long.

Plumbing leaks under a raised floor

If the water is there in dry weather, check the plumbing before you spend a dollar on drainage. A supply leak runs all the time, because that line is always under pressure. A drain leak only runs when something drains, so it shows up after showers, laundry or the dishwasher.

The meter test catches supply leaks, and anyone can do it. Turn off every faucet, the ice maker, the irrigation and the washing machine. Find the water meter and watch the small leak indicator on its face, usually a little triangle or star. If it turns with everything off, water is leaving the system somewhere. For a slower leak, write down the reading, go an hour without using any water, and read it again. Any change means a leak.

A drain leak takes a look instead. Run a full tub or a load of laundry, then go under the house with a flashlight and look at the soil below that fixture. Drain leaks often smell, too. If you go under, tell someone first, and stay out if the water is deep or touches any wiring, an outlet or a pump.

Either way, the repair belongs to a licensed plumber. Motmot is not a plumbing company. When one of our jobs needs plumbing work, a licensed master plumber we hire does that part. The good news on a pier and beam house is access, since the lines usually hang in the crawl space where a plumber can reach them with no tunnel and no slab breakout. If the house sat over a leak for months, though, the clay under it may have swollen or settled, and measuring that part is our job.

What standing water does to piers, sills and clay

A puddle that drains in a day or two is a warning. Water that stands for weeks starts taking the house apart from the bottom up.

Wood goes first. Sill plates and beam ends sit lowest and closest to the wet soil, and wood that stays above roughly 20 percent moisture content can rot. A softened sill crushes under the load it carries, and the joists bearing on it settle with it. Wet wood also draws subterranean termites, and our guide to wood rot versus termite damage shows how to tell the two apart. The EPA moisture and mold guidance says to dry water-damaged materials within 24 to 48 hours to keep mold from growing. A crawl space that stays wet for a month is far past that.

The steel suffers too. Steel shims and pier hardware rust in damp air, and a rusted shim stack can slip long before anything looks broken.

Then the soil moves. The clay under the middle of a raised house usually holds a fairly steady moisture, because the floor shades it from sun and rain. Soak it and it swells, and the interior piers can ride up while the perimeter stays put. Moving water also carries soil away, and a channel running under the perimeter beam can wash out the support along its edge. Inside the house it shows up as a floor that bounces near an outside wall, or doors that start sticking after the wet season. When the framing has already gone soft, floor joist and sill repair covers what replacing it involves.

The order we fix it in

Every fix for crawl space water sits somewhere between cheap work outside the house and expensive work underneath it, and the order matters. Anything you do inside sits on top of whatever water is still arriving.

Downspouts and gutters come first. Extensions that carry the water at least six feet out cost little and often solve the whole problem on their own.

Regrading comes next. That means rebuilding the soil along the house so it falls away at 6 inches in 10 feet, using compacted clayey fill rather than sand or loose topsoil.

Where the lot will not cooperate, like a side yard squeezed between two houses or a neighbor's yard that drains onto yours, a French drain or a swale catches the water before it reaches the skirt. Our drainage correction page covers how we choose between them. A sump pit with a pump is for the last bit, when the crawl floor sits below the yard and there is no way to drain it by gravity.

Only after the water stops arriving does a ground cover make sense. A polyethylene sheet over damp soil slows the moisture rising into the joists, and on some houses it is a cheap, sensible addition. Laid over water that is still coming in, it hides the problem and keeps the wet trapped under the plastic.

Vents come last. On most raised houses here they can stay open almost all year, because once the water stops arriving, moving air is what dries the soil and framing back out. Close them on hard-freeze nights and open them again after. The exception is a muggy stretch when the outside air is plainly wetter than the air under the floor. Our guide on whether crawl space vents should be open or closed goes through the seasons.

Full encapsulation and crawl space dehumidifiers treat humid air, not water that is still running in, so for a wet crawl space here they are rarely the first answer. Motmot does not sell either one. If you want one, that is a job for an insulation or HVAC contractor, and so is wet insulation sagging out of the floor or a duct that sweats. Our side of it is the drainage, the framing, the piers and the level of the floor, and our crawl space moisture control page lays out that scope.

How we assess it on a free inspection

On a raised house the free inspection includes going under the floor. We look at where the water sits and how high it has marked the piers, probe the sills and beam ends for soft wood, check the shims, and take floor elevations read in tenths of an inch. Outside we walk every downspout, check the grade along each wall and note where the yard drains. If the pattern points to plumbing, we tell you to call a plumber before anything else.

About a third of the time there is no repair to recommend, and a wet crawl space can land there too. If the answer is a downspout extension and some fill dirt, we say so. If the water has already softened the framing or moved the piers, you get a written scope in the order above, with the drainage first.

We inspect homes in Bexar, Travis, Williamson, Hays, Comal, Guadalupe, Kendall, Medina, Bastrop and Caldwell counties, and in that area the inspection is free. Book it here or call (210) 816-0034. And if the water came with a storm and new cracks in the walls, our guide to cracks after heavy rain covers what to check next.