Learning Center · Diagnosis
Why Is Water Coming Up Through My Concrete Floor?
WHERE WATER ON A SLAB FLOOR USUALLY COMES FROM
| What you notice | Likely source | Who to call |
|---|---|---|
| Wet in dry weather too, or a warm spot | Supply line leak under the slab | Licensed plumber |
| Damp after showers or laundry, sewer smell | Drain line leak under the slab | Plumber, with a hydrostatic test |
| Wet along outside walls after storms | Rain or irrigation getting under the edge | Drainage correction |
| No puddles, but flooring lifting or musty carpet | Vapor moving up through the concrete | Flooring installer |
| Film on tile on a muggy morning | Condensation from humid room air | Lower the indoor humidity |
Why is water coming up through my concrete floor? In a Texas slab house it usually comes from a supply or drain line leaking under the slab, rain or irrigation water getting under the slab edge, moisture moving up through the concrete itself, or humid air condensing on a cool floor. The timing tells you which. Water that is there in dry weather, or a warm wet spot, points to a plumbing leak and a licensed plumber. Water that shows up after storms along the outside walls points to drainage. A damp film on a muggy morning is condensation, and the foundation has nothing to do with it.
Start with when and where the water shows up
Before anyone tests anything, write down when the floor gets wet and exactly where. Timing sorts the sources faster than any tool.
Water that is there in dry weather and wet weather alike, and comes back after you dry it, is coming from something that runs all the time, and under a slab that means a pressurized supply line. A warm spot on the floor, often noticed barefoot or by a pet that suddenly loves one patch of tile, points to the hot water line. Water that shows up only after storms, or after the sprinklers run, and sits along the outside walls is coming from outside. A damp sheen across the floor on a humid morning, beading on tile or sealed concrete, is condensation. Musty carpet, or flooring that lifts with no puddles at all, is usually moisture moving up through the concrete.
Rule out the sources above the floor before you blame the slab. A water heater pan, a washing machine hose, a toilet seal, a dishwasher line or an air conditioner condensate drain can all leak onto the floor and wick under the flooring, so the wet spot shows up a few feet from where the water started. Pull the flooring back at the edge of the wet area if you can. Water that is wettest at the concrete and drier on top came from below, and water that is wettest on top came from something in the room.
A simple test separates the last two. Tape a square of clear plastic, about two feet on a side, flat to bare concrete with all four edges sealed, and leave it a day or two. Moisture under the plastic is coming up through the slab. Moisture on top of it is condensing out of the room air.
A plumbing leak under the slab
Every slab house has two plumbing systems under the floor. Supply lines carry pressurized water in, so a pinhole leaks day and night. Drain lines carry wastewater out by gravity, so they leak only when water runs down them, which is why a drain leak often shows up as a damp spot that gets worse after showers, laundry or the dishwasher, sometimes with a sewer smell.
The water meter is the first check for the supply side. Shut off every fixture in the house, including the ice maker and the sprinklers, and watch the small leak indicator on the meter for 15 minutes. If it turns with everything off, water is going somewhere. The drain side needs a plumber's test, and our guide to the hydrostatic plumbing test explains how it works and what a failed result means. Finding and fixing the pipe is a licensed plumber's job, and in Texas that license comes from the Texas State Board of Plumbing Examiners.
Our part is reaching the pipe. When the leak is under the slab, a licensed master plumber does the plumbing work, and our crew digs the tunnel from outside or opens the breakout that gets the plumber to it, then backfills it in compacted layers. A tunnel leaves the floors inside untouched, and under-slab tunneling covers how that works.
Rain and irrigation getting under the slab
When the floor gets wet after storms, start outside. Look for downspouts emptying against the wall, soil or flower beds sloping toward the house, beds closed in with edging that hold water against the slab, and sprinkler heads spraying the foundation. Water that ponds against the slab edge soaks down beside the beam, and some of it finds its way under the concrete, then up through cracks in the slab, around pipes where they come up through the floor, or at the bottom of exterior walls.
The ground under a slab can hold that water in place. The Foundation Performance Association's maintenance and inspection guide describes how dense clay with a more porous layer above it can trap water in that upper layer. Many slabs sit on just that arrangement, a layer of fill over native clay, so water that gets in at the edge can sit under the floor after a heavy rain. The same guide lists leaking plumbing, dripping air conditioner condensate lines and misdirected gutter water as the local wet spots that cause uneven foundation movement, and says to fix them as soon as possible.
The fixes are outside the house. Downspout extensions that discharge at least six feet out, a grade that falls at least 6 inches in the first 10 feet, beds that drain away from the wall, and sprinklers aimed at the lawn instead of the slab. When water keeps arriving below the surface after the simple fixes, that is drainage correction, with regrading, area drains or a french drain scoped to where the water comes from.
Moisture through the slab, and condensation on top of it
Concrete is not waterproof. It passes water vapor slowly from the damp ground below toward the drier room above, which is why codes call for a plastic vapor retarder under a house slab. The residential code San Antonio adopted requires one between the slab and the ground beneath it, with the seams lapped at least 6 inches. Older slabs may have a thin one, a torn one or none at all.
Vapor rarely makes puddles. What it does is damage what sits on the concrete. Glue-down flooring lets go, wood floors cup, carpet smells musty, and white mineral salts show up along cracks and at the edges of the room. Those are flooring problems, and a flooring installer can test the slab and choose a floor or a sealer that suits it. They are not a sign the foundation moved.
Condensation is the other harmless one. A tile or sealed concrete floor runs cooler than the room, and on a muggy day, or when warm, wet air arrives right after a cold snap, water beads on it the way it beads on a cold glass. It shows up across open areas and near doors, mostly in the morning, and goes away when the air dries or the air conditioning runs. Lower the indoor humidity and it stops.
When the water is also moving the foundation
Water under a slab does more than wet the floor. Our clay swells as it takes on water, and a leak that keeps one patch of clay wet under the middle of the house can push the slab up into a hump over it. Inside, that shows up as interior doors that start sticking, a rise you can feel crossing a room, and cracked tile spreading out from one area. We cover that pattern in leak heave. Water running under the edge can do the opposite, carrying soil out until the slab corner has nothing under it.
The order matters, and the water comes first. Fix the water, survey the floor elevations, give the wet clay a few weeks to dry back, and measure again before anyone talks about piers. Much of the heave from a leak eases on its own once the water stops, and piering a slab while it is still swelling locks the problem in. The full sequence is in foundation repair after a plumbing leak.
What to do today
If you suspect a supply leak, shut off the main water valve until a plumber can test it. Dry the floor, take dated photos of the wet area, and note when it comes back. Check the meter. Walk the outside of the house during the next rain and watch where the water goes. If flooring or walls are getting wet, call your insurance agent early, because sudden water damage is often covered even when the pipe itself is not, and does homeowners insurance cover slab leaks walks through what a Texas policy usually pays for.
Where we fit
We are a foundation company, not a plumbing or flooring company. We dig the tunnels plumbers work in and close them back, we correct the drainage that puts storm water against the slab, and when the water has moved the foundation, we measure it and fix it. The free inspection includes a ZipLevel elevation survey at 40 or more points, read to a tenth of an inch, plus a walk of the drainage, so you learn whether the slab moved and where the water is coming from. About a third of our inspections end with no repair needed. If the problem is a pipe, a gutter or a damp slab under new flooring, that is what we will tell you.
The inspection is free in Bexar, Travis, Williamson, Hays, Comal, Guadalupe, Kendall, Medina, Bastrop and Caldwell counties. Book it below or call (210) 816-0034.
