How to Tell If You Have a Slab Leak: 9 Warning Signs

A slab leak is one of the few plumbing problems you usually can't see. The pipe is buried inside or beneath your home's concrete foundation, so the water goes into the soil, not onto your floor. Most Texas homeowners only find out when something else changes: the bill, the feel of the floor, a sound, a smell. This guide walks through the nine signs that matter most, how to tell which ones point to a slab leak and which have a simpler explanation, and the one test you can do yourself before you pay anyone.

Why slab leaks are hard to spot

Most homes in Texas are built on a concrete slab. The water supply lines and the drain lines either run through the slab or sit in the soil just beneath it. When one of those pipes develops a pinhole, a cracked joint or a split, the water escapes downward and sideways into the ground. It can run for weeks or months before any of it shows up inside the house.

That is why the early signs are indirect. You're not looking for a puddle. You're looking for evidence that water is going somewhere it shouldn't, or that something under the floor has changed temperature or moisture.

Sign 1: Your water bill jumped and your habits didn't

This is the most common first clue. A pressurized supply line leaks 24 hours a day, so even a small leak adds up. If your bill rises by a noticeable amount and you can't connect it to guests, a new pool, a hot summer of watering or a running toilet, take it seriously.

One high bill can have many causes. Two high bills in a row with no explanation is a pattern worth testing.

Sign 2: The water meter moves when everything is off

This is the closest thing to proof you can get on your own. Your meter has a small leak indicator, usually a little triangle, star or dial that spins at even the tiniest flow. If it moves while every faucet, appliance and irrigation zone is off, water is leaving your plumbing.

We walk through the full test below. If you only do one thing after reading this article, do the meter test.

Sign 3: A warm spot on the floor

A patch of tile, wood or carpet that feels warm when the rest of the floor doesn't is a classic sign of a leaking hot-water line under the slab. Hot water escaping into the soil warms the concrete above it. Pets often find these spots before people do, so if the dog has started sleeping in one particular place, check it with your hand or your bare feet.

Warm spots tend to show up between the water heater and the kitchen or bathrooms, because that is where hot lines run.

Sign 4: The sound of running water when nothing is on

In a quiet house, late at night, you may hear a faint hiss or trickle in a wall or under the floor. If every fixture is off and you still hear water moving, a pressurized line may be leaking. Sound travels well through concrete and pipes, so the noise isn't always right above the leak, but it tells you something is flowing.

Sign 5: Your water heater runs constantly

A water heater that seems to fire up far more often than it used to, or never quite catches up, may be refilling a hot-water line that is leaking. The heater is doing its job, heating new water to replace what's escaping. Combined with a warm spot or a higher gas or electric bill, this points strongly at the hot side of the system.

Sign 6: Damp carpet, cupped wood or loose tile

Water under the slab can wick up through the concrete. You might notice carpet that feels damp along one wall, padding that smells musty, hardwood boards that cup or lift at the edges, or tile that starts to sound hollow. These signs usually appear near plumbing walls, at the edges of rooms or close to bathrooms and the kitchen.

Before blaming the slab, rule out the obvious: a leaking toilet seal, a tub drain, a dishwasher connection or an AC condensate line can produce the same symptoms.

Sign 7: A musty smell or mildew you can't trace

Moisture that stays trapped under flooring or behind baseboards eventually smells. If a room smells musty even after cleaning, and especially if the smell is strongest near the floor, hidden moisture is a likely cause. The EPA advises drying wet materials within 24 to 48 hours to prevent mold, and a hidden slab leak keeps them wet much longer than that.

Sign 8: A sewer smell or slow drains

Not every slab leak is on the water supply side. Drain lines under the slab, especially cast-iron drains in homes built before about 1980, can crack or rust through. A broken drain lets sewer gas and moisture into the soil beneath the house. Signs include a sewer smell inside, drains that are slow in more than one fixture, and damp areas near bathrooms.

Here's the catch: a drain leak never shows on your water meter, because the water has already been used. If you smell sewage and your meter test is normal, the drains need their own test. Our guide to how slab leak detection works explains the hydrostatic test used for drains.

Sign 9: New cracks or doors that stick

Water under a slab can wash out or soften the soil, letting one part of the foundation settle. New cracks in tile, drywall or brick, or doors that suddenly stick, can be a sign of that movement. They can also be caused by ordinary clay movement, which is very common in Texas. The point is not to panic, but to test the plumbing before paying for foundation work.

The 2-hour meter test

You can do this with nothing but your water meter and a phone camera.

1. Turn everything off. Faucets, showers, the dishwasher, washing machine, ice maker, water softener and every irrigation zone. If you have a pool with an auto-fill, turn that off too.
2. Find the meter. In most Texas neighborhoods it sits in a box near the street or sidewalk. Lift the lid carefully; spiders and wasps like these boxes.
3. Record the reading. Take a photo of the numbers and the leak indicator.
4. Wait one to two hours without using any water at all.
5. Read it again. If the numbers changed, or the leak indicator is moving, water is escaping.

To narrow it down further, close your home's main shut-off valve (usually where the line enters the house) and watch the meter again. If it still moves, the leak is in the yard line between the meter and the house. If it stops, the leak is inside the house, and the slab is a strong candidate.

What else could it be?

Before booking slab leak detection, check the cheap and common culprits:

• Toilets. A worn flapper can leak silently into the bowl. Put a few drops of food coloring in the tank and wait 15 minutes without flushing. Color in the bowl means a leak.
• Outdoor faucets and hose bibs. Look for drips and wet soil.
• Irrigation. A stuck valve or broken head can waste a lot of water.
• Water softeners. A softener stuck in its regeneration cycle can keep running water to the drain.
• The water heater. Check the relief valve discharge pipe and the base of the tank.

If all of these check out and the meter still moves, the leak is hidden, and finding it is a job for detection equipment.

How a slab leak is actually found

A licensed plumber who specializes in leak detection follows a set process: confirm the leak with the meter and a pressure test, isolate which line is leaking, trace that line's route under the floor, then pinpoint the exact spot with acoustic listening, thermal imaging or tracer gas. The spot is marked on your floor before anyone thinks about opening concrete. You can read every step in plain English on our Our Process page.

Detection typically costs $150 to $600, depending on the size of the home, how many lines need testing and whether drains need a hydrostatic test. You get the price before the visit and a written report at the end, which is useful for insurance and for asking your water utility about a leak credit.

When to call right away

Some situations can't wait for a meter test:

• Water coming up through the floor
• A strong sewer smell inside the home
• The meter spinning quickly with everything off
• A large area of floor that is wet or warm

Shut off the main water valve and call the same day. For slower signs, such as one warm spot or a bill that has crept up for two months, book detection within the week. Water under a slab doesn't fix itself, and the longer it runs, the more soil it can move.

The bottom line

One sign on its own is a reason to look closer. Two or more, or a meter that moves with everything off, is a reason to have the lines tested. The goal is simple: find the exact leak before anyone breaks your floor looking for it. If you're in Texas and want a licensed plumber to test your home, see the cities we serve or call us to talk through what you're seeing.