Signs of a Slab Leak (and How to Confirm One)

A slab leak is a supply or drain line failure under or inside the concrete slab your house sits on. There is no visible path for the water to reach you, so the symptoms arrive indirectly and in a fairly reliable order. Catching it in the first group is the difference between a plumbing repair and a plumbing repair plus flooring plus remediation.

This Old House groups the signs the same way in its slab leak guide updated April 7, 2026, listing a slight increase in the water bill, faint sounds of running water when fixtures are off, and minor drops in water pressure as the early signals; damp or warm areas on the floor, mold or mildew, and visible water damage under flooring or carpet as the more noticeable ones; and warped flooring, cracks in the foundation and persistent moisture around the home's perimeter as advanced damage.

• Early: a water bill that steps up and stays up. Not a one-month spike, but a new, higher normal with no change in household use.
• Early: the faint sound of running water, hissing or rushing when the house is silent and nothing is switched on. Easiest to hear at night.
• Early: a small, general drop in water pressure throughout the house rather than at one fixture.
• Mid: a warm patch on the floor. A leak in a hot-water line heats the concrete above it, and on tile you can often find the spot with the back of your hand or a bare foot.
• Mid: damp spots, or carpet and flooring that feels cool and slightly wet in one area with no spill to explain it.
• Mid: a musty, earthy or mildew smell in one room, or mold appearing in a corner with no water source above it.
• Mid: the water heater running more than it used to, if the leak is on the hot side and it is constantly reheating water going into the ground.
• Late: flooring that warps, cups, lifts or loosens - wood swelling, tile grout cracking, vinyl bubbling.
• Late: new cracks in the slab, in interior walls, or doors and windows that suddenly stick as the foundation moves.
• Late: persistent moisture or soft soggy ground around the exterior foundation line, or an unexplained patch of lush grass against the house.

Every sign above has other possible explanations. The meter test does not: it measures whether water is leaving the system. Run it before you spend anything, because the result changes the conversation with a plumber completely.

1. Turn off every water-using fixture and appliance: taps, showers, dishwasher, washing machine, ice maker. Switch the irrigation controller off, not paused. Ask everyone not to flush.
2. Find your water meter, usually in a ground box near the curb. Open the lid carefully - these boxes attract insects and snakes in Texas - and clean the glass.
3. Watch the low-flow indicator, the small triangle, star or wheel on the dial. If it is turning at all with nothing running, water is escaping continuously. That is a confirmed leak.
4. If it is still, write down the full reading, wait thirty minutes with everything off, and read again. Any change means water moved.
5. Now locate it roughly. Close the main shut-off valve at the house and repeat. If the meter stops, the leak is inside or under the house. If it keeps running, it is in the service line between meter and house.
6. Split hot from cold. Close the cold-water inlet valve on top of the water heater and repeat again. If the meter stops, the leak is on the hot side - which in a slab home very often means a hot line under the concrete.
7. Dye-test every toilet before concluding anything. A worn flapper passes water continuously and silently, and it is the most common cause of a mystery bill. Food coloring in the tank that appears in the bowl within twenty minutes without flushing is your answer.
8. Write down everything you found. Those four facts - meter moving, stopped or not with the main closed, stopped or not with the heater isolated, toilets clear - are what a plumber most wants to know.

Several problems produce slab-leak-like symptoms and cost far less to fix. Ruling them out is worth twenty minutes, because nobody wants to pay for concrete work to solve a toilet flapper.

The running toilet is first on the list, and it accounts for a large share of unexplained water bills. It is silent, continuous, and invisible unless you dye-test.

Irrigation is second. A cracked lateral or a stuck zone valve puts water into the ground where nothing shows, and in a hot North Texas summer a damp area near the house reads as normal watering.

A failed pressure regulator is third and the most commonly missed. If static pressure at an outdoor spigot reads over 80 psi, you have a problem that both mimics other faults and actively causes leaks. Our water pressure regulator guide covers the two-minute gauge test.

Then there are the non-plumbing explanations for the late-stage signs. Foundation movement and cracking in expansive North Texas clay happens without any leak at all, driven by drought and rain cycles. Air conditioning condensate lines back up and wet a floor. A hot water line running through the slab on its normal route warms a floor in a corridor without leaking. None of these rules out a slab leak - but a meter that does not move does.

• A running toilet: dye-test every toilet in the house first.
• Irrigation leaks: shut the controller off completely and re-test.
• Appliance supply hoses: dishwasher, washing machine and refrigerator ice maker lines.
• Outdoor hose bibbs and vacuum breakers, which split in freeze-thaw cycles.
• A failed pressure regulator reading over 80 psi, which stresses the whole system.
• A blocked air conditioning condensate line wetting a floor or ceiling.
• Normal foundation movement in expansive clay soil, which cracks slabs and walls without a leak.
• A water softener or filter stuck in regeneration, running continuously to drain.

Slab leaks are far more common in North Texas than in much of the country, and the reasons are geological as much as plumbing.

Expansive clay soil is the main one. It absorbs water and swells, then dries and shrinks, and the Dallas climate delivers that cycle hard - long dry summers followed by heavy rain. The slab rides on soil that is moving, and the rigid supply and drain lines cast into it or running beneath it get stressed at every joint and bend. Over enough cycles, a joint opens or a line develops a pinhole.

Housing stock is the second. The region's enormous postwar and 1960s to 1980s build-out used copper supply lines under slabs and cast iron drains, both now at or past the age where failures cluster. Homes built from the late 1970s to the mid-1990s may have polybutylene instead, which has its own failure mode at the fittings. Newer construction uses PEX and runs it overhead where possible, which is a large part of why new homes see fewer slab leaks.

Water chemistry and pressure finish the picture. Mineral-rich North Texas water scales the inside of lines, and a static pressure above the 80 psi code limit stresses every joint in the system continuously - including the ones under the concrete that nobody can inspect.

The practical consequence is a vicious circle: a slab leak puts water into expansive clay under your foundation, the clay swells, the slab moves, and the movement stresses the remaining pipe. It is the reason a slab leak is worth locating quickly even when it seems small.

Everything above is safe for a homeowner and genuinely diagnostic. Beyond it, the tools and the trade change, and in Texas plumbing work must be performed by a licensed plumber.

• The meter confirms a continuous leak, the toilets dye-test clean, and irrigation is off. That combination needs professional detection.
• The meter stopped when you isolated the water heater, which points at a hot line under the slab.
• You have found a warm patch on the floor, with or without a meter result.
• The meter keeps moving with the house main closed, which puts the leak in the service line.
• Flooring is lifting, warping or cupping, or you can see mold. The leak has been running a while and remediation may be needed alongside the repair.
• New foundation cracks have appeared, or doors and windows have started sticking. Get the plumbing assessed first, because wet soil under a slab is a plumbing cause with a structural consequence.
• Any plumber proposes cutting concrete before locating the leak. Locating comes first, and it is the cheaper half.

Dallas Best Plumber is a free referral service. We do not set prices, we do not perform work, and nothing here is our quote. Below are published figures from independent sources for locating and repairing a slab leak, named and dated.

The important thing the table shows is that detection is cheap relative to repair, and repair varies enormously by method. That is why confirming the leak properly is the step that protects your money.

Dallas Best Plumber is a free referral service, not a plumbing company. Run the meter test, dye-test the toilets, and note whether the leak is on the hot side. Then tell us what you found and we match you with a licensed, insured plumber serving your ZIP who can locate the leak and quote the repair. The match is free and there is no obligation to hire.

Those test results are worth money to you. A plumber arriving with the leak already narrowed to the hot side, inside the house, spends their time confirming rather than searching - and detection time is usually billed.

Our slab leak repair cost guide covers the published figures and the repair methods in more detail, including what homeowners insurance does and does not usually cover. If the meter is running hard and you can see water, shut the main off first; our shut-off guide covers where the valve is in a slab-built Texas home.