Plumbing decision guide / 8 min read
How do you know if you have a slab leak?
Most slab leak pages list the symptoms and then tell you to call somebody. The symptoms are worth knowing, and they do tend to arrive in the same order. But there is also a test you can run yourself in about fifteen minutes, with no tools, and it answers something the symptom list cannot: whether the problem is under your slab at all.
Short answer
The signs arrive in a rough order: a water bill that climbs though nothing in the household has changed, the sound of running water when every fixture is off, then a warm patch on the floor, which only a hot water line can cause. Damp flooring, a musty smell in one room, and falling water pressure follow. None of those confirm anything on their own. The confirmation is your water meter. Shut off everything in the house that uses water, then watch the low flow indicator. If it moves, water is leaving your system. Then close the shutoff where the line enters the house and read the meter again. A meter that keeps turning puts the leak outside, in the service line or an irrigation line. A meter that stops puts it inside your plumbing, which is what makes the slab a candidate.
The signs show up in a fairly consistent order
The water bill climbs first, though nothing in the household has changed. Then you hear water running when nothing is on, usually at night when the house is quiet. Only after that come the signs people actually call about: a warm patch on the floor, a room that smells damp, flooring that cups or lifts. Foundation cracks come last, and they mean the least of any of it.
You do not have to wait for any of it. Go read your water meter. That check needs no tools and no appointment, and it gives you an answer nobody can talk you out of.
What each sign is telling you
| What you notice | What it points at | Weight |
|---|---|---|
| Water bill up, habits unchanged | Continuous flow going somewhere it should not | High. A slab leak runs around the clock, and continuous flow is what a monthly bill picks up best. |
| Running water sound with everything off | Water moving under the floor, or a fixture leaking through | High, but it is a running toilet just as often. Rule that out first. |
| Warm spot on the floor | A hot water line under the slab | High and specific. Cold lines make no warm spot. |
| Damp carpet, cupping wood, hollow sounding tile, musty smell in one room | Moisture arriving from underneath | High, if the ceiling above is dry. |
| Water heater running more, or a recirculation pump cycling constantly | Hot water lost continuously | Medium to high. Pairs with the warm spot. |
| Pressure lower than it used to be | Volume lost before it reaches the fixture | Medium. Also a failing pressure regulator or a half closed valve. |
| Lawn green and soft in August, or water at the slab edge | A line losing water outside the house | Medium. The meter test tells you which line. |
| Cracks in drywall, brick, or the slab | Ground movement | Low on its own. See the soil section below. |
The meter test, step by step
The test takes about fifteen minutes, needs no tools, and separates the four things a slab leak gets confused with.
1. Shut the water off inside. Every tap and shower off, the dishwasher and washing machine off, the ice maker off at the valve behind the refrigerator, the irrigation controller off, and the pool auto fill off. Nobody flushes during the test.
2. Find the meter and read it. It is usually a lidded box at the curb or near the front property line. Look for the row of digits and the small triangle, star, or dial beside them. That is the low flow indicator, and it spins on a flow far too small to move the digits.
3. Watch the low flow indicator. Any movement at all, with the house shut down, means water is leaving your system. A leak of real size shows within a minute or two. A very slow one will not show that fast, which is what step 6 is for.
4. Close the main shutoff for the house and read again. Use the valve where the line enters the house, not the one in the meter box, because the reading only means something if you closed the right valve. If your irrigation has its own shutoff and backflow assembly in a separate box, close that too and note whether the meter changes when you do.
- Meter still turning: the leak is between the meter and that valve, which puts it in the service line or in an irrigation line teed in ahead of the house. It is not under your slab.
- Meter stops: the leak is downstream, inside the house plumbing. The slab is now a candidate, and so is a wall, an attic run, or a fixture.
5. Reopen the main, close the cold inlet valve on top of the water heater, and read again. If the meter stops or slows sharply, the leak is on the hot side. If it runs at the same rate, the leak is on the cold side. Hot lines under a slab are the more common failure, and knowing the side before anyone arrives changes how the visit goes.
6. If nothing moved and you still suspect a leak, run the slow version. EPA’s WaterSense program gives the method: check your water meter before and after a two hour period when no water is being used, and if the meter changes at all, you probably have a leak.
Two cautions. If the meter box lid is stuck or full of water, leave it for the plumber or the utility. And a drain or sewer line leak under the slab will not move the meter at all, because a drain only carries water when something upstairs is draining. A still meter with damp floors is a real combination, and it points at the drain side.
Ruling out the four decoys
Toilets first. A silent flapper leak is the most common false alarm by a distance, because it makes no sound and hits the bill in exactly the shape a slab leak does. EPA’s dye test settles it: put a few drops of food coloring in the tank, and if any color shows in the bowl after 10 minutes, that toilet is leaking. Do every toilet in the house before you go further.
Irrigation comes next. Sprinkler lines usually tee in upstream of the house shutoff, which is why step 4 catches them. EPA puts an irrigation system leak 1/32 of an inch in diameter at about 6,300 gallons of water per month, easily enough to produce the bill that started your search.
Third is the water heater’s temperature and pressure relief valve. It is built to release water, and a failing one weeps continuously. It draws from the supply, so it does move the meter, which makes it a genuine decoy. Check the pipe running down from the tank. There is more on that valve in repair or replace a water heater.
The last one is air conditioning condensate. A coil running through a North Texas summer produces a steady stream of water, and a blocked condensate line puts that water where it should not be. It is easy to eliminate: condensate only appears while the system is running, and it never moves the meter, because that water was already inside the house as humidity.
Why the ground is part of the story
The USDA’s official series description for Houston Black, the clay that runs through the Blackland Prairies and the eastern part of the Grand Prairies of Texas, records high shrink swell potential. In dry periods that clay cracks half an inch to four inches wide, 1.3 to 10 cm, at a depth of 12 inches. Those cracks remain open 90 to 150 cumulative days in most years.
That is the documented soil behavior. What follows is mechanism rather than a cited finding, and the difference is worth stating plainly. Ground that opens cracks that wide does not move uniformly. A slab poured on that ground rides the movement, and so do the water lines cast into the slab or buried beneath it. Stress concentrates at joints, at bends, and where a line passes through the slab edge. That is how the failure happens. None of that says how often it happens, and anyone who hands you a percentage on this has invented it.
The same soil behavior explains the last row of the table. Ground that moves seasonally on its own gives cracking a strong explanation that has nothing to do with plumbing. Cracks alone should send you to the meter, not to an excavator. Cracks alongside a moving meter are a different conversation.
What a leak detection appointment involves
Nobody should be cutting your floor to go looking. Detection narrows in steps:
- The technician isolates the system and puts it under gauge pressure, hot separated from cold. Which side loses pressure, and how fast, says where to hunt and roughly how big the failure is.
- Supply and drain are different problems and different tests. A drain line under a slab is checked by plugging and filling it, not by pressurizing it.
- The line’s route is traced, so the mark ends up over the pipe rather than over where the pipe was assumed to run.
- The leak is pinpointed with a ground microphone worked across the floor, with inert gas detected where it surfaces, or with a thermal camera reading a hot line leak through the floor.
You should end up with a mark on the floor, the method that produced it, whether it is supply or drain, which side it is on, and a statement of confidence. Ask for that last one out loud. “How sure are you, and what happens if we open the floor and it is not there?” is a fair question, and the answer tells you plenty.
Spot repair, reroute, lining, or repipe
| Approach | What it is | What pushes the decision toward it |
|---|---|---|
| Spot repair | Open the floor at the mark, cut out the failed section, replace it | One leak, precisely located, on a line with no history, in a floor you can open without wrecking a kitchen |
| Reroute | Abandon the run under the slab, run new pipe overhead or through walls | A second leak on the same line, pipe that has already failed once, or a leak under cabinetry, an island, stairs, or tile you cannot match |
| Lining | Cure a liner inside the existing pipe so the slab is not opened at the leak | Access is the problem rather than the pipe. Whether your line qualifies depends on diameter, material, and bends, so make the contractor confirm that yours does |
| Repipe | Replace the supply system | Repeated failures across different runs, or supply pipe that has reached the end of its life throughout the house |
The second leak decides more than the first one does. A single failure in a sound line is a spot repair question. A second failure on the same run is telling you the pipe is the problem rather than the hole, and a third trip into the floor is the outcome a reroute exists to prevent, even though each repair on its own looks like the smaller decision.
One question to ask regardless of method: whether your slab is post tensioned, meaning steel cables run through the concrete under tension. Cutting into one without locating the cables first is dangerous and can do structural damage. Ask how the contractor intends to find out which kind of slab you have. A contractor who has not raised it before picking up a saw has not done this often enough.
Call your insurer before the demolition, not after
The Texas Department of Insurance states that most homeowner and renters policies cover sudden and accidental water damage, and that policies usually will not cover damage from gradual leaks. A slab leak sits awkwardly across that line, since by its nature it has been running a while before you notice. TDI also notes that you usually must report water damage that was hidden from view within days after you first see it, so the clock starts the day you spot it, not the day the pipe failed.
So call the carrier before anyone opens the floor. The condition of the floor before demolition is evidence, and it exists only once. Then read the water damage section of your own policy rather than anyone’s general summary, including this one.
Who you let cut into your slab
Plumbing in Texas is licensed by the Texas State Board of Plumbing Examiners, and its consumer guidance gives you two checks worth running before you sign. The board states that the written estimate and invoice should contain the Responsible Master Plumber’s license number and the name, address and telephone number of the Texas State Board of Plumbing Examiners. If that number is not on the document in front of you, ask why.
Then use it. The board’s guidance is to enter the license number and verify both that the license is valid and that the master plumber has a certificate of insurance on file. That second item is the one nobody volunteers, and it matters more here than on almost any other plumbing job, because the work cuts into a structural element of your house. The board also advises putting all warranties and estimates in writing. On a slab leak, that document should name the detection method, where exactly the leak was located, the repair approach, and who restores the floor.
Sources
- U.S. Environmental Protection Agency, WaterSense, Fix a Leak Week. Source for the water meter check, the toilet dye test, and the irrigation leak figure.
- U.S. Department of Agriculture, Natural Resources Conservation Service, Official Series Description, Houston Black series. Source for shrink swell potential, crack width and depth, crack duration, and the range of the series.
- Texas Department of Insurance, when water damage and mold are covered by insurance. Source for sudden and accidental coverage, the gradual leak exclusion, and the reporting timeline.
- Texas State Board of Plumbing Examiners, consumer information. Source for what belongs on a written estimate and invoice, and what to verify against the license number.
No figure for how often slab leaks occur, or how long one takes to cause damage, appears on this page. We could not open a primary source for either, so we did not print one.
Sources
- Fix a Leak Week U.S. Environmental Protection Agency, WaterSense
- Fix a Leak Week, toilet dye test and irrigation leak figure U.S. Environmental Protection Agency, WaterSense
- Official Series Description, HOUSTON BLACK series U.S. Department of Agriculture, Natural Resources Conservation Service
- When are water damage and mold covered by insurance? Texas Department of Insurance
- Consumer Information Texas State Board of Plumbing Examiners
Common questions
Can a slab leak be on a drain line instead of a water line?
Yes, and it behaves so differently that the usual advice actively misleads you. A supply line under the slab is pressurized, so it leaks continuously and it will move your water meter, which is exactly what makes the meter test work. A drain or sewer line under the slab only carries water when something upstairs is draining, so it can soak the ground beneath your floor for months without ever touching your water bill or moving the meter by a hair. So a dead still meter alongside damp flooring, a persistent musty smell, or a section of floor that has started to move is not an all clear. It points you at the drain side instead. The tests are different too. A drain line is checked by plugging it, filling it, and watching whether the level holds, not by pressurizing it the way a supply line is. If you have symptoms under your feet and a meter that will not budge, say exactly that when you book the visit, because it changes what equipment the technician brings.
How can I check for a slab leak myself without calling a plumber?
Shut off everything in the house that uses water, including the ice maker, the irrigation controller, and any pool auto fill, then go read your water meter. Look for the small triangle, star, or dial next to the digits. That is the low flow indicator, and it spins on a flow far too small to move the numbers. If it is turning with the house shut down, water is leaving your system somewhere. Now do the step almost nobody publishes: close the shutoff where the line enters the house, not the valve in the meter box, and read the meter again. If it keeps turning, the leak is between the meter and that valve, which puts it in the service line or an irrigation line rather than under your slab. If it stops, the leak is inside the house plumbing, which puts the slab in play. One more fork is worth running. Reopen the main, close the cold inlet valve on top of the water heater, and read again. A meter that stops or slows sharply has just told you the leak is on the hot side, which is the more common slab failure. If nothing moves, run EPA's slower version: check the meter before and after a two hour period when no water is being used, and if the meter changes at all, you probably have a leak.
What happens if you have a slab leak and leave it alone?
It does not stop, and that is the whole problem. A supply side slab leak runs continuously, so the water loss and the damage under your floor both compound while nothing looks urgent from the surface. People usually notice the bill first, then damp flooring, then a musty smell in one room. The part contractors rarely raise is the insurance side. The Texas Department of Insurance states that most homeowner and renters policies cover sudden and accidental water damage and that policies usually will not cover damage from gradual leaks. Gradual is the category a slow leak drifts into the longer it runs. TDI also notes that you usually must report water damage that was hidden from view within days after you first see it, so the reporting clock starts the day you notice rather than the day the pipe failed. We are not going to tell you how many weeks you have, because there is no honest number for that, and it depends on the size of the leak and what sits under your floor. What is fair to say is that the easiest version of this problem to deal with is the one caught on a water bill.
How do I tell a slab leak from a running toilet or a sprinkler leak?
Toilets first, because a silent flapper leak is the single most common false alarm, and it hits your water bill in exactly the same shape a slab leak does. EPA's dye test settles it: put a few drops of food coloring in the tank, and if any color shows up in the bowl after 10 minutes, that toilet is leaking. Irrigation is next, and it is not a small category: EPA puts an irrigation system leak 1/32 of an inch in diameter at about 6,300 gallons of water per month. The clean way to separate irrigation and the service line from anything under your slab is to close the shutoff where the line enters the house and watch the meter, because a meter still turning means the water is escaping before it ever reaches your house plumbing. Two more decoys are worth knowing. A weeping temperature and pressure relief valve on the water heater draws supply water, so it will move the meter and look convincing. Check the discharge pipe running down from the tank. Air conditioning condensate looks like plumbing water but never moves the meter and only appears while the system is running.
Does homeowners insurance cover a slab leak in Texas?
Read your own policy, because the wording varies between carriers and yours is the only one that governs your house. The general shape, from the Texas Department of Insurance, is that most homeowner and renters policies cover sudden and accidental water damage, while policies usually will not cover damage from gradual leaks. A slab leak sits awkwardly across that line by its nature, since it has typically been running for some time before anyone notices. TDI also states that you usually must report water damage that was hidden from view within days after you first see it, which is why the timing of your call matters as much as what you say in it. The practical move that almost nobody makes is to call the carrier before anyone opens the floor, because the condition of the floor before demolition is evidence, and it exists exactly once. Separately from the damage question, ask your carrier in plain terms how the policy treats the work of getting at the pipe, since access and the repair itself are handled differently from one policy to the next.
What actually happens at a slab leak detection appointment?
Nobody should be opening your floor to go looking, and if that is the plan, stop the visit. A real detection is a sequence of narrowing steps. The system is isolated and put under gauge pressure, hot separated from cold, so which side loses pressure, and how fast, tells the technician where to hunt and roughly how big the failure is. Whether it is a supply leak or a drain leak gets settled early, because that changes the method. A drain line under a slab is tested by plugging and filling it rather than pressurizing it, and a drain leak will not show on your water meter at all. Then the pipe route is traced, so the mark ends up over the actual line rather than over where the line was assumed to run. The leak itself is pinpointed acoustically with a ground microphone, with inert tracer gas detected where it surfaces, or with a thermal camera on a hot line leak. What you should leave with is a mark on the floor, the method that produced it, whether it is supply or drain, which side it is on, and a plain statement of how confident the technician is. Ask that last question out loud, along with what happens if the floor is opened and the leak is not there.
Can a slab leak be repaired without breaking up the floor?
Sometimes, and the answer turns on access rather than on the leak itself. Rerouting abandons the run under the slab entirely and puts new pipe overhead or through walls, which avoids cutting concrete and is often the right call when the leak sits under cabinetry, a kitchen island, stairs, or tile you could never match. Lining cures a new liner inside the existing pipe, but whether your run qualifies depends on the pipe's diameter, material, and how many bends it has, so make the contractor confirm that your specific line is a candidate. A spot repair, which does open the floor, is still the sensible answer when there is one leak, it has been precisely located, the line has no history of failure, and the floor above it is something you can open and close. The rule worth carrying is that the second leak decides more than the first one does, because a second failure on the same run is telling you the pipe is the problem rather than the hole. Ask one more question regardless of method: whether your slab is post tensioned, meaning steel cables run through the concrete under tension. Cutting into one of those without locating the cables first is dangerous and can cause structural damage.
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