Two different failures wearing one name
Slab leak is one phrase covering two problems with different symptoms, different detection methods, different urgency and different repair logic. Most consumer content conflates them, which is why so much advice does not fit the situation it is given for.
Supply-side leaks
These are the under-slab hot and cold water supply lines — historically copper, sometimes polybutylene or CPVC, and in newer houses PEX, often run overhead instead. The defining feature is that they are pressurised twenty-four hours a day, so a supply leak runs continuously whether anyone is home or not, putting a large, steady volume of water into the soil directly under the foundation. Copper under a slab here typically fails by pinhole pitting and by abrasion against concrete or aggregate.
Drain-side leaks
These are the under-slab drain, waste and vent lines: cast iron in older houses, PVC in newer ones, occasionally clay. They are not pressurised, so water escapes only when something upstream drains, which makes the leak intermittent and the volume lower. It also makes them far subtler — frequently no change in the water bill, no sound, no warm spot. Many are discovered the wrong way round: a foundation engineer finds movement, orders a plumbing test to rule out water, and the test fails. The leak may have been running for years.
For a buyer the distinction cuts like this. A supply-side leak is usually detected first and is often causing the movement. A drain-side leak is usually found second, after the movement. A proposal that does not say which one you have has skipped the first question.
How a slab leak shows up in the house
Supply-side leaks announce themselves eventually, and the signs are cheap to check:
- An unexplained jump in the water bill.
- The sound of running water with every fixture off.
- A warm or hot patch on the floor, which points at the hot line specifically.
- Reduced pressure at fixtures.
- Damp, cupped or discoloured flooring, or a musty smell.
- Localised heave. In expansive clay a pressurised leak is an unintentional soaker hose running under one part of the slab. The clay swells and that part of the house lifts.
That last one is the North Texas signature. In most of the country a slab leak is a plumbing problem with water damage attached. Here it is a plumbing problem with a foundation problem attached, because Blackland Prairie clay swells when it takes on water. A leak concentrates moisture in one place, year-round, indifferent to the season the rest of the yard is in.
Drain-side leaks present far more quietly, and the evidence is indirect: floors out of level in one area, doors out of square on one side of the house, cracking that concentrates rather than distributing. Those are foundation observations, and they are how an under-slab drain leak most often first appears in an inspection report — not as a plumbing finding at all.
What actually finds a slab leak
Every method below is plumber or leak-specialist work. None is in the standard inspection, and each has a limit that decides what it can honestly tell you.
- The water meter test. With all water off, watch the meter's leak indicator. The EPA's WaterSense programme puts it plainly: check the meter before and after a two-hour period when no water is being used, and if it changes at all you probably have a leak. Free and effective, but supply side only — a drain leak is not pressurised and will never move a meter — and it will not tell you where. Two false positives dominate: a running toilet flapper and an irrigation valve. The EPA toilet check is a few drops of food colouring in the tank, looking for colour in the bowl after ten minutes (EPA WaterSense).
- Static or hydrostatic drain test. A plug is set at the cleanout on the city side, the drain system is filled to cleanout height, and the level is watched for a drop, typically over half an hour or more. It is the most sensitive tool for small drain leaks. Its limits: drain side only, it shows that the system leaks but not where, it needs an accessible cleanout, and on badly corroded cast iron the test itself carries real risk, since a static head on a thin-walled line can cause or worsen a failure.
- Isolation testing. Sectionalising the drain system with plugs to narrow a failed static test to a run. Invasive, needs access points, adds time.
- Supply pressure test. Isolate and pressurise the supply piping and watch for decay. Supply side only, and locating still needs a second method.
- Acoustic listening. Microphones on the slab pick up pressurised water escaping. Supply side only, because a non-pressurised drain leak makes no sound. Degraded by ambient noise, floor coverings and depth.
- Tracer gas. Inert gas introduced into an isolated line, detected where it surfaces. Slab coverings interfere.
- Thermal imaging. Best on hot-side supply leaks. A screening aid rather than a locator, and it images surface temperature, not water.
- A sewer camera. Shows the condition inside a drain. It cannot find a leak.
Read that list for what it implies: no single method covers both sides of the system, and none both confirms and locates in one step. Confirmation and location are separate jobs, and a quote that prices them as one is worth questioning.
What an inspector can and cannot determine
Within the plumbing standard of practice (22 TAC 535.231), an inspector shall report the location of the water meter, the location of the main water supply shutoff valve, the static water pressure, and the visible material used for supply and drain lines. The inspector shall report as Deficient the presence of active leaks, water pressure exceeding 80 PSI, the lack of a pressure reducing valve where it does, deficiencies in water supply pipes and waste pipes, deficiencies in fixture drain performance, and deficiencies in water supply as determined by viewing functional flow in two fixtures operated simultaneously.
Static pressure is worth pausing on. It is one of the few genuine measurements in the whole plumbing section, high pressure is a real contributor to supply line failure, and a missing pressure reducing valve is among the cheapest corrections on any inspection report.
The rule is equally clear about the other side of the line. The inspector:
- Is not required to inspect anything buried, hidden, latent, or concealed (22 TAC 535.227) — an exact description of an under-slab line.
- Is not required to operate any main, branch or shut-off valves, so an inspector cannot isolate the house the way a plumber can. An inspector may still observe a meter moving. Observing is not testing.
- Is not required to inspect water mains, and does not perform or interpret static, isolation, pressure, acoustic or tracer gas testing.
- Does not determine causation, because an inspection is not intended to be a comprehensive investigation or exploratory probe into the cause or effect of deficiencies.
So the honest answer to can an inspector detect a slab leak is: sometimes, indirectly, never conclusively. An inspector can report a creeping meter, a warm spot on a floor, damaged flooring, poor fixture drainage, pressure over 80 PSI, and a floor and door pattern consistent with localised heave. Each is a reason to bring in a licensed plumber. None is a diagnosis.
Post-tension slabs, and a myth worth killing
Two things get muddled about slab leaks in post-tensioned foundations, the North Texas norm from the mid-1980s onward.
First: the repair is harder and riskier, not the leak. A post-tensioned slab is held in compression by steel tendons stressed after the concrete cured, and cutting a tendon releases stored energy and permanently removes engineered capacity. Any interior cut therefore needs tendon location first, by ground-penetrating radar or electromagnetic scanning, and often an engineer directing the cut and the repair detail. That is why tunnelling from outside is chosen so often here. The Post-Tensioning Institute publishes the framework in Technical Note 21, the residential standard as PTI DC10.5, and maintenance guidance in DC10.2.
The myth to correct: post-tension slabs cannot be repaired, and you can never cut one. Both are wrong, and both frighten people out of sound houses. Post-tension slabs are cut for plumbing repairs routinely, with tendon location, engineering input and proper repair detailing. The accurate statement is narrower: cutting a post-tension slab without locating the tendons first is dangerous and can permanently damage the foundation. That is a procedure requirement, not a prohibition.
Second: a stiffer slab hides symptoms longer. A conventionally reinforced slab tends to crack and telegraph distress early and locally. A post-tension slab is designed to resist the tensile stresses of soil doming and dishing without cracking as readily, so a localised moisture change underneath may produce less visible interior evidence for longer, and the movement that eventually appears is spread over a larger area. Nobody can say how many years a post-tension slab will mask a leak.
Which came first, and what to do about it
The relationship between slab leaks and foundation movement runs both ways, and both one-directional versions are wrong.
Leak to movement. A pressurised leak under a slab in Houston Black clay is a continuous, localised water source; the clay swells and that part of the slab lifts. That is differential heave with a cause you can switch off. Movement to leak. Seasonal shrink and swell shears rigid under-slab drain lines and pulls cast iron joints apart; Texas A and M AgriLife lists broken pipes among the damage expansive soil movement causes.
Which came first is frequently unresolvable, and here is the liberating part: it usually does not need to be resolved to decide what to do. The sequence is the same either way.
Stop the water first. If the meter is creeping with everything off, that is a same-week call to a licensed plumber, not a negotiating point. A pressurised leak is a continuous soil-saturation event in soil that swells when wet.
Then test the drains, then evaluate the foundation. On a slab house showing movement, a plumbing test comes before any pier proposal. A contractor who proposes piers without one has skipped a step, and the local market makes that easy: North Texas has vastly more foundation repair companies than geotechnical engineering firms, and the party writing the proposal is paid for the piers, not the diagnosis.
Fix the pressure while you are at it. Static pressure over 80 PSI with no pressure reducing valve is cheap to correct and a genuine driver of supply line failure — one of the few slab-leak preventives that actually exists. And be realistic about scope: cost is driven by which side of the system is involved, the depth and location of the failure, whether the fix is a spot repair, a re-route or a replacement, and whether access is by tunnel or interior cut. On supply lines the common eventual remedy is re-routing overhead.
Common questions
Can a home inspector detect a slab leak?
Not conclusively, and not by testing. A Texas inspection is a limited visual survey, and inspectors are not required to inspect anything buried, hidden, latent or concealed — which is exactly what an under-slab line is. What an inspector can do is report the things that point at one: a water meter moving with fixtures off, static pressure over 80 PSI with no pressure reducing valve, a warm spot on a floor, damaged or cupped flooring, poor fixture drainage, and a floor-slope and door pattern consistent with localised heave. Each of those is a reason to bring in a licensed plumber. None of them is a diagnosis.
What is the difference between a supply-side and a drain-side slab leak?
Supply lines are pressurised all the time, so a supply leak runs continuously and pushes a lot of water into the soil quickly; it shows up as a jumping water bill, running-water sound, a warm floor spot, low pressure, or localised heave. Drain lines are not pressurised, so a drain leak only releases water when something drains; it is slower, quieter, often produces no bill change, and is frequently discovered only after a foundation engineer orders a plumbing test. They are found by different methods — meters and acoustics for supply, static and isolation testing for drains — and a proposal that does not say which one you have has skipped the first question.
Will a static test damage my old cast iron pipe?
It can, and that risk is real rather than theoretical. A static test fills the drain system to cleanout height and puts a standing head of water on pipe that may already be thin at the invert from decades of corrosion. On badly channelled cast iron that load can cause or worsen a failure. Some owners decline the test for exactly that reason. The counter-argument is that a line that fails under a static head was not going to survive normal service for long either, and finding out before closing is better than finding out afterwards. It is a judgement call worth discussing with the plumber before the plug goes in.
Can a post-tension slab be cut to repair a leak?
Yes. The claim that post-tension slabs cannot be repaired or cut is a myth, and it frightens people out of sound houses. What is true is that the slab is held in compression by stressed steel tendons, and cutting one releases that energy and permanently removes engineered capacity. So the tendons must be located first, by ground-penetrating radar or electromagnetic scanning, and an engineer typically directs the cut location and repair detail. Because that adds cost and risk, tunnelling from outside the perimeter is often chosen instead, which avoids the concrete entirely.
My foundation is moving. Should I get piers or a plumbing test first?
The plumbing test comes first, every time. Water under a slab in expansive clay causes movement, and lifting the foundation over a line that is still leaking repairs the symptom while the cause keeps running. Any foundation proposal written for a slab house with movement and no plumbing test has skipped a step. This is worth insisting on because the local market makes it easy to skip: North Texas has far more foundation repair companies than geotechnical engineering firms, and the party writing the proposal is compensated for the piers rather than for the diagnosis.
How do I check for a slab leak myself?
Start with the meter. Turn off every fixture and appliance that uses water, including the ice maker and the irrigation controller, and check the meter's leak indicator before and after a two-hour period with no water used; if it moves at all, water is leaving the system. Rule out the two big false positives first: put a few drops of food colouring in each toilet tank and look for colour in the bowl after ten minutes, and isolate the irrigation system at its valve. Remember the limit — this test only works on pressurised supply lines. A drain leak will never move the meter, and finding one requires a plumber and a static test.