A hydrostatic test is the standard way to prove whether the drain lines under a concrete slab still hold water. The under-slab waste system is isolated at the cleanout, filled to the level of the slab and watched: if the level holds, the system is sound, and if it drops, something down there is leaking. It is the test that lenders, insurers, foundation engineers and buyers ask for by name in San Antonio, because guessing about pipework under a foundation is how people end up paying twice.
A hydrostatic test answers one question and answers it definitively. It does not tell you where the fault is or what it will cost to fix — that comes next, with a camera.
What the test proves, and what it does not
The value of a hydrostatic test is that it turns an argument into a measurement. Everything about under-slab drainage is invisible, so opinions multiply. A level in a standpipe either holds or it does not.
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It proves the drain system holds
Every waste line beneath the slab is tested together, as one sealed system, at the pressure a full slab-height column of water applies.
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It does not locate the fault
A failed hydrostatic test tells you there is a leak in the system. Finding where it sits takes a camera and a sonde, which is the next visit, not this one.
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It says nothing about the supply lines
Pressurised hot and cold lines are a separate system tested a different way. A house can pass a hydrostatic test and still have a supply-side slab leak.
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It is not a foundation inspection
It tells an engineer whether plumbing is contributing to soil movement. It does not assess the foundation itself, and no plumber should claim otherwise.
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It is repeatable
The same test run before and after a repair is what demonstrates the repair worked, which is why insurers ask for both.
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It produces a document
Date, address, who ran it, what was isolated, the starting level and the result. That is the part other people act on.
The five reasons people order one
Almost every hydrostatic test we run traces back to one of these, and knowing which one you are in changes what you do with the result.
| Situation | What the test is for |
|---|---|
| Buying a house | Finding out during the option period whether the drains under the slab hold, while the price is still negotiable |
| Before foundation work | Engineers want plumbing ruled out before lifting a slab, because a leaking drain will simply move it again |
| After foundation work | Lifting a slab can break lines that were intact. A hydrostatic test afterwards is what proves it did not |
| An insurance claim | Adjusters ask for a pass or fail as evidence of a plumbing cause rather than gradual settlement |
| Unexplained damp or movement | Damp with no bill increase points at the drain side, and this is the test that settles it |
The house-purchase case is the one worth pressing. A standard inspection does not include a hydrostatic test, and under-slab drain repair is one of the larger bills a property can hand a new owner. An hour during the option period is the cheapest that question will ever be answered.
How the test is run
It is a simple procedure done carefully, and the care is in the isolation rather than the filling.
- We locate the main cleanout. If the house does not have an accessible one, fitting one comes first, and we tell you that before the visit rather than on arrival.
- An inflatable test ball goes into the line downstream of the house and is inflated to seal the system off from the city main.
- The under-slab drains are filled with water until the level reaches the top of the slab. That column is what puts the system under load.
- The level is marked and left. We watch it for a set period rather than glancing at it — a slow drop is still a fail, and slow drops are the common ones.
- A level that holds is a pass. A level that falls is a fail, and we note how far it dropped and how quickly, because the rate tells you something about the size of the fault.
- The test ball is deflated and removed, the system drains normally, and you get the result in writing before we leave the property.
One caution worth stating plainly: a hydrostatic test puts the drain system under more load than daily use does. On very old or already compromised cast iron there is a small risk the test itself reveals a weakness by causing it to let go. That is a real trade-off, we will raise it before testing a house of that age, and it is also an argument for knowing rather than not knowing.
It failed. What happens next?
A failed hydrostatic test is information, not a verdict on the house. What follows is a sequence, and each step narrows the cost rather than raising it.
First the fault gets located. A camera goes down the line with a sonde transmitting its position, so the break is marked on the ground with paint and its depth recorded. Until that is done, nobody can honestly quote a repair, and anyone who does is quoting a guess.
Then the repair route is chosen. Under-slab drains are reached either by tunnelling in from outside, which leaves the floor untouched, or by opening the slab at the marked point. Which one makes sense depends on the depth, the distance from an exterior wall and what is above the fault. Both get priced.
Then the test is run again. A second hydrostatic test after the repair is what proves the system now holds, and it is the document an insurer, a lender or a buyer actually wants. We include the retest in the repair rather than billing it separately.
What a failed test usually turns out to be
A hydrostatic test failure is rarely exotic. Five faults account for most of what the camera then finds under San Antonio slabs.
A cracked closet bend is the most common of all — the elbow beneath a toilet, cast iron, corroded at the point where it turns. It is also among the easier repairs, because the access is directly under a fixture rather than in the middle of a room.
Root intrusion at a joint comes next. Cast iron and clay were laid in sections with joints that roots find, and a root mass holds the joint open even after the roots themselves are cut out. Then there is channelling: cast iron corrodes along the bottom of the bore until the pipe wall is simply gone at the invert, and water leaves through the floor of the pipe.
A belly — a sagged section holding standing water — will sometimes pass a hydrostatic test and still cause repeat blockages, which is exactly why the camera follows the test rather than replacing it. And a broken hub, usually from slab movement, gives you a clean fail and an obvious location.
None of these are unusual in a house of a certain age here, and all of them are repairable without the house coming apart.
Hydrostatic test, camera inspection and leak detection are three different things
These get used interchangeably, including by people who should know better, and the confusion costs homeowners money.
| What it answers | What it cannot tell you | |
|---|---|---|
| Hydrostatic test | Do the under-slab drains hold water, yes or no | Where the fault is, or anything about supply lines |
| Camera inspection | What the inside of the line looks like and where a fault sits | Whether a hairline crack is actually passing water |
| Leak detection | Where a pressurised supply line is losing water | Anything about the gravity drain system |
In practice they are used together. A hydrostatic test proves the drains are the problem, a camera finds where, and slab leak detection covers the supply side when the test comes back clean and water is still going somewhere.
What sets the price of a hydrostatic test
The test itself is a modest, predictable job. These are the things that move it, and we confirm which apply before we book.
| Factor | Why it matters |
|---|---|
| Whether there is a cleanout | No accessible cleanout means installing one first, which is a separate job with its own price |
| One system or two | Larger homes sometimes have separate drain systems that have to be isolated and tested individually |
| Locating after a fail | The camera and sonde work that follows a failed hydrostatic test is priced separately and only happens if you want it |
| Access | A cleanout behind a locked gate, under decking or beneath landscaping adds time before the test starts |
Foundation companies, engineers and the order things go in
If a foundation contractor has quoted you for piers, a hydrostatic test belongs before the work, not after it, and a good engineer will say the same.
The reason is simple. A leaking drain washes soil out from under a slab, and that washout is a cause of movement rather than a consequence of it. Lift a slab that is still being undermined and you have bought yourself the same problem again on a slower clock. Ruling plumbing in or out first is what makes the structural work durable.
Afterwards is equally important for the opposite reason. Raising a foundation moves everything cast into it, and rigid cast iron drain lines do not always survive being lifted. A hydrostatic test after the piers is what tells you whether a sound system is still sound, and it is far cheaper to find that out immediately than two years later.
We are happy to run the test with the engineer or the foundation contractor present, and we give the result to you rather than to them.
Buying a house in San Antonio? Put it in the option period
A general home inspection does not include a hydrostatic test. The inspector runs water and confirms it drains, which tells you nothing about whether the pipe under the slab is holding it.
- Anything built before about 1980, where cast iron under the slab is at the end of its service life.
- Any house with visible foundation repair history, whether or not it was disclosed.
- Any property with large mature trees close to the footprint.
- Any house where the seller has recently replaced flooring in one room only.
- Any purchase where a repair bill after closing would genuinely hurt.
The test takes about an hour and the result is yours, not the seller’s. A pass removes the largest invisible unknown on the property. A fail gives you a document and a repair estimate to negotiate with, which is a far stronger position than a suspicion.
When a hydrostatic test is not the right next step
We would rather turn down a booking than sell a test that cannot answer your question.
If your water bill has jumped and nothing indoors is damp, the problem is almost certainly on the pressurised side — a supply line under the slab or out in the yard — and a hydrostatic test will pass while the leak carries on. Electronic leak detection is the right call there.
If a single fixture is slow and everything else drains normally, that is a branch line or a fixture trap, not a system-wide failure. Drain cleaning answers it for a fraction of the cost.
And if there are no symptoms, no transaction, no foundation work planned and no insurance claim running, a hydrostatic test is a reasonable thing to defer. The useful version of this service is the one ordered when the answer changes a decision.
Hydrostatic testing across San Antonio and the surrounding towns
We run tests across San Antonio, Helotes, Boerne, Bulverde, Schertz, Converse, Stone Oak, Alamo Ranch and Somerset. Slab-on-grade construction is close to universal here, which is why a hydrostatic test is a routine item rather than an exotic one.
Same-week appointments are normal, and option-period work gets prioritised because the clock on those is real. If the test fails, slab leak repair and sewer line repair are the two pages worth reading next.
Work is carried out by licensed plumbers under our Responsible Master Plumber, license RMP-43333, issued by the Texas State Board of Plumbing Examiners. You can verify that license yourself.
What San Antonio homeowners say
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Frequently asked questions
What is a hydrostatic test?
The under-slab drain system is sealed off at the cleanout and filled with water to slab level. If the level holds, the drains are sound. If it drops, something below the foundation is leaking. It is a pass or fail on the whole system at once.
How long does it take?
About an hour for a typical single-family home, including setup and the watching period. Larger houses with more than one drain system take longer because each has to be isolated and tested on its own.
Does a hydrostatic test check my water lines too?
No. It tests gravity drains only. Pressurised hot and cold supply lines are a separate system and need pressure testing and electronic leak detection instead.
Can the test itself break my pipes?
On very old or already deteriorated cast iron, there is a small risk that the added load reveals a weakness. We raise that before testing a house of that age. It is worth weighing against the alternative, which is not knowing.
My house has no cleanout. Can you still test it?
Not until one is installed, and that is a separate job we will price for you first. A cleanout is worth having regardless — without one, every future drain problem is harder and more expensive to reach.
What does a failed hydrostatic test mean for the value of the house?
It means there is a repair to price, not that the house is unsound. Plenty of San Antonio homes of a certain age fail, get repaired and pass a retest. What it changes is who pays, which is why timing it inside the option period matters.
Will my insurance pay for the test?
Sometimes, as part of investigating a claim. Policies differ. What is consistent is that adjusters want the written result, so keep the report whoever ends up paying for it.
Do I need one after foundation repair?
We would, yes. Lifting a slab can break drain lines that were previously intact, and finding out immediately is far cheaper than discovering it later. Ideally the same test is run before the work as well, so there is a before and after.
Is a camera inspection not enough on its own?
No, and this is the common misunderstanding. A camera shows you what the pipe looks like. It cannot tell you whether a hairline crack is actually passing water. Only a hydrostatic test proves the system holds.
Can I be present for it?
Yes, and we would encourage it. The test is visual — you can see the level in the standpipe yourself — and watching it is the easiest way to understand what you are paying for.
What if the test passes but I still have damp?
Then the drains are not the cause and the next place to look is the supply side or an external source such as irrigation or an air conditioning condensate line. A pass narrows the search, which is useful in itself.
Do you retest after doing the repair?
Yes, and it is included rather than charged again. A repair that has not been retested is an assertion; a second hydrostatic test is proof.
Get a yes or a no on the drains under your slab.
We isolate the system, run the test while you watch, and hand you the written result before we leave.
Silverback Plumbing & Drain provides licensed plumbing and drain service for homes and businesses throughout the San Antonio area. From emergency repairs to routine maintenance, we diagnose the problem, explain the fix, and get it done right the first time.
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Mark Vargas, Responsible Master Plumber #43333 Licensed through the Texas Board of Plumbing Examiners P.O. Box 4200, Austin, Texas 78765 (800-845-6584)