Warning signs

Is it the soil, or a plumbing leak?

Almost every page on this site tells you to get a hydrostatic test before paying for piers. This is the page that explains why. A leak under a slab moves a foundation entirely on its own — and piering around one means buying the same repair twice.

How a leak moves a foundation

In two opposite directions, depending on what is leaking and what it is leaking into.

A supply line is pressurised, so it leaks continuously — all day, every day, whether anyone is home or not. On expansive clay that means water added relentlessly to one area, the clay swelling, and that part of the slab lifting. The result looks like heave rather than settlement.

A drain line is not pressurised and only discharges when something upstream is used. Its damage is usually erosive: water escaping intermittently carries soil away, leaves a void beneath the slab, and the structure drops into it.

The distinction that actually diagnoses it

Clay movement is cyclical — the same door binds every August and frees up by spring, year after year, because the soil is responding to the weather. Leak-driven movement does not care what month it is. It is concentrated near the leak rather than spread around the perimeter, and it carries on through a wet spring that should have relieved it. If your symptom appeared and stayed, in one area, out of step with the seasons, that pattern is worth telling a contractor about before they quote.

Signs of a leak under the slab

Several of these have nothing to do with the foundation at all, which is rather the point — they point at a cause the cracks cannot.

  • A water bill that climbed with no change in how much you use. The clearest single indicator of a supply leak.
  • The sound of running water with every tap and appliance off.
  • A warm patch on the floor — a hot water supply line, and one of the few signs that locates itself.
  • Flooring that is damp, lifting or buckling without a spill to explain it.
  • A mildew smell, or mold appearing low on a wall.
  • One patch of grass greener than the rest, close to the house, through a dry spell.
  • Low water pressure that has developed rather than always been there.
  • Movement that appeared quickly in one area rather than gradually across the house.

Plenty of houses with leaks show none of these. Absence of symptoms is not evidence of absence, which is why the test exists.

What the hydrostatic test actually covers

A plumber inserts an inflatable test ball through a cleanout, inflates it to block the line, and fills the drain system with water up to slab level. If the level holds, the drains are sound. If it drops, they are losing water somewhere. Typically $250 to $600, and a couple of hours.

Two limits that matter and are rarely spelled out:

Hydrostatic test Supply pressure test
What it checks Drain, waste and vent lines Pressurised incoming water lines
When those leak Only when something is used Continuously, day and night
Tells you there is a leak Yes Yes
Tells you where it is No — locating is separate No — locating is separate

So a passed hydrostatic test clears half your plumbing, not all of it. If your symptoms point at a constant water source — the bill, the warm spot, water running with everything closed — ask specifically for a supply-side pressure test as well, because the hydrostatic will not find it.

Neither test locates the leak. That is a further step: isolating the system section by section, a camera survey, or electronic and acoustic detection, commonly $300 to $800 on top.

Why it comes before the piers

Three reasons, and the third is the one people do not expect.

  1. You will pay twice. Piers stop the house sinking further; they do nothing about water still swelling or eroding the soil underneath. The movement continues and the repair gets repeated.
  2. Access gets harder. Pier work and backfilled access pits can make reaching an under-slab line later more difficult and more expensive than it would have been beforehand.
  3. It can change the diagnosis entirely. Fix the leak, let the soil work back toward equilibrium over the following months, re-survey — and sometimes there is no foundation repair left to do.
Three ways a contractor reaches a support point Three cross-sections of a house on a slab. The first shows a pier installed from outside at the perimeter. The second shows an interior pier reached by cutting through the slab from inside the house. The third shows an interior pier reached by hand-digging a tunnel underneath the slab from outside. Route 1 Perimeter Dig outside, press, backfill Machine access. Fastest and cheapest per support point. Route 2 Through the slab Cut a hole in the floor Flooring out, concrete cut, patch after. Post-tension cables must be located first. Route 3 Tunnel underneath hand-dug Dig in from outside, under the slab Interior untouched, but priced per linear foot of tunnel before any pier goes in.
A support point on the perimeter and the same support point in the middle of the house cost very different amounts, because of what it takes to reach it. This is the single biggest reason two quotes for one house can differ by a multiple — check whether both include interior work before comparing the totals.

If the test is positive, the repair faces the same access question a pier does: tunnel in from outside, or break through the slab indoors. Tunneling costs more per foot and avoids demolition inside the house. On older cast iron or clay drain lines, it is also worth asking whether a spot repair is likely to be the first of several, because at a certain age re-routing the whole run can be better value than chasing failures one at a time.

The insurance angle

Worth understanding because it changes who pays, and it is commonly misread.

Standard Texas homeowners policies typically exclude damage from earth movement, settling and expansive soil — which covers most foundation movement. They may respond where damage was caused by a sudden and accidental discharge of water, which a failed pipe can be.

Separately, many policies include some form of access or tear-out cover: paying to break through and reinstate the slab or flooring needed to reach a covered leak, even where the pipe itself is not covered. That provision gets overlooked constantly and can be worth a great deal on a job whose main expense is getting to the pipe.

Read your own policy, and notify your carrier before work starts rather than after. We are a referral service, not an insurance adviser, and your carrier is the only party who can tell you what you actually hold.

Why a good contractor raises this against their own interest

This is the one recommendation in the entire process that can cost a foundation contractor the job. If the test comes back positive, you stop, call a plumber, and the pier contract shrinks or disappears.

A contractor who recommends it anyway is telling you something useful about how they work. One who never mentions it has either not thought about it or would rather not know. Ask them directly: do you want a hydrostatic test before the piers go in, and what would you do if it came back positive? The answer is informative whichever way it goes, and the question costs nothing.

What the testing and repair cost

The first two rows are small numbers that regularly save the large ones below them. General industry ranges, not quotes.

About these figures

Foundation Repair IQ does not quote prices and does not perform repairs. The figures below are general industry ranges published for orientation only — not quotes, not estimates for your property, and not binding on any contractor. Foundation pricing is set by the contractor after an on-site evaluation, and your actual cost may fall outside any range shown here.

Work General range Notes
Hydrostatic test $250 – $600 Drain lines only — not your supply lines
Supply-side pressure test $150 – $500 The other half of the plumbing
Leak location $300 – $800 Finding it is a separate job from proving it
Tunneling to reach a line $150 – $250 per linear ft Avoids opening floors inside the house
Spot repair through the slab $1,500 – $5,000 Plus flooring reinstatement
Re-route or replace under-slab drains $4,000 – $15,000+ Often better value on aged cast iron or clay
Elevation survey $300 – $800 Re-survey after the leak is fixed, before piering
Engineer's evaluation $400 – $1,000 The opinion with nothing to sell you

Estimates for planning only. See the cost guide, slab repair costs and cost per pier for the foundation side.

General information, not an assessment

This page describes how under-slab leaks tend to interact with foundations and what the standard tests do. It is not a diagnosis of your home and not advice about your insurance position — your carrier and your policy govern that. A licensed plumber can establish whether you have a leak; a licensed contractor or a Texas-licensed professional engineer can establish what your foundation is doing.

Questions about slab leaks and foundations

What is a hydrostatic test and what does it cost?

It is a pressure-free test of the drain system under your slab. A plumber inserts an inflatable test ball through a cleanout, inflates it to block the line, then fills the system with water up to slab level. If the level holds, the drains are not leaking. If it drops, they are. It generally runs $250 to $600 and takes a couple of hours. Two things it does not do: it does not tell you where the leak is, which is a separate locating exercise, and it does not test your pressurised water supply lines at all. Those need a separate pressure test. A great many homeowners come away from a passed hydrostatic test believing their plumbing has been cleared when only half of it has.

Does a hydrostatic test check my water supply lines?

No, and this is the most useful distinction on this page. A hydrostatic test checks the drain, waste and vent system — the pipes that carry water away, which are not under pressure and only leak when something is being used. Your supply lines are the pressurised pipes bringing water in, and they leak continuously, day and night, whether anyone is home or not. A supply leak is tested differently: the line is isolated and pressurised, and the gauge is watched for a drop. If your symptoms point at a constant water source — a jumping water bill, a warm patch on the floor, the sound of running water with every tap closed — ask specifically for a supply-side pressure test, because the hydrostatic test will not find it.

Can a plumbing leak really cause foundation movement?

Routinely, and in two opposite directions depending on the soil and the pipe. On expansive clay, a leaking supply line adds water continuously to one area, the clay swells, and that part of the slab lifts — heave rather than settlement. On sandier or silty ground, or with a leaking drain line discharging intermittently, moving water carries soil away and leaves a void, and the slab drops into it. Either way the damage is concentrated near the leak rather than spread around the perimeter, and it does not follow the seasons. That is the single most useful thing to notice: clay movement cycles with the weather, leak movement does not care what month it is.

Should I fix the leak before the foundation?

Almost always, and it is the reason this test appears on nearly every page of this site. Install piers around an unaddressed leak and the water keeps working — the soil keeps swelling or keeps washing away, the house keeps moving, and you pay for the foundation repair a second time. There is also a sequencing cost: pier work and backfilled access pits can make later plumbing access harder and more expensive than it would have been. And occasionally the test changes the diagnosis completely. Fix the leak, let the soil settle back toward equilibrium over following months, re-survey, and sometimes there is no foundation repair to do at all.

Will insurance cover a slab leak?

It depends on your policy and on what exactly you are claiming for, and your carrier is the only one who can answer it. The general shape in Texas is that standard homeowners policies typically exclude damage from earth movement, settling and expansive soil — which is most foundation movement. They may respond where damage was caused by a sudden and accidental discharge of water. Separately, many policies include some form of access or tear-out coverage, which can pay to break through and replace the slab or flooring needed to reach a covered leak even where the pipe itself is not covered. That provision is commonly overlooked and can be worth a great deal. Read your own policy, and report it to your carrier before work starts rather than after.

My foundation contractor never mentioned a plumbing test. Should I worry?

It is worth raising, because this is the one recommendation in the whole process that can cost a foundation contractor the job. If the test comes back positive, the homeowner stops, calls a plumber, and the pier contract may shrink or disappear. A contractor who recommends it anyway is telling you something good about how they work. One who does not either has not thought about it or would rather not know. Ask directly whether they want a hydrostatic test before the piers go in, and what they would do if it came back positive. The answer is informative regardless of which way it goes, and it costs nothing to ask.

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