Slab Leaks: What They Are, How They're Found and Fixed, and What They Cost

A slab leak is a two-way emergency on Texas clay. It is a break or pinhole in a pipe buried under your concrete slab — and because the escaping water can swell the clay into a dome or wash it out into a void, a hidden leak does not just damage plumbing, it moves your foundation. The moving foundation then cracks more pipe, which feeds more water into the soil: a vicious cycle. That is why the one rule that runs through every section below is fix the leak first. Drive a pier under a slab that a leak keeps re-wetting and you are stabilizing soil that will move again. This page is the hub for the whole subject — what a slab leak is, how it is detected and repaired, what it costs, and whether insurance pays — and it hands each of those off to a dedicated deep dive. For how a leak actually moves a foundation (the cause), we link to our plumbing-leaks page and stay in our lane here: the leak itself.

Bottom line up front

A slab leak is a plumbing failure with a structural shadow. The plumbing part is straightforward — a pipe under the slab is leaking, and a licensed plumber can find it and fix it. The structural part is what makes it urgent and what most homeowners underestimate: on the expansive clay under most of San Antonio, water escaping under the slab is the single most common way a foundation gets moved without anyone touching it.

So the whole subject organizes around one sequence, and it is the recommendation that repeats in every section: fix the leak first, then assess the foundation. Confirm and repair the plumbing, let the soil restabilize, re-survey the elevations, and only then stabilize whatever is still out of tolerance. Reverse that order — pier or re-level while the leak is still running — and you pay to stabilize soil the water will move again. Often, fixing the leak is the entire repair: once the water source is gone, the clay settles and no structural work is needed at all.

The good news embedded in that sequence is how cheap the front end is. Detection runs roughly $150–$600 and happens before any concrete is broken, so a plumber opens one square of slab directly over the leak instead of jackhammering a floor to search. The repair itself averages around $2,280–$2,300 nationally, in a wide band of about $630–$6,750 depending on access, pipe material, and how many leaks there are. And a couple of inexpensive devices — a pressure-reducing valve, a smart shutoff — change the odds on the most expensive failure mode.

This page is the map. Each major question below is summarized to roughly the same depth and then linked to its dedicated page:

• Detection — how the leak is found before the jackhammer. Full detection guide →
• Repair — the six ways it is fixed and how to choose. Full repair guide →
• Cost — what each step runs in 2026, and the foundation fork. Full cost breakdown →
• The foundation link — how a leak moves a house. Plumbing-leaks cause page →
• Insurance — when a sudden burst is covered and a gradual leak is not. Insurance guide →

What a Slab Leak Is

A slab leak is a leak in a water pipe — either a pressurized supply line or a gravity drain or sewer line — that runs beneath or is embedded within your home's concrete slab foundation. The defining feature is the concrete: the pipe is trapped under the slab where you cannot see it, so the leak reveals itself through secondary symptoms rather than a visible drip.

This is overwhelmingly a slab-on-grade problem, which makes it a San Antonio problem. In warm Southern and Western states where deep frost footings are not required, homes are commonly built on a single slab poured directly on the prepared ground, and the plumbing is installed before the concrete goes down. A building inspector approves the drain system, the supply lines are run, and then the slab is poured over the top — sealing the pipes in for the life of the house.

Two different pipe systems live under that slab, and they fail differently.

• The supply side is your pressurized hot and cold water lines, kept at an ideal pressure of about 50–70 psi. Historically these ran through the slab in soft copper; increasingly they run overhead through walls and the attic in copper or PEX. Because they are always under pressure, a supply leak pushes water out continuously.
• The drain side is the Drain-Waste-Vent (DWV) system: gravity sewer and waste lines that carry water out only when something is draining, sloping at roughly a quarter-inch per foot toward the municipal main. These are typically PVC, ABS, or — in older homes — cast iron, and the main sewer line is the deepest pipe under the house.

That split drives the single most useful distinction in the whole subject.

Pressure-side versus drain-side leaks

Pressure-side (supply) leaks push water out continuously under pressure, so they saturate the soil around one spot quickly and tend to announce themselves relatively fast — a jump in the water bill, warm spots on the floor from a hot-water line, a drop in water pressure. They cause real damage, but they are found faster precisely because the water never stops flowing.

Drain-side (gravity) leaks are far more veiled. They leak only when water is actually draining through the line, so there is no constant pressure to lose and often no continuous sound to hear. A drain leak can persist for years showing no obvious indication, surfacing instead as foul-smelling or murky water and slow soil washout. The hidden ones are the dangerous ones for a foundation, because they wet or erode the soil for years before anyone notices.

Knowing which side is leaking is the first real diagnostic decision, because it dictates every tool that comes after it — a point our detection page builds its entire toolkit around.

What Causes Slab Leaks

Pipes under a slab fail for a handful of well-understood reasons. Most cases are corrosion of one kind or another, but mechanical wear, bad installation, pressure, and — in Texas especially — the soil itself all contribute. The table sorts them.

The common causes of slab leaks, from water chemistry to soil movement. Most cases are corrosion; in Texas, expansive clay adds a mechanical cause on top.

Two of these deserve emphasis for a Texas audience. Excessive water pressure is both a direct cause of pipe failure and the one cause a homeowner can cheaply eliminate — which is why a pressure-reducing valve shows up again under prevention. And expansive-clay swell and shrink is the cause that ties this whole subject back to your foundation: the same soil movement that snaps a pipe is the soil movement that moves a slab. The uplift figures vary — different authorities cite numbers from roughly 5,000 psf to far higher — but the direction is not in dispute. For why the clay reacts so strongly to moisture, and how that same movement bends a foundation, our plumbing-leaks cause page and the broader foundation-repair hub carry the detail; here it is enough to know the clay can break the pipe.

Warning Signs

Because the pipe is hidden, a slab leak is something you infer from a pattern of clues rather than see directly. No single sign is proof, but several together are enough to call a specialist. Here is the full list homeowners actually notice:

• An unexplained spike in the water bill with no change in use
• The sound of running water when every fixture is off
• Warm or hot spots on the floor — heat from a hot-water-line leak transferring through the flooring
• Damp, warped, discolored, or buckling flooring — vinyl planks popping up, warped hardwood, loose tile, persistently damp carpet
• Mildew, musty odors, or visible mold
• A sudden drop in water pressure
• Cracks in walls, baseboards, floors, or the foundation
• Pooling water or soggy patches inside or around the home's perimeter
• Foundation movement — doors that stop closing, uneven floors, soil depressions near the foundation edge

The water-meter check — do this first

Before you call anyone, run the one test that costs nothing. Shut off every fixture and water-using appliance in the house, then go look at your water meter — specifically the small leak-indicator dial or triangle many meters carry. If it is still creeping with everything off, water is escaping somewhere it should not, and on a slab home that "somewhere" is often under the concrete.

The meter test pairs naturally with the symptom list: a moving meter plus a warm floor spot, or a moving meter plus a bill that jumped, is a strong signal. It is also the screening step a homeowner can legitimately own — the detection page covers the professional tools that take over once the meter confirms something is wrong. One reason not to sit on a positive result: per US EPA mold and moisture guidance, mold can develop within 24 to 48 hours of water intrusion, so a confirmed under-slab leak is not a problem to watch and wait on.

The Two-Way Relationship With Your Foundation

This is the part that turns a plumbing problem into a structural one, and it cuts in both directions.

A leak moves the foundation. A chronic under-slab leak delivers concentrated water to one part of the soil. On the expansive clay under most of San Antonio, that wetted spot swells and lifts — a localized heave or center-lift dome you can sometimes feel as a high point in the floor. A severe drain leak can do the opposite: a steady flow of escaping water erodes and washes soil into the broken line, removing support beneath that spot so it settles instead. Heave on one side, settlement on another — and either way the result is differential movement, part of the slab moving while the rest stays put, which is the precise pattern that cracks slabs, brick veneer, and interior finishes.

The foundation then breaks more pipe. It runs the other way too. The same swell-and-shrink cycle that wets and dries the clay exerts repeated push-pull stress on the buried pipes. Swelling clay lifts and shifts lines; shrinking clay opens voids that pull support away; rigid sections crack and joints separate, and low spots ("bellies") form that trap waste. So soil movement breaks the pipe, the leaking pipe feeds more moisture into the soil, and the soil moves more. It is a vicious cycle, and recognizing it is what dictates the order of operations.

That order is the cardinal rule of this entire subject: fix the leak first, before any structural stabilization. Underpinning a slab while the leak still runs just stabilizes soil the water will re-wet — the piers are seated correctly and the slab shifts anyway, because the cause was never addressed. Fix the leak, let the soil settle, re-survey, then stabilize only what remains out of tolerance.

This page deliberately keeps the mechanism brief, because it belongs to the cause cluster, not the slab-leak hub. For the full derivation — how a supply leak versus a sewer leak moves a slab, the leak-or-soil decision table, and why older San Antonio homes are most exposed — see our plumbing-leaks page, and for the symptoms this movement produces, the signs of a sinking foundation.

Detection (Overview)

The most important thing about detection is when it happens: before the jackhammer. A few hundred dollars of listening, camera, and pressure work pinpoints the leak so a plumber opens one small square of concrete directly over it — instead of breaking up a floor to search. A contractor who wants to start breaking concrete "to find it" has the order backwards; detection finds the leak, demolition only reaches it.

The work runs in two stages. First, confirm a leak exists and on which side — a quick pressure test for the supply lines, or a hydrostatic (static isolation) test for the drains, in which a plumber plugs the sewer at the cleanout, fills the lines to slab level, and watches whether the water holds. Then pinpoint the exact spot with the tool that matches the pipe: acoustic ground microphones for the hiss of a pressurized metallic leak, an infrared camera for the warm signature of a hot-water line, a sewer camera for a cracked or root-invaded drain joint, and tracer gas for plastic pipe where acoustic fails. No single method does the whole job — the combination is what makes the find reliable.

Detection is also cheap relative to what it protects: roughly $150–$600 all-in, with the slab-leak-specific average near $280. That is small money against the thousands an unnecessary slab break can cost, which is the entire reason detection comes first.

This is a summary; the mechanics of each method, how pros sequence them, and the post-tension scanning requirement live on the dedicated page. Full slab leak detection guide →

Repair (Overview)

Once the leak is pinpointed, there are six common ways to fix it, and the right one depends on how many leaks there are, where they sit, the pipe's condition, and whether you want to keep your floors. None is universally best.

The six slab-leak repair methods and roughly what they cost. The decision turns on the number of leaks, access, and whether you keep your floors.

The one rule worth committing to memory cuts across all six: if you have already spot-repaired two or three leaks on the same line, stop chasing and reroute or repipe. Each spot repair on a deteriorating pipe only buys time until the next pinhole, and the running total quietly passes what a reroute would have cost.

There is a hard safety gate in front of any method that cuts concrete. Many slab homes are reinforced with post-tension cables — steel tendons held under enormous tension inside the slab — and cutting one to reach a pipe can cause structural failure. Before anyone jackhammers or cores a post-tension slab, it must be scanned or X-rayed to map the cables so the cut lands clear of them. This is not optional, and it is one of the central reasons slab-leak repair is not a do-it-yourself job.

The method-by-method decision logic, the access tradeoff between tunneling and breaking the slab, and the order-of-operations detail all live on the dedicated page. Full slab leak repair guide →

What It Costs (Overview)

A slab-leak repair usually lands in the low thousands — the national average is about $2,280–$2,300 — but that headline hides a range with a very wide mouth, roughly $630–$6,750, because the same job can be a few hundred dollars or push toward five figures. Three things move the number: how hard the pipe is to reach, what it is made of, and whether the leak has already moved your foundation.

The cost stacks up in layers:

• Detection is the cheapest line — about $150–$600, average near $280 — and the highest-leverage, because it keeps the demolition small.
• Repair is the main event, from a $250–$850 spot fix to a $4,000–$15,000 whole-house repipe, with the expensive methods dominated by concrete, not plumbing — breaking and re-pouring slab, or tunneling beneath it.
• The foundation fork is the variable that changes everything. If the leak only damaged pipe and finishes, you are pricing plumbing. If the escaping water heaved or undermined the slab enough to move the foundation, you are also pricing underpinning — roughly $1,000–$3,000 per pier, $5,000–$20,000 for partial work, and $20,000–$80,000 for a full perimeter.

All of these are 2025–2026 planning ranges, not quotes — slab-leak pricing varies by region, access, the number of leaks, and pipe material, so your only path to a real number is on-site bids, ideally after an engineer has told you which side of the foundation fork you are on. The full breakdown, the variables behind each line, and the structural-cost handoff live on the dedicated page. Full slab leak cost breakdown →

Will Insurance Cover It?

Maybe — but far more narrowly than most homeowners assume, and the whole question turns on one distinction: sudden versus gradual.

A sudden, accidental pipe break — a supply line that bursts — is often a covered peril. In that case dwelling coverage may pay to tear out and replace the slab to reach the pipe, and to repair the resulting water damage to floors and walls. What it generally will not pay for is the broken pipe itself, and it will not pay at all for a gradual leak, long-term seepage, corrosion and wear-and-tear, or earth movement and settling — the standard exclusions that knock out the large majority of Texas foundation claims by cause.

Because coverage hinges on cause, insurers routinely send an adjuster or engineer to determine whether a leak was sudden or ongoing; if it reads as gradual, the seepage and wear-and-tear exclusion ends the claim. This scrutiny is not arbitrary — per the Insurance Information Institute (Triple-I), water damage and freezing is the second-most-frequent homeowners insurance claim, at roughly 24% of claims (about 1 in 67 insured homes files annually), with an average claim around $15,400. Carriers look hard at the most common claim they pay.

That is why the document that decides your claim is the same one that decides the foundation fork: a sealed engineer's cause-of-loss report establishing what actually happened. There is also a clock — mold can develop within 24 to 48 hours of water intrusion per US EPA guidance — so shutting off the water and documenting the damage fast protects both the claim and the house. The full coverage breakdown, including tear-out provisions and service-line endorsements, is on our insurance guide.

Prevention

You cannot move your house off the clay, but you can change the odds on the most expensive failure mode for a fraction of a repair. Five measures, roughly in order of leverage:

• Pressure-reducing valve (PRV) — about $250–$400. Keeps static water pressure below the 80 psi that stresses pipes and joints — which code requires above 80 psi anyway — and trims water use as a bonus. It directly removes one of the most common causes of pipe failure.
• Water softening for hard water — about $1,500 on average. Hard water accelerates scale and corrosive mineral attack that shorten system life; softening slows it.
• Smart leak-detection shutoff — about $500–$800. Installs on the main line, learns your usage, alerts your phone on an anomaly, and can automatically shut the water off before a leak runs for days under the slab. Several insurers discount premiums or reimburse part of a deductible for installing one, so ask your agent.
• Routine monitoring. Check the water meter with all fixtures off, watch the leak-indicator triangle, and treat any unexplained bill spike as a signal — the same screening described under warning signs, done as a habit.
• Proactive repipe of problem materials. Replace polybutylene, aging galvanized steel, and deteriorating cast iron before they fail — especially if leaks have recurred or an insurer signals non-renewal.

Two of these — the PRV and the smart shutoff — are inexpensive against a slab-leak repair that averages $2,000-plus and can fork into five-figure underpinning. Prevention does not eliminate the risk; it changes the math on the failure that costs the most. For the broader home-maintenance routine these fit inside, including drainage, grading, and moisture management around the slab, see our maintenance checklist.

FAQ Note

The FAQ below answers what San Antonio homeowners ask most about slab leaks — what one is, how to spot it, how urgent it is, whether it will damage the foundation, whether insurance covers it, whether you can stay in the home during repair, whether DIY is wise, whether a leak can cause a sinkhole, and what causes slab leaks in Texas. For the three deep dives this hub summarizes, follow through to detection, repair, and cost; for how a leak moves a foundation in the first place, the plumbing-leaks cause page; and before you commit to any structural work, an independent engineer's report.

Get Matched With a Vetted San Antonio Foundation Specialist

If you suspect a slab leak — a meter that keeps moving with everything off, a warm spot in one room, a water bill that jumped, or a dome you can feel in the floor — the right next step is a leak-detection visit, not a sales pitch to start breaking concrete. We'll match you with a vetted San Antonio specialist who detects before they demolish, and point you to an independent engineer and licensed plumber if the foundation may have moved. The match is free, the quote is no-obligation, and we don't take a fee from you. We screen for the right sequence above all: the leak found before any concrete is cut, post-tension slabs scanned before any cut, honest insurance documentation, and structural work scoped by a sealed engineer only on what is still out of tolerance after the water is off — because on a slab home, fixing the foundation before the leak just guarantees you pay again.