A slab leak is a leak in a pressurized water supply line running under or through your foundation; a sewer leak is a leak in the unpressurized drain line carrying waste away from the house. Testing tells them apart: a supply-side pressure test isolates a slab leak, while a hydrostatic test on the drain system plus a sewer camera identifies a sewer leak. The symptoms overlap enough that guessing is unreliable, and the two repairs are very different in scope and price.

Both problems are common in Dallas-Fort Worth for the same underlying reason: expansive clay soil moves seasonally, and pipe cast into or beneath a slab does not tolerate that movement forever. This page covers the symptom differences, exactly what each test proves, the order a competent plumber runs them in, and what each repair path typically costs.

SYMPTOMS SIDE BY SIDE

Use this to form a hypothesis, not a conclusion. Several symptoms appear in both columns, which is precisely why testing exists.

The clean discriminators are at the top and middle of that table. A warm spot on the floor combined with a running-water sound and a spiking bill is a supply-side slab leak until proven otherwise. Sewage odor with multiple fixtures backing up is a drain-side failure until proven otherwise. Everything else is ambiguous.

THE THREE TESTS AND WHAT EACH ONE PROVES

There is no single test that answers everything. Each of these proves one specific thing, and they are used in sequence.

Supply-Side Pressure Test

The plumber closes the main shutoff and pressurizes the supply system, then watches whether it holds. If pressure drops with every fixture closed, water is escaping from the pressurized side somewhere. This confirms a slab leak exists but does not locate it. Typical cost is $150 to $350.

Hydrostatic Test On The Drain System

The drain system is plugged at the cleanout and filled with water to a set level, then monitored. A dropping level means the drain system is losing water below grade. This confirms a sewer or under-slab drain leak. It also does not locate anything. Typical cost is $200 to $500. Our hydrostatic testing guide covers the procedure in detail.

Isolation Testing

Once a system has failed a test, isolation narrows it down. The plumber divides the system into sections and re-tests each one, so a whole-house failure becomes a specific run of pipe. This is where the diagnosis gets useful and where a lot of unnecessary demolition gets avoided. Typical cost is $300 to $700.

Camera Inspection

A camera goes into the drain line and shows the actual condition: the break, the belly, the root mass, the corrosion. It is the only method that lets you see the defect rather than infer it. Cameras work on drain lines, not on pressurized supply lines, which is why supply-side slab leaks are located acoustically or with tracer methods instead. Typical cost is $200 to $400.

THE DIAGNOSTIC ORDER WE FOLLOW

Order matters. Running the wrong test first costs money without narrowing anything.

Interview and inspect. Symptom history, water bill trend, age and construction of the home, any foundation work. This alone often points strongly to one side.
Meter check. With every fixture off, does the meter still move? Movement indicates a pressurized-side loss, which points to a slab leak and lets us skip straight to supply-side testing.
Test the indicated side first. Supply-side pressure test if the meter moves or there is a warm spot; hydrostatic drain test if there is odor or backups.
If the indicated side passes, test the other side. A pass is useful information, and it is why both systems sometimes get tested.
Isolate. Break the failed system into sections and re-test to narrow the location.
Camera the drain line if the failure is drain-side, or run acoustic and tracer location if it is supply-side.
Scope the repair with options and written pricing before anything gets opened up.

If you already know you are dealing with a supply-side failure, our slab leak article goes deeper on that side, and leak detection services covers the locating work.

REPAIR PATHS AND TYPICAL COSTS

These are typical Dallas-Fort Worth ranges, not quotes. The gap between the two columns is the reason a correct diagnosis is worth paying for.

One point worth understanding: for repeat supply-side slab leaks, rerouting overhead is frequently cheaper and more durable than repeatedly opening the slab, because it removes the pipe from the soil movement that keeps breaking it. For drain-side failures in aged cast iron, the decision is usually about how much of the system is at end of life, which our sewer line replacement guide works through.

WHO OWNS WHICH PIPE

A frequent and expensive surprise: the homeowner is generally responsible for the service lines on their property, including the sewer lateral running from the house to the connection at the city main. The municipality maintains the main itself. That means a break in your yard is typically your bill even though the line carries waste to a public system.

The boundary and the responsibility split vary by city, and it is worth confirming with your own utility before assuming. Fort Worth publishes its water and wastewater service information through the Fort Worth Water Department, and other DFW cities publish equivalents. If a leak is at or very near the main, calling the city first is worth doing before paying for private diagnosis.

WHY DFW SEES SO MUCH OF THIS

Three regional factors compound. First, expansive clay soil across North Texas swells when wet and shrinks when dry, moving slabs and the rigid pipe attached to them through a seasonal cycle. Second, a large share of the housing stock in Dallas, Fort Worth and the inner suburbs predates the mid-1970s and has cast iron drain lines now beyond fifty years of service. Third, the region’s drought-and-downpour pattern makes the soil movement more extreme than a steadier climate would produce.

Newer suburbs like Frisco and much of McKinney have PVC drain systems and see far fewer drain-side failures, though the same clay soil still produces supply-side slab leaks. Established areas of Dallas and Fort Worth see both.

MISDIAGNOSES THAT COST REAL MONEY

Most of the money wasted on these problems is not wasted on repairs. It is wasted on repairs aimed at the wrong system. Four patterns account for the majority of it.

Treating a sewer smell as a drain clog. Repeated snaking clears the symptom for a few weeks at a time while a cracked line keeps leaking. Several hundred dollars a visit, several visits a year, and the underlying break never gets found because nobody ever ran a camera.
Opening the slab on a guess. Visible water surfaces at the low point of the floor, which frequently is not above the break. Breaking concrete before isolating and locating means opening the wrong spot and paying to open a second one.
Assuming foundation movement is a foundation problem. A long-running under-slab leak saturates and destabilizes the soil supporting the slab. Foundation work performed without first testing the plumbing can be undone by the leak that caused the movement in the first place. Testing before foundation work, not after, is the cheaper sequence.
Spot-repairing a system at end of life. On sixty-year-old cast iron, fixing one break often just relocates the next failure a few feet down the line. The right question is not what the cheapest repair is today, but how much serviceable pipe is actually left.

The common thread is that each one skips the diagnostic step. Testing costs a few hundred dollars; every mistake above costs thousands.

FREQUENTLY ASKED QUESTIONS

How do I tell a slab leak from a sewer leak myself? Two quick checks. Turn off every fixture and watch the water meter: if it still moves, you likely have a pressurized supply leak, meaning a slab leak. Separately, note whether you smell sewage or have multiple fixtures backing up, which points to the drain side. Neither check is conclusive, but together they usually indicate which side to test first.

Can I have both at the same time? Yes, and in older homes it is not rare. Both systems are exposed to the same soil movement and the same age. This is why a plumber will sometimes recommend testing both rather than stopping at the first failure found.

Does homeowners insurance cover it? Coverage varies significantly by policy. Many Texas policies cover resulting water damage while excluding the cost of repairing the pipe itself, and some exclude damage from long-term seepage. Read your policy and talk to your carrier before work starts, because carriers often require documentation such as a hydrostatic test result.

How urgent is it? A supply-side slab leak is actively releasing pressurized water under your foundation continuously, so it should be addressed quickly. A sewer leak is slower but introduces waste into the soil under the home and undermines the slab over time. Neither improves on its own.

Will the test damage old pipe? A properly conducted test stays within normal residential operating pressure and does not damage sound pipe. It can reveal that a deteriorated system is already failing, which is the information you are paying for.

Do I need to test if I already know where the water is? Usually yes. Visible water shows where it surfaced, which is often well away from where it escaped the pipe. Testing and locating prevent opening the wrong section of slab.

HOW LONG YOU CAN WAIT

The honest answer differs by which problem you have, and it is worth being specific rather than alarmist.

A supply-side slab leak is continuously releasing pressurized water into the soil beneath your foundation. That soil is the thing holding your slab level. Left alone, the leak both raises your water bill every day and progressively undermines the support under one part of the house. This is the one that should be diagnosed within days, not months.

A drain-side leak is not pressurized, so it releases water only when fixtures are used. That makes it slower, but it introduces wastewater into the soil under the home and it does not stop. The realistic horizon is weeks to months rather than days, and the cost of waiting is usually measured in how much additional soil saturation has to be dealt with when repairs finally happen.

In both cases the diagnostic step itself is cheap relative to the repair. Testing early rarely costs anything except the test; testing late frequently means paying for foundation consequences on top of the plumbing.

GET A DEFINITIVE ANSWER

We diagnose in the order above, tell you which system failed and why, and give you written repair options before anything is opened. Licensed and insured, flat-rate pricing, same-day and 24/7 availability across Dallas-Fort Worth.

Call +1-214-444-3697 or request a diagnostic visit. City pages: Plano leak detection, Dallas leak detection, Fort Worth leak detection. Buying a home? See our pre-purchase plumbing inspection guide.

Slab Leak vs Sewer Leak: How Testing Tells Them Apart in DFW