The most reliable way to tell if you have a water leak underground is the meter test: shut off every fixture in the house, then watch your water meter. If it is still moving, water is escaping somewhere it should not be, and on a buried line you will almost never see it happen.
That is what makes underground leaks expensive. They run for weeks before anything looks wrong. The average household wastes nearly 10,000 gallons of water a year to leaks according to WSSC Water, and the EPA puts the national total at close to 1 trillion gallons annually, with 9% of homes leaking 50 gallons or more per day.
We run one to two water line jobs a day across Montgomery County. These are the six signs that show up first, in the order homeowners usually notice them, plus what to do the moment you spot one.
How to Tell If You Have a Water Leak Underground: The Short Answer
Watch for these six signs:
- A wet, soft, or unusually green patch in the yard that never dries out
- A water bill that jumps with no change in how you use water
- The sound of running water when every fixture in the house is off
- Water pressure that has been slowly fading for months
- The lowest toilet in the house gurgling, or drains that keep backing up
- A warm spot, damp flooring, or a musty smell on a slab floor
One sign is worth checking. Two together usually means a buried line has already failed. The meter test in the section below confirms it in about ten minutes.
The Six Warning Signs of an Underground Water Leak
1. A Wet Spot in the Yard That Never Dries
This is the one that brings in the most calls. A soft, spongy, or standing patch that hangs around through a dry week is water surfacing from a break below.
Look for grass that is greener and growing faster than everything around it. A pressurized water line is essentially irrigating that spot. On a sewer line, you may get the same wet patch with an odor.
The trap here is location. Water travels along the outside of the pipe and surfaces wherever the ground lets it, which can be twenty or thirty feet from the actual break. The wet spot tells you a leak exists. It does not tell you where to dig, which is why underground leak detection comes before any shovel does.
2. A Water Bill That Spikes for No Reason
Nothing changed in the house, nobody filled a pool, and the bill doubles. On a service line running under a yard or driveway, that bill is often the only symptom for months.
A useful comparison: pull up your last four bills and look at the same month last year. WSSC’s own high bill adjustment process keys off exactly that comparison, how far your current average daily consumption sits above the same season a year ago. If this month is triple last year’s, you have your answer, and property owners carry responsibility for leaks on private property either way, so save the invoice and permit paperwork for the adjustment request.
Occasionally WSSC spots the usage before the homeowner does and makes the call. Treat that as a starting point, not an answer. It confirms water is leaving the system without telling anyone where.
3. You Can Hear Water Running When Nothing Is On
Pick the quietest time of day, turn everything off, and put an ear to the floor of the basement or against the wall where the service line enters. A steady hiss, a faint rush, or an intermittent trickle is pressurized water pushing out through a break.
Homeowners catch this one more often than they expect, especially at night. It is the crude version of what our crews do with ground microphones, listening for where the escaping water is loudest. If you can hear it at all, the leak is usually substantial.
4. Water Pressure That Has Been Slowly Fading
A leak bleeds pressure off the line before it ever reaches your fixtures. If pressure dropped off gradually over months rather than overnight, a buried line belongs on the suspect list along with a corroding main and a partially closed valve.
We wrote a full diagnostic on this if pressure is your main complaint: what causes no water pressure in Clarksburg homes walks through the six most common culprits.
5. The Lowest Toilet Gurgles, or Drains Keep Backing Up
This sign points at the sewer side rather than the water line, and it is worth knowing the difference because the fix is different. A water line is pressurized, so it pushes water out. A sewer line is not, so it backs water up.
The signs of a sewer line problem are their own short list:
- The lowest drain or toilet in the house gurgles when you run water anywhere else
- Backups return within weeks of being snaked
- More than one fixture drains slowly at the same time
- A wet patch in the yard that carries an odor
- Sewage smell in the basement or around the cleanout
In older Montgomery County neighborhoods, roots are usually the reason. Big mature trees, or trees planted too close to where the cleanout or tap connection runs, work into the joints in clay and cast iron pipe. A sewer camera inspection is what tells you whether you are dealing with roots, a break, or a line that has already collapsed behind them. If it needs replacing, sewer line repair and replacement covers the options, and our guide to trenchless versus traditional sewer repair explains when we can do it without opening a trench.
6. A Warm Spot, Damp Flooring, or a Musty Smell on a Slab
If the leak is in a line running under a concrete slab, the signs move indoors. Watch for a patch of floor that feels warm underfoot, which points at a hot water line, flooring that buckles or stays damp, carpet that will not dry, or a mildew smell with no visible source.
Homeowners panic at the words “slab leak” because they assume we are going to rip up an entire room. That is often not the case. Depending on the run, we can reroute the line, pull new pipe, or use another method that leaves most of the floor alone. Our slab leak detection and repair page walks through how we find them with acoustic and thermal detection and what each repair approach involves.
The Ten-Minute Meter Test That Confirms It
Any of those six signs is a reason to suspect a leak. This is how you prove one, for free, in about ten minutes. It runs in two stages, and the second stage is the one that matters because it tells you whether the problem is buried outside or sitting inside your walls.
Find the meter first. In most WSSC-served homes around here it sits in a pit near the curb or the property line under a metal or plastic lid, and in some older homes it is in the basement near where the line enters. Lift the lid and clear out the leaves and water so you can read the face.
Stage one, is water moving at all:
- Shut off everything that uses water. Faucets, toilets settled and quiet, dishwasher, washing machine, ice maker, humidifier, and any irrigation zone or timer.
- Read the meter. Write the number down. Most meters also have a small triangular or star-shaped leak indicator that spins at even a trickle, so watch that too.
- Leave it alone for 20 to 30 minutes and use no water whatsoever.
- Read it again. Any movement at all, or a leak indicator that turned, means water is escaping somewhere in your system.
Stage two, inside or outside:
Now find the main shut-off valve where the service line enters the house and close it. That valve separates everything indoors from the buried line running out to the street. Repeat the wait and the reading.
- Meter still moving with the house valve closed: the leak is outside, on the buried service line between the meter and the house. That is the underground repair, and it is the one you want located before anyone digs.
- Meter stops when you close the valve: the leak is somewhere in the interior plumbing. Still worth fixing, much less involved.
Rule out the most common false alarm. A toilet that refills silently will fail this test and look exactly like a hidden leak. WSSC recommends a dye tablet check: drop a tablet in the tank, wait 15 minutes without flushing, and if color bleeds into the bowl the flapper is your culprit and not a buried pipe.
One thing not to do with the result: dig. Depth, unmarked utilities, and the way water surfaces far from the actual break make exploratory excavation more expensive than locating the leak properly. Utility marking is required by law before any excavation in Maryland, and only a licensed master plumber holding a permit can legally do underground water or sewer work here. If your test points outside, our walkthrough of a real Potomac job where the line ran the length of a football field under a shared driveway shows what happens next.
Why Underground Lines Fail in Montgomery County Specifically
Two things decide whether a buried line gives out: what it is made of, and how long it has been sitting in our soil and our water. When you tell us where you live and roughly when the house was built, we can usually narrow down what is under it before we ever pull up.
What Is Probably Buried Under Your Home
- Bethesda, Chevy Chase, and Rockville: older housing stock, usually galvanized steel or copper
- Gaithersburg: copper, plus polybutylene in homes built while it was still being installed
- City of Frederick: mostly copper, with the surrounding county varying by how old the neighborhood is
Beyond those, on water service lines we also run into PVC, grey pipe, well pipe, and black poly. On the sewer side it is clay, cast iron, Orangeburg, and PVC.
Why Each One Eventually Fails
- Copper develops pinhole leaks, and this is the one that surprises homeowners most. Local water chemistry contributes. A line that performed fine for thirty years starts weeping from a dozen tiny points at once.
- Galvanized steel rusts from the inside out. The corrosion narrows the pipe until pressure noticeably drops, and eventually it perforates.
- Polybutylene is not installed anymore for good reason. It grows brittle with age and tends to let go at the fittings.
- Clay and Orangeburg sewer lines fail at the joints and seams, which is exactly where tree roots find their way in.
- Cast iron scales and corrodes, narrowing the line and catching everything that goes down it.
Water chemistry matters more here than most homeowners expect. In Poolesville the mineral content is the worst in our service area, and appliances wear out faster because of it.
None of this means your line is failing. It means the age of your home is a real clue about what is buried under it, and a leak in a 1985 copper service line is a different conversation than a leak in a 2015 poly line.
What Happens After You Spot a Sign
Confirming a leak exists is the part you can do yourself. Everything after that needs equipment and a permit.
The next question is always whether the leak sits on your side of the tap connection or WSSC’s, because that decides who pays. We settle it with valve isolation, line locating, and credentialed access to WSSC’s WERI records rather than guesswork, and on sewer lines, the camera makes it obvious. Our post on how we determine whether a leak is on your side or WSSC’s walks through that whole process on a real job, including why trenchless took $2,500 off the quote.
Two things worth knowing before you call anyone:
- Only a licensed master plumber holding a permit can legally perform underground water or sewer work in Maryland, and the work has to be inspected.
- Ask whether the company owns its leak detection and locating equipment. Plenty of shops subcontract it or skip it and dig instead. We run our own sonic detection, cameras, and locators, which is why we end up being the second or third plumber on a lot of these jobs.
When the work is done, underground excavation carries a three year warranty, labor carries one year, and water and well line materials carry a lifetime warranty. Financing through GreenSky is available, which matters because a buried line almost never fails at a convenient time.
Scope and options live on our water line repair and replacement page. Outside Clarksburg, we also serve Gaithersburg, Germantown, Rockville, Potomac, Damascus, North Bethesda, and Frederick.
Frequently Asked Questions
How Do You Tell If a Water Leak Is Underground or Inside the House?
Close the main shut-off valve inside the house, then watch the meter. If the meter keeps moving with the house valve closed, the leak is on the buried service line between the meter and the house. If it stops, the leak is somewhere in the interior plumbing.
Can an Underground Water Leak Go Unnoticed for a Long Time?
Yes, and that is the norm. A buried line can leak for months with no visible sign except a slowly rising bill, because the water disperses into the soil instead of pooling where you would see it. This is why the meter test matters more than a visual inspection.
What Are the Signs of a Slab Leak?
A patch of floor that feels warm underfoot, flooring that buckles or stays damp, carpet that will not dry, a musty or mildew smell with no visible source, an unexplained bill increase, and sometimes the sound of running water under the floor. Slab leaks rarely require tearing out a whole room.
Who Do I Call for an Underground Water Leak?
A licensed master plumber. In Maryland, only a master plumber holding a permit can legally perform underground water or sewer line work, and the work has to be permitted and inspected. Ask whether they own their own leak detection and locating equipment, because plenty of shops subcontract it or skip it and dig instead.
What Is the Difference Between a Water Line Leak and a Sewer Line Leak?
A water line leak is pressurized clean water escaping, so it shows up as a rising bill, falling pressure, and a wet spot with no smell. A sewer line problem is not pressurized, so it shows up as backups, gurgling from the lowest drain in the house, and a wet patch that smells. The tests are different too: water lines get acoustic detection, sewer lines get a camera.
Can Tree Roots Cause an Underground Leak?
On sewer lines, constantly. Roots find the joints and seams in older clay and cast iron pipe, and the usual culprits are big mature trees or trees planted too close to where the cleanout or tap connection runs. Snaking with a cutter head clears them short term, jetting with a root treatment buys longer, and a camera tells you whether the pipe underneath is still sound.
How Long Can I Wait to Fix an Underground Leak?
Not long, and the bill is the smallest part of why. Water escaping under pressure erodes the soil around it, which can undermine a driveway, a walkway, or a foundation over time, and a saturated area only gets more expensive to excavate. If your meter is moving with the house valve closed, treat it as this week’s problem rather than this season’s.
Is It Worth Paying for Leak Detection, or Should We Just Dig?
On a short, accessible line, an experienced crew can often scope the repair without a full detection, and we will tell you when that is the case. On a long run, under a driveway, near a street, or anywhere access is difficult, detection pays for itself immediately by shrinking the excavation. Guessing wrong on a buried line is far more expensive than finding it first.
The Takeaway
Underground leaks do not announce themselves. They show up as a soggy patch, a strange bill, a sound in a quiet basement, or a floor that will not dry. Any one of those is worth ten minutes at your water meter.
If the meter is moving, do not wait for it to get worse and do not start digging. We own our leak detection, camera, and locating equipment, and we like being the second or third plumber on the job because finding what other crews could not locate is the part of this work we are best at.
Four generations, family owned and women owned since 2008, three master plumbers on staff, WSSC #140, MHIC #118444, and 4.8 stars across more than 500 reviews. Our after-hours calls are answered by our own team, not an outsourced service.
Let’s get started. Call (301) 972-2223 or book online.
Let’s Flow.
