12 Ways to Detect a Supply Plumbing Leak Under a Slab Foundation

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Think you might have a leak, but you’ve looked in all the easy to see places? No leaky toilet flappers, dripping faucets, or sprinkler heads shooting water sky high? Maybe the leak is in one of the the hardest places to detect…below your slab foundation. Read on for twelve ways to detect a supply plumbing leak under a slab foundation. We are going to go through all the potential signs so you might even detect it before the first atrocious water bill (step 11) hits your mailbox!

We’re going to go from the most obvious to worst case.

1. Check the plumbing lines going out of the water heater.

Do you see anything? Do you hear anything? The sound of rushing water is the first clue you are on the right track.

This is the place you’d rather hear the sound–you can shut of the hot water without shutting off the entire house. Given you don’t feel “lucky” at the time, but it is the best of the bad answers if you have a traditional trunk and branch system as opposed to a really awesome manifold system. In this case, you still can still flush the toilet while you determine the source and fix the leak.

If you don’t hear anything then…

2. Check the main plumbing stack coming into the house.

Do you see anything? Do you hear anything?

I will confess I do not go out and listen to the water main going into my house on a daily basis (or weekly, or monthly, or yearly for that matter). It is a good idea to add it as part of your regular maintenance routine. Or, if you are not a maintenance person or travel quite often, go all out on a whole house water monitor (as I will be next week).

This is also a good point to note just because you did not hear anything at the water heater does not mean there is not a problem with the hot water line. Perhaps the leak is too small, or, even worse, the leak occurs between the main stack and the water heater. A water leak before the unit is just as bad as a cold water line leak in terms of using sound to detect the problem.

The next few of these detection methods really only apply to hot water line leaks before we head into worst case scenarios.

3. Pet detection.

If you have a cat or a dog, finding them unusually comfortable in an unusual spot is a highly effective way of detecting the location of a hot water leak.

Honestly, the cat found my under slab foundation leak before I noticed. He detected the warm spot on the floor leading away from the water heater.

It was only after hearing water running constantly from the plumbing coming from the water heater that I checked the odd spot on the floor.

4. Feel the floor yourself.

Even if you do not have a feline-warm-spot-magnet mechanism to determine the location, it would have been easy to find the warm spot in comparison to a cold water leak where there is no difference of temperature from the water leaking into the slab.

5. Cold showers?

Another sign of a hot water supply line leak is when the water in the shower would have been noticeably colder. My guess is at this point you should definitely be hearing the water rushing out of the water heater. The water heater is probably constantly running too.

6. A consistent drop in water pressure.

You may notice your water pressure is slowly dropping. As the problems gets worse, the lackadaisical water coming out of your faucets sends you straight to a plumber named Google. The problem becomes undeniable as your water pressure precipitously declines to a calamitous drop to nearly nothing.

7. Call in a forensic plumber.

These gals/guys will bring in their detection equipment and, in addition to finding the location of the leak, map out your plumbing lines. Once you have a leak strong enough to detect, this step is inevitable with a truck and branch system. You need to know where the problem is and where your invisible lines are run so you know where to put in a bypass.

An ‘X’ with painters tape marks the spot!

8. Put in a whole house monitoring system.

Can’t find a problem, but not willing to wait for the leak to get worse? A Moen Flo or an Uponor Phyn will tell you how bad your plumbing is leaking.

9. Popped tiles or soggy carpet.

Concrete is porous. Even though the plumbing lines run below the slab, a leak is a constant source of water soaking into the concrete.

The mortar gluing your tile will eventually disconnect from the saturated cement, starting with a creaky tile. You can also be unpleasantly surprised with a damp sock or squish between your toes if the leak appears in a carpeted room.

10. Mold / mildew smell.

Sometimes you’ll smell it before you see it or feel it. If you smell a funk odor, it just might be water seeping up through the concrete into your flooring or walls.

11. Keep tabs on your water bill.

I know many of us automate this bill and don’t look at it. However, at least take a peek…like you review your purchases monthly before paying off your credit card.

Did it unexpectedly go sky high? You might be the new neighborhood car wash and might want to invest in some Wyze cams…or you most likely have a leak.

12. Run your own test with the water meter.

Make sure no water is running (intentionally) anywhere in the house. Then go check your water meter outside. It is time to call the forensic plumber if it is still turning.

Have you ever had the unfortunate experience of a supply line plumbing leak under a slab foundation? Did you detect it a different way? Please share your experience in the comments below!

I just want to clarify how supply line plumbing should be run, mostly because I didn’t know about it before I was forced into caring. Supply lines are supposed to be run before the slab is poured. That is, whatever kind of pipe, be it PEX, copper, or CPVC, they are supposed to be run in the dirt. Then they will be covered in order to separate the supply lines coming into the house from the slab. It’s expected the ground will shift over time. There is a saying in construction: it’s not a matter of “if” you get a crack in your slab foundation but “when”. By keeping the supply lines separate from the concrete, the supply lines are less likely to break when the slab moves.

I realize the picture shows drain lines instead of supply lines, but the idea is the same–the lines are all buried below the slab, not embedded in it. Dig a trench, drop the sleeved lines in said trench, cover with dirt and pour the concrete.

Of course, there are always times when the builder doesn’t follow the correct procedure and pours the concrete right over the pipes.

You can see the leaks caused in part by the shifting of the foundation on this side of the house.

On another line, you can see where this is not the first time this pipe has been fixed, due to the fact that the supply line bubbles OUT above the foundation.

FYI on top of an FYI…even if the pipes are properly installed underground beneath the slab, they should also be placed within a conduit. This protects the plumbing from exterior element corrosion, like the pH of the dirt or plant roots.