Testing For A Leak Under Slab Of Your Building: A Helpful Guide

Opinion & Expertise
January 24, 2020

by David Balkan


Many water leaks are difficult to verify or pinpoint. However, a leak under slab of a house or commercial building is about the most difficult to diagnose. Of course one of the main reasons is that the point of the leak is not visible. But there are other factors that complicate a leak under slab as well. In many cases it takes careful testing simply to determine if the leak is from your sewer line or your water line. In many cases these types of leaks are completely misdiagnosed, which results you having expensive work being done, and with no positive benefit.under-slab-water-leak

Want to know the telltale signs that you may have a leak under your slab floor? There are 3 basic signs you may have a leak:

1. In some cases you will hear a rushing or hissing sound under the floor inside your building. It may even sound like the sound a radiator makes when steam leaks out. The louder it is, the closest the leak is to the surface of your slab.

2. The 2nd and most obvious signs of a leak under the floor or slab is puddling or damp spots. You may assume someone spilled some water, but if these puddles are there all the time, you probably have a leak.

3. The 3rd sign is if you water meter is spinning when no one is using water in the building. This is typically caused by a running toilet. But if you eliminate the possibility of a running, you may have an inside leak. Taking care of this leak is imperative, as all the water leaking is going to be billed to you on your next water bill.Your water bill may exceed the cost of the actual repair!

Make sure the leak under slab is not a sewer issue

Leaks under slabs of buildings can even be confusing as to which type of pipe is leaking. Visible above ground leaking water can be from backed up sewer lines that are under pressure. Sewer leaks under pressure can appear to be just like a fresh water leak from a water line. With that being the case, the first step is to make sure your house sewer isn’t backed up. If you’re not sure how to locate your house trap, read How To Find The Sewer Trap And Locate Your House Sewer.

Checking for a house sewer backup

After making sure the city sewer is not backed up (this is known as a public sewer surcharge) open the sewer plug on the street side of your trap. Every main line trap should have 2 plugs or caps. Always open the street side plug on your trap 1st.  The reason for that is if there is a stoppage in the trap itself. If there is a trap stoppage, having the street side plug open allows for an outlet for the backed up waste water. You can then safely open the house side plug. If all waste water is flowing smoothly, it is unlikely your leak is from the sewer.

sewer cleanout
Typical NYC approved double vent main house trap

A 2nd way to check for a sewer backup on a main or branch line drain is by doing a dye test. Simply place some food coloring, or clothing  dye, inside each plumbing fixture in your basement. Then introduce running water. If you have a branch line drain backup, the dye will appear on your floor after about 10 to 20 minutes.

Verifying a water leak is not from outside your Home

Water under pressure will travel to the point of least resistance. that means that leaks from outside your home can appear to be inside your home, and visa versa. What can assumed to be a leak under slab, can very well be a water service line leak outside of your home. That being said, you should test for a water line leak. The 1st step to do that is to close the main control valve closest to the inside of your home. You would then have to drain down the plumbing in your home by opening a sink on the upper level, and a sink on the lowest level. Doing this will allow all the water in your building to drain out, and prevent a misdiagnosis from a running toilet, etc.

Simple tools amplify the sound of water leaks for easy detection

Patience is a key element in accurate leak verification

To verify a leak is not from under your building slab, you need to exhibit patience. If you have a leak from a water line under your slab, it takes quite a while for the water to completely drain out from the leak. It is recommended to at least wait 20 to 30 minutes after turning off the water. Obviously, if the leak stops after that period of time, you know there is a leak under slab. For more information about accurate water  leak testing read A 10 Step Guide To Water Leak Testing.

If the leak does not stop, chances are that you have a water service line break. You can actually place your ear on your water service line and listen for the leak. It is probably best to call a professional to do the leak test, or call your local water authority. A Professional plumber or DEP investigator will have specialized listening tools.

Testing for the location of Your leak

In almost all cases water leaking from out of a pipe makes noise. It can be a hissing sound on small leaks, to a rumbling sound on larger leaks. There exists specialty acoustic equipment that can “sound out” a leak, and locate it. Typically it has to be a quiet environment, so the leak testing may have to take place before or after-hours. There are even specialty tools that work on vibration, or are heat sensing. The point is that if your leak is in a sensitive or unknown area, professionals have the tools to pinpoint it.

leak testing
Acoustical leak testing tool

Repairing a leak under a slab of Your building

Most repairs of leaks under your building slabs are not costly, or too difficult. Most piping under a slab is relatively shallow, and fairly accessible once a small hole is dug. Of course there are exceptions to this, but they are rare. As an example, in most homes the underground house drain is only 2′ deep under the slab, or less. If your have water lines under your slab, they may only be 1′ or less in depth.

If your leak is on a water line, frequently it can be re-routed above the slab, this preventing any breaking at all. This is particularly helpful if your home was built on filled in ground, or is under a finished living space. These situations are why contacting a Licensed Plumber can pay dividends. A licensed professional plumber can save you both money and unnecessary damage to your finished surfaces.

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David Balkan

David Balkan is the CEO of Balkan Sewer And Water Main, the largest and most trusted service in New York City. David is extremely active in various NYC plumber organizations being the Chairman of the Subsurface Committee in the Master Plumbers Council, and Vice President of the Subsurface Plumbers Association. In addition David’s expertise is respected by officials of New York City agencies such as the NYC DEP, NYC DOB, and the NYC DOT. He frequently provides valuable input on a variety of industry related matters.

Dave Balkan

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