How to Fix a Slab Leak

Slab leaks occur in the pipes underneath the slab foundation. There are supply line slab leaks and drain line slab leaks. On supply line slab leaks, the homes hot and cold water supply distribution system is plumbed underground.

Each fixtures supply piping is extended above the ground and configured into a manifold assembly whereby the fixture is supplied with hot and cold water. Continuing the journey, the hot and cold supply lines go back down into the ground and come up at another fixture in a looping fashion.

This system is designed to keep from having any tees or connections underneath the concrete slab. The problem with this system is that eventually, due to wear and tear as well as oxidation, pinhole leaks develop somewhere in the water supply line and access for repair becomes extremely difficult underneath a concrete slab. Symptoms of a slab leak include wet floor or carpet, large water bills, a low-level hissing sound can sometimes be heard inside wall cavities as the pipes carry the noise along with them inside the walls and cabinets.

The wet spot could be cold or warm to the touch. Here is a thermal image photograph of a pinhole leak in a hot copper water supply line underneath a concrete floor. This particular floor had a wood floor installed over the concrete floor.

The thermal camera pinpoints the location of the leak, and the resulting location justifies a re-route of the supply line instead of a point repair beneath the wood flooring and concrete.

The first thing you should do to verify if you have a slab leak and whether it’s on the hot or cold side of your water supply is to go out to the curb and remove the lid to the water meter.

There is a slot in the top of the cover that you can insert a large screwdriver to pry the lid up and out. Be prepared as it is pretty heavy. You should be wearing some protective gloves and have a small towel and flashlight handy. The inside of this water meter box is known by many professionals to harbour insects and spiders. Very often you may find black widow spiders that have a toxic venom so use care when accessing this meter box. On top of the water meter, you will see a gauge. If you don’t then the cover is on it and you will have to open the protective cover by lifting on the bottom of the cover plate, and it should respond and swivel open. Now you are looking at your meter gauge, wipe it off with the towel, and you should see a large red pointer that measures the flow of water travelling through your meter. One full turn around the dial equals one cubic ft of water (1 cubic foot of water is equal to 7.48 gallons of water). The small triangular metering device that spins the fastest measure very small amounts of flow.

Now for the test. Go inside the house and make sure no one is using any hot or cold water and verify that the toilets are not leaking from the flappers or filling up as this will throw off the results of the test. Before you shut the water off to your home, go to the water heater and see if there is a motor pump connected along the waterline from the bottom or any motor is in line with the water line. This is known as a recirculation pump, and if you turn the water off while this pump is plugged in and in the run mode, there is a possibility you can destroy the motor. So unplug it until you are done with the testing. If it has a timer on it, then you just reset the time when you are finished with testing.

Now, go to the service shutoff valve that controls the water flow coming from the street and entering your home. Turn that valve completely off. If the valve is a lever ball valve, it should be easy. If it is a globe valve or conventional gate valve, then you should be very gentle as you turn it off and don’t over tighten because if it’s over several years old. There is a chance that the gate valve can break in the off position requiring immediate replacement if you plan on using water for the rest of the evening.

Now that you have the water turned off at your service valve at the entrance to your home examine the meter. Look at the small triangular metering indicator. Does it move? How fast? If it moves and continues to move in the same direction, you have a leak on your service line somewhere between the water meter and your water service at the entrance to the home.

It’s important to note that if your scenario shows a leak on your service line, then attempting to find the location of your leak in the home and knowing that its appropriately repaired by the end of the repair will be impossible. You have to first fix the leak on the service line and stabilise the reading at the water meter so that it shows no flow to the water service.

Once you have accomplished this, reopen the service lever ball valve or gate valve all the way open and look at the meter again. Do the indicators move in the same direction consistently? If so, turn off the sprinkler shut off valve that feeds your sprinkler manifold. Did it stop spinning? If so, you probably have a leak somewhere between the service shut off valve and your sprinkler valves. The valves themselves could be leaking. Check them… Are they wet? Is one in particular wet? Find and repair the leak and or fix the leaking valves and re-verify that with the meter indicators.

Ok, now turn the sprinkler shutoff valve to the off position and look at the meter indicators. Do you see movement? If not, the water you have discovered in your home may not be from a water supply line source so do some more investigating. What device uses water and is closest to the wet spot?

Ok, let’s assume you did see the indicator moving and we now see a constant movement turning the same direction on the indicator. Now we have determined that something is calling for water past your service line valve and past your sprinkler shut off valve. The next test is to determine if the leak is a hot side leak or a cold side leak. Once in a while, it’s possible to have both simultaneously, but it is very rare.

Proceed to the water heater. Unplug the recirculation pump if one is installed anywhere on the water heater. Never run the recirculation pump while you have the water shut off, you may burn up the pump! Now, turn off the shut off valve on top of the cold inlet of the water heater. Go back outside and check the water meter for movement on the indicator. Did the small indicator stop spinning? If so, you have a slab leak on the hot supply side. If, on the other hand, the meter indicator continues to spin with the water heater shut off valve in the closed position. You have a cold slab leak for sure, and after fixing the cold slab leak, the test for the hot water slab leak must be performed once again to rule out the possibility of a hot slab leak. It’s rare to have both a cold and a hot slab leak simultaneously. However, they do exist.

Congratulations! You have just diagnosed your home for a possible slab leak. Repairing a slab leak is a challenging and difficult job and is not recommended for a do it yourself project. We recommend you call your local plumber to handle this repair!