Repairing the Slab Leak

When my foundation repair consultant, Richard Nash, suggested I have a plumber out to check for a slab leak, he made one criteria clear.

“Have them wait at least two hours to see if the water level drops.”

Here is what he meant. A lot of the slab leaks in houses built in the 1970s as mine was occur in the out-going or sewer lines. And that is because the cast iron pipes used under the concrete slab foundations are starting to fail.

The plumbers come in and pull up one of the toilets and set it aside. Then they basically plug up the sewer lines and run water from faucets until they can see water backing up into the line where the toilet was. They let the water come up to just below the floor tile then shut it off.

Then the waiting begins to see if the water level drops. If it does, you have a leak. Sometimes it drops right away and sometimes it takes awhile. That’s why Mr. Nash advised a two hour window to see what happens.

If the water level in the sewer line does not drop after two hours it’s good news. (Do a happy dance.) You do not have a leak in your out-going lines.

Mine did drop and it took about a minute. In fact the plumbers had a hard time getting the water up to the slab, the leak was so bad. Not happy news. I definitely had a slab leak.

Slab Leak on Incoming Water Lines

Checking to see if you have a leak under your concrete slab foundation in your incoming water lines is something most homeowners can do themselves. Turn off ALL valves and faucets, make sure NO water is running. This includes water lines to appliances such as water heaters and ice makers.

Go get a reading on your water meter. Wait a while, a few hours. Go do something else. Go to a movie. Re-organize the sock drawer. Rotate your hats. Then check your water meter again.

If the meter has moved, showing water flow, you have a leak. FINDING the leak under the slab foundation is a thing best left to the plumbers who have the equipment to do so.

In sewer lines the plumbers often send a video camera attached to a plumber’s snake through the clean-outs to locate the leaks. My situation was complicated by old school cast iron joints that had 90 degree angles that prevented the camera from getting under the foundation. It was like hitting a wall.

So job one was retrofitting the clean-outs with new PVC lines that had a sweep (or a curve) to them which allowed the camera to pass.

They found the first slab leak where my laundry closet plumbing ties in with the guest bathroom. The best way to access the leak was to move the washer and dryer  and go through the floor under the wall between the two rooms.

Photo of hole in the slab foundation and floor.  A four foot square hole was jack hammered through the floor and slab. Then piles of dirt were dug out to expose the plumbing. It was loud and dirty work.

The plumbers found a good sized hole in the joint area of the cast iron sewer line and replaced all the cast iron they could reasonably get to with new PVC. With all the plumbing lines exposed, they could visually check for incoming water leaks and there were none.

Photo of plumber repairing slab leak.

After making the repairs under the laundry and guest bath, they did another pressure test. Another slab leak. (Oh great. More money.)

This leak was under the foundation where the kitchen and master bath plumbing ties together and the best way to reach it was by tunneling from the outside. They ran into very wet clay soil while digging under the foundation.

Photo of tunnel dug to access slab leak

Again, the plumbers found cast iron sewer pipes that were failing. Admittedly, some of the holes you see in this picture were caused by the force required to pull the lines out, but the smaller ones were the source of the slab leaks.

Photo of broken cst iron pipe causing slab leak

The cast iron was replaced with PVC as much as was practical, the system pressure tested again and this time no more leaks were detected.

The bottom line was, before the repairs, every time we washed dishes, took showers or baths and did the laundry, we were leaking water under the slab foundation. This had probably been going on for years.

So, the result of a slab leak in clay soils could well be upheaval in the interior part of the foundation which causes the outer edges of the slab to appear to drop or settle and will cause the same signs of foundation movement..cracks in the brick or sheet rock, and doors and windows that don’t work right.

What I’ve learned from this experience is this: before you take on the cost of foundation repair get a hydrostatic pressure test done on your plumbing system. You’ll have a better picture of what is going on under your slab as you may have upheaval.

If you do have piers put in, get another pressure test done afterwards to make sure no plumbing was pulled apart during the jacking and leveling process. Reputable foundation repair companies will work with you on that and may even offer it as part of their service.

My foundation repair consultant Richard Nash says I should wait six months to a year to see what happens as the clay soil under my foundation slowly dries out.

I don’t know, I sure would like to fix those drywall cracks. They be ugly.


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My Foundation Slab Leak-Part One


    • May 22, 2014

    My condo association is performing hydrostatic tests this month. My bedroom walls have all cracked and the front of the room has risen higher than the rest. So, I’m curious: if a leak is found and fixed, why would they consider putting piers into my bedroom floor if it has already risen higher? Shouldn’t they wait to have it (soil and building)settle back down to determine if piers are needed?

  1. Yes, they should wait. It could take at least six months for the soil to settle back.

    • CuriousGirl
    • July 23, 2014

    How much did the replacement/repairs cost and how big is your home?

  2. It’s a modest home, about 1200 square feet, and the cost was 4K.

    • Diane
    • June 24, 2015

    My home is also 1200′. Was 4000 the price of the plumber or foundation or the total price altogether?

  3. 4,000 was the cost ofthe foundation repair. The slab leak repair was almost twice that. They had to dig under the house in two areas (one inside) and replace most of the cast iron pipes with PVC. Very expensive.

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