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Where is the water under my concrete slab coming from?

Answered by Jeffrey Anderson ~ June 3, 2013 ~ No Comments » | Respond to this question

We have a moisture problem in our concrete slab and don't know where it is coming from. We had glued down engineered hardwood floors that starting coming up off of the floor. We had leak detection company check for plumbing leaks and they found none. Where could the water underneath the slab be coming from and is it a major problem?

- Brian

Jeffrey Anderson

Brian, ruling out an under slab plumbing leak was a good first move as that would have been my fist guess as well. I have encountered quite a few over the years. They can be difficult as well as expensive to repair,

The simplest cause of your water problem might be the site drainage around the home. If the ground slopes toward the slab in any areas, the water flowing in the home's direction during a rain could be more than the drainage system can handle. If this appears to be the situation, creating some drainage swales in the yard or installing a French drain should solve the problem. I would also suggest checking to ensure that all of your gutter downspouts are directing their flow away from the home.

Unfortunately, my next guess as to the cause isn't quite as easy to detect. In many parts of the country, contractors install a drainage system consisting of drain tile and gravel under a home's concrete slab. It's possible that the ends of the drain tile where they exit from under the slab may have gotten blocked by debris or even grass clippings. Removing the blockage may allow the water to drain.

However, if the plumbing contractor installing the under slab water and drain lines or the concrete contractor were not careful, they may have damaged the drain tile in the interior part of the slab in one or more spots. This eliminates the drain tile as any easy exit point for the ground water that exists under just about every slab, and static pressure can force it up through the concrete.

Repairing a crushed drain tile can be difficult as the location may be tough to find, and it normally requires tearing up part of the concrete slab. In this case, installing a French drain around the perimeter of the slab may be the best solution.

Determining the cause of under slab water intrusion and remedying it is often a trial and error process. I recommend holding off on replacing your flooring until you are sure that the issue has been corrected. You should also be careful when hiring a contractor to fix the problem. They should have extensive experience in dealing with foundation and slab water leaks.

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