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Community and Q&A

What would cause water to migrate up through a slab and “sidewalk”?

Obeeswax | Posted in General Questions on

We have lived in a slab Foundation house for almost two years. When it rains after a dry period we will have enough water come up through slab that it takes a squeegee to get it under control. This happens right at the carport door entry and about a foot away from a half bath wall, as well as into the first six or seven feet of parking area.

My first guess is that the downspout was directed into a pipe that may have been routed under the slab and that has failed somehow. We have noticed it more since we had the well housing and pipes raised to stop sediment from getting into our water lines.

Any ideas or suggestions in problem solving would be greatly appreciated

Deepest regards

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  1. GBA Editor
    Martin Holladay | | #1

    When you say that this happens "about a foot away from a half bath wall," are you talking about indoors or outdoors?

    If all of this is happening outdoors, my next questions are:

    1. Are the areas in question -- the carport area and the parking area -- above the surrounding grade or equal to grade?

    2. Does the grade adjacent to these problem areas have any slope (away from the concrete)?

  2. Obeeswax | | #2

    Thank you Martin, it’s outdoors but covered by the roof, long ranch house.

    The walkway is equal to the grade and even though the carport is 4”lower it is mostly equal with the grade, and some below grade The entire yard needs to be re-graded as the soil is higher than or equal to the driveway in many places.and nature has done her job and soil has built up

    The water comes up about 25’ from the exterior carport wall. My current plan or rather thought, is to disconnect the downspout from the pipe in the ground and run the water about ten to fifteen feet from the house as I don’t have a camera that can go down the pipe. Maybe a diverter would be better. Not really sure on the best approach.

    Kindest regards

  3. GBA Editor
    Martin Holladay | | #3

    This sounds like a classic grading issue. Slabs and sidewalks should always be elevated with respect to the nearby grade, and the grade should (if possible) always slope away from a house or garage in every direction.

    That's easier to do in Vermont than it is in Kansas or Florida. These days, most contractors understand the need to elevate a slab. But some homes built in the 1940s or 1950s are not properly elevated.

    A tough problem to fix.

  4. Obeeswax | | #4

    Thanks Martin! We live in NC so it’s red clay all around. The house was started in 1969 and finished in 1970. I imagine if they put a vapor barrier down, it’s most likely disintegrated by now, even with no sun hitting it. Perhaps we need to bite the bullet and have 20 to 40 feet around the entire house regraded before we start building the shop and storage building. Should we also be thinking of a French drain around the house? We are in our late 50’s so this will be our last house unless we end up in a convelescent place.

  5. GBA Editor
    Martin Holladay | | #5

    Q. "I imagine if they put a vapor barrier down, it’s most likely disintegrated by now, even with no sun hitting it."

    A. That's unlikely. A subslab vapor barrier (polyethylene) will last indefinitely. But most contractors don't put polyethylene under sidewalks or carports.

    Q. "Perhaps we need to bite the bullet and have 20 to 40 feet around the entire house regraded before we start building the shop and storage building. Should we also be thinking of a French drain around the house?"

    A. It's almost impossible to give advice without a site visit, but feel free to post photos if you think that the photos can properly depict what's going on.

  6. Obeeswax | | #6

    Thank you so much! Don’t know if these are any good but it’s best I can do. Some with the slight grade won’t go small enough for me and still show the very slight grade

  7. Obeeswax | | #7

    The first shows the majority of the walkway having capillary action.
    The second is the step up from the parking pad
    Then we have the walkway around the house with the soil being at or slightly above the grade of the walkway
    And finally the corner downspout which may be adding to the problem

  8. GBA Editor
    Martin Holladay | | #8

    Most of the photos don't provide much additional information -- but the one showing the lawn adjacent to your concrete walkway says it all.


  9. Obeeswax | | #9

    Thanks Martin! Our plan is to bring the grade down about three inches and to slope it away from the house and the well. But why would the water start wicking up over twenty feet away and not near the grade issue in the beginning?

  10. GBA Editor
    Martin Holladay | | #10

    Good question; I'm not sure. It's possible that the soil under your walkway simply becomes saturated, and that your walkway lacks a capillary break (e.g., 4 inches of crushed stone) under the concrete, and lacks polyethylene. That would allow water to rise easily through cracks in the concrete.

  11. Obeeswax | | #11

    Thanks again Martin! I know that the little sidewalk did not have gravel under it as we had to cut a bit out to run the new electric for the well but that doesn’t mean they didn’t put it under the slab. I guess the next time the old plumbing backs up, we will have the plumber run a camera to see why it keeps messing up and I’ll ask them to put it down the spout drain as well. My guess is that it runs at an angle and connects to the washing machine drain and two more downspouts that feeds into the trees. Maybe the big oak in the front yard has sent feeder roots under the carport. They didn’t keep plans when this house was built so everything is a guess.

    Martin, your help has been invaluable, thank you so much!

  12. user-2310254 | | #12


    Do your gutters fill with leaves during the fall? If so, it's possible some of those leaves are falling into your downspouts and ending up in the buried drain lines. I used to have a house with this type of arrangement and had to unblock the lines on a regular basis.

  13. Obeeswax | | #13

    Thanks Steve! I just helped my neighbor clean his out. When my parents were alive, we had guards put in the gutters as I couldn’t keep up with Fall. Our gutters are still clean!

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