About 4 years ago my wife & I purchased a home. It was approximately 10 years old at that time. The foundation is a concrete slab.

Approximately 2-3 years before we purchased the house, the previous owner had an addition built on the back of the house. They poured a new slab for this addition. It appears that the slab was not appropriately treated to resist moisture.

Fast forward to the present - we notice dark spots in the wood floor in the addition. It appears we have moisture coming up through the slab.

We have had two plumbers and a landscaper come in to look at it. There is no plumbing in the addition and both the plumbers and landscaper have suggested there may be old irrigation lines under the addition. We did investigate this possibility and discovered that there are old sprinkler lines, but they have been disconnected and capped. There was an issue with the neighbor over-watering his lawn, with the result that water was pooling next to the addition. We have spoken to him about this and it has been resolved. We also did some landscaping to slope the ground away from our house and back into his yard. Finally, we also installed gutters so when it does rain, the water is directed away from the house and directly into drains. I should mention that we are in Southern California, and the water table in our area is very deep and there is little danger of heavy rain.

After all this, I am reasonably certain we have addressed the source of the moisture, but the damage is done & it's now time to undertake repairs.

We are going to have a professional replace the flooring. We have gotten two estimates so far, but frankly, none of these flooring professionals seem to have a clue what to do. We want to remove the old damaged flooring, eliminate mold (if any), apply some sort of waterproofing to the slab, and replace the flooring. We did some research into products for sealing the slab, but neither of the contractors we've talked to seem to know anything about them.

Here's what I understand needs to be done:

  1. rip out old flooring
  2. clean the slab with bleach to remove mold (if any)
  3. allow the slab to dry out
  4. apply something to seal the slab against future moisture incursion
  5. apply flooring

I have two questions: first, is this plan correct? Am I missing any steps or are there any that are not correct? And second, what sort of products can be used to seal the slab? A plastic sheet barrier? Some sort of "paint on" sealant?

  • 1
    If it were me, I would stop at point #3 for a while to see what is going on. Maybe for 6 months or longer. You need to get a handle on exactly where water is coming in and at water kind of rate. I would not start thinking about waterproofing or new flooring until I had that locked down.
    – DMoore
    Mar 18, 2015 at 17:22
  • could it be condensation from the room, not seepage from under the slab? Put in a dehumidifier for a few weeks to test. $150 to $200 on amazon for a nice one.
    – ssaltman
    Mar 18, 2015 at 17:50
  • @ssaltman - not likely. I live in S Calif where it's sunny & 70 every day with low humidity. Also, there's no evidence of condensation anywhere else in the house - even after a shower it's unusual to see condensation on the windows in the bathroom!
    – Kryten
    Mar 18, 2015 at 18:01
  • @Kryten we have almost an identical issue as yours that I've asked in this question here diy.stackexchange.com/questions/62232/… I also live in Socal. Very interested to hear what solution finally worked for you and if you were able to prove the source of the issue now that its been over 2.5 yrs. Please LMK.
    – AKN
    Feb 12, 2021 at 4:53

2 Answers 2


A plastic or EDPM rubber sheet will work, if the flooring installers don't puncture it. That pretty well limits your flooring to a floating floor, but that's not a great impediment.

Nothing painted on to the inside of a concrete structure will stay put if water is coming from the outside (more commonly seen as failed attempts at "waterproofing the inside of a basement" but the principle is the same. Painted on products only work from the outside.

On the other hand, if you have actually resolved the sources of moisture, there should not be any more.


Products like Dricore are designed for scenarios like this. Basically its a piece of plywood with a rubber base. The base has channels in it to allow small amounts of water to run under the floor. These work in scenarios where a bit of moisture is unavoidable. The downside is the cost and can run $2-5 per square foot, but an upside is extra insulation on the floor which makes it feel warmer.

Is it possible that your water table is high? If so, the only permanent solution would be a sump pump so that the water can collect and be pumped away before it makes contact with the concrete.

  • The question states specifically "the water table in our area is very deep"
    – Ecnerwal
    Mar 19, 2015 at 13:53

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