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I recently bought a house with laminate flooring in a partially-below-grade room that sits on a concrete slab. The owners ripped out carpeting in the room and installed the laminate prior to putting the house on the market. The house is ~40 years old, and the owners (who were the original owners) claim that the carpet never appeared to be moist during that time.

Subsequent to our going under contract on the house but prior to settlement, there was a lot of rain and the laminate apparently started bowing. The sellers (without consulting us first) decided that the problem was due to moisture absorbing into the slab, so they replaced the gutter above the room, re-graded the side of the house, installed a French drain, and replaced the bowed floor boards. I have now been living in the house for about a month, during which we've received a lot of rain, but everything seemed to be fine. That is, until a few days ago when we received a downpour of about 2 or 3 inches of rain in the span of about 2 hours. A couple days later I noticed that some of the floor boards had started to bow. It is in a relatively isolated area, approximately in the same place (I am told) where they had previously bowed. Therefore, I am assuming this is not due to expansion gap issues.

We are contacting the contractors who did the work for the previous owners to see if they have a warranty/guarantee, however, in the event that that does not work, what is the next step? Could it be that the French drain and gutter modifications were insufficient? We don't want to replace the floor boards just to have this happen again in another few months...

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Is there a moisture barrier under the laminate flooring? Is there any other apparent water damage in the general area? What is the humidity like in the room? –  Tester101 Jul 11 '11 at 15:23
    
Since I did not install it, I am not sure if there is a moisture barrier. The only other water damage is some rotting window trim on the exterior of the window in the room (the window is original to the house, and we are in the process of replacing it). The drywall in the room appears to be completely dry. There is no apparent water damage at all anywhere in the interior of the house. The humidity in the room seems relatively low. –  ESultanik Jul 11 '11 at 15:28
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2 Answers

There are 3 potential moisture issues:

  • standing water leaking in via the wall/footer
  • moisture migrating through the slab
  • high humidity

You need to first figure out which of those 3 is happening.

If the water is leaking in, that's an issue of gutters, grading, draining, etc. That needs to be fixed on the outside, first.

If moisture is migrating through the slab, then you may have a high water table. If that's the case, I wouldn't bother finishing the basement. If it's just a bit of moisture, you could get by with putting down plastic underneath the laminate flooring.

If it's high humidity, then this might be the gap issue. Engineered floors shouldn't expand/contract as much as real wood, but they still need plenty of space around the edge of the room to expand. Check for that. Otherwise, get yourself a quality dehumidifier and have it run 24/7.

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My first thought is that laminate flooring on top of concrete is seldom a good idea, especially if you're in an area where basements are common. There are ways to make it work, all of which require some subflooring and a waterproof membrane, but one of the cardinal rules of carpentry is that wood does not touch concrete, for exactly the reason you are seeing; concrete absorbs water, and will wick it into the wood which will then swell (and eventually rot, if it cannot breathe). You also cannot nail directly into the slab, so forget about the standard installation method with a flooring nailer to secure the planks. So, if you are going to stick with the laminate, you MUST have a waterproof but breathable barrier such as TyPar or Tyvek between the concrete and any wood product. That may include running it up the side of the wall for a few inches either behind the baseboards or even behind the drywall of the room. A plastic sheet such as vapor barrier is a no-no; it will trap both water and air, preventing the wood breathing and accelerating rot if and when water does get under it. You should probably also have an OSB subfloor, so you have something to nail the boards down into.

If all of this is present under your floor and the floor is still bowing, I would go ahead and pull up your baseboard and check the expansion gap. There should be AT LEAST 1/4" between the sides or ends of the board and the wall. I would also feel the drywall and/or studs behind that; do they look or feel damp? If so, your problem is actual water leakage; water is somehow soaking or leaking through the roof or exterior wall in this area and dripping or seeping down to the floor. To fix the floor, you must first fix the leak, or it's just going to happen again with the next heavy rain.

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