Carpet wet under plastic chair mat

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After taking up the plastic chair mat under a desk chair, the carpet is wet. Not damp, but wet. What are the chances this is a result of moisture in the carpet from a cleaning over a year ago versus a leak in the slab? We were re-plumbed a few years ago so there shouldn't be an active pipe there, but this room is next to a bathroom. The carpet is dry all around the wet area.
  14 answers
  • If you have a leak in your plumbing you can determine that pretty quickly. Make sure your faucets and valves are off throughout the house. Then go to the water meter and using a felt tip pen put a tiny dot on the arrow that spins around and note the numbers on the dial. Then go out for several hours so your not tempted to flush or wash anything. After a few hrs come back and check the meter. Did the numbers change or did the arrow that spins move from where you placed the dot? If so you have a leak. If not the moisture is from another source. I doubt its left over moisture from a carpet cleaning. If it had been the carpet would be moldy and have an odor. If your slab is damp because of a poor vapor barrier which is common in that type of construction the dampness is coming up from under the ground and is not a leak. I believe this is what is happening in your case. The fix can be easy and it also can be hard. Removal of the carpet is required followed by a good cleaning of the exposed cement. Then a good coat of water damp proofing cement is in order. Allow to dry well then new carpet. Perhaps this is the time to have that new ceramic tile floor installed along with radiant heat. Check with your insurance company on this. It may be covered.
  • Jennie H Jennie H on Apr 23, 2014
    Thank you Woodbridge. Will do that!
  • Terry's Flooring Terry's Flooring on Apr 24, 2014
    It's a leak for sure ....
  • Jennie H Jennie H on Apr 24, 2014
    Thank you, Terry.
  • Harry Harshaw Harry Harshaw on Apr 25, 2014
    Jennie, concrete will always continue to wick water and some parts of the country call it it sweating. Anytime moisture under the slab comes through its vapor. This vapor condenses most where it cannot escape. Under rugs with glues, plastic and other locations. A slab without a vapor Barrier will sweat under rugs. Here is a test. Take a 3' X 3 ' 6 mil plastic. Lay it down on the concrete. Get some black duct tape or other super adhesive tape. Tape the entire perimeter of the plastic. Leave it there. This will show you if you have vapor condensing into water under the b plastic. The concrete will darken and alot of water may appear on the underside.
  • Harry Harshaw Harry Harshaw on Apr 25, 2014
    Jennie, some things can be done to retard vapor transmission through concrete. Let me know how your test goes and send me some pictures. I will make some recommendations after results.
    • See 1 previous
    • D'Lain Braunig D'Lain Braunig on Apr 14, 2021

      I have been having the same problem since I put a big plastic mat over the carpet by my bedside so that I could roll around a bedside computer table to use while sitting up in bed. I had thought that maybe my window behind bed had a leak but the carpet was never wet by the baseboard. I cut away all carpet under the bed but the carpet by my bedside still gets wet and is now stinky like mildew. I scrubbed it last night with a good pet carpet deoderizing cleaner but still smells like mildew after leaving fan blowing on it all night. So, I just used Lysol spray all over carpet to try to get the smell out. So, I guess that the concrete is sweating. Who would have thought. I will surely miss that mat .

  • Jennie H Jennie H on Apr 25, 2014
    Thank you, Harry. We've left the plastic mat off, and the carpet has dried now. Not sure what kind of clue that may be? We haven't taken the carpet up yet to see underneath it.
    • Harry Harshaw Harry Harshaw on Apr 26, 2014
      No problem glad we could help. Between tip top and aquatight comments you should be able toget a good handle on your situation.
  • Whenever we install laminate flooring in a basement or a slab home, or when we do an evaluation to determine if moisture is coming from the floor or wall or its a result of just high humidity in a room we perform a test that is not much different then what your plastic mat had done. We tape a piece of six mill plastic using duct tape onto the wall or floor area and leave it for a few days. If moisture is coming up or though the wall or floor we will find that there is water trapped in the plastic between it and the cement. This tells us that moisture source is from the wall or from below the slab. If the plastic is wet on the surface to the touch then we know that elevated humidity is allowing moisture to condense on the cooler surface of the wall or floor. In your case the plastic has trapped the moisture that comes from below the cement floor and was allowed to condense under the mat. This resulted in the carpet soaking up the moisture causing it to become wet or damp. The fix is water sealing the cement floor. And evaluate the exterior of the home for water ponding or gutters overflowing or leaders not piping the water away from the house. The idea is to keep the surrounding soils as dry as you can in an effort to keep the dampness from getting under the slab. It will not completely solve any water or dampness coming up from below the slab, but it will help.
  • Jennie H Jennie H on Apr 27, 2014
    That is a helpful explanation. Thanks, Woodbridge Environmental. And there was condensation on the underside of the plastic mat along with the carpet being wet.
  • Sue carroll Sue carroll on Jun 06, 2014
    I am on a slab and had wet spots (some up to a foot diameter) come and go in various rooms and dry all around them. Had various professionals come out and it turns out my water heater was leaking from the bottom and the water was going to the lowest points. I've replaced it and haven't had them since. It might be something you want to check?
  • Jennie H Jennie H on Jun 09, 2014
    Thank you for your suggestion!
  • Sage Sage on Jan 11, 2017
    You should call a plumber that can accurately pinpoint if there is a slab leak. The longer it waits, the worse the cost will be.
  • R Bullard R Bullard on Oct 20, 2019

    In my home office, an office chair sits on a 2 mm vinyl chair mat. The mat sits on heated terra cotta tile. Water accumulates between the floor mat and the tile. What solutions to this water issue exist and what damage is the accumulation likely to cause.


    Installation was as follows: Over an 87-year old existing six-eight inch slab (the vapor barrier for which is unknown and cannot be changed) we poured an additional six inch slab, which, after a 6-8 week cure period, was covered with an electric heat mat, 1/4-1/2 inch of self-leveling concrete, standard thin set and 1-1/2 centimeter thick sealed terracotta tile. Today the temperature-relative humidity inside the office are 70º-47%; the outside temperature-relative humidity are 53º-86%. A 12-inch split stone wall surrounds the old, original floor slab and the crawl-space adjacent to the slab floor is completely dry.

  • Deb K Deb K on Apr 14, 2021

    Hi Jennie, I can't see it being from a carpet cleaning that long ago, so you should check for a leak of any kind, was the moisture smelly? or fairly fresh, could be your slab, or water heater

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