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Question DetailsAsked on 2/3/2017

our ground level cement slab with tile gets moist will subfloor help?

This is used as a bedroom and we would like to get rid of the moisture and mold. We've been told a subfloor will help. What type is recommended? Can we carpet over it?

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2 Answers


Nope - not unless the subfloor space has an aggressive moisutre removal system AND the subfloor system is highly water and rot resistant like ground-contact timber or concrete. Otherwise will just likely move most of the mold and rot occurrence from the floor to the subfloor. You need to get rid of the moisture - and if it gets noticeably damp or wet now with a tile surface, you have water at or very close to the bottom of the slab, so you probably have four realistic choices:

1) one which might or might not work - greatly increase the air circulation at the floor and the overall ventilation in that room, though it would have to vent at least mostly outdoors or you would expect to get moisture problems wherever that moist air goes

2) normally, depending on HOW wet the slab is, a dehumidifier will NOT solve the problem - might well mitigate the mold issue in the room if you can drop the room humidity below about 45-50% at all times, but the floor will likely still be damp if it is now - and bare concrete (including polished concrete) or tile or stone are about the only surfaces that will not readily and severly form mold in that condition.

3) long-term solution, though expensive (commonly several thousand $ minimum) is to either put a french drain system around the outside of the foundation (and simultaneously put waterproofing membrane on the outside ofthe foundation to keep the walls dry), placed at least at or preferably a foot or two BELOW the bottom elevation of the foundation footing (but a foot or two outside it), to drain the groundwater level down well below the slab - then IF that reduces the slab moisture enough, put down 6 mil or heavier plastic vapor barrier on the slab or tile, totally fastened and sealed at the foundation walls. Then put an inorganic flooring material over it - either an all-plastic laminate or vinyl plank floor for instance (preferably one that can be taken up and cleaned and reused if flooded like some Armstrong and Pergon products for example, or an inorganic very open-weave highly breathable carpet with no pad or waterproof pad like the bubble-wrap type - something insensitive to water and moisture and easily taken up if needed. For simplicity and economy, I recommend preferably easily handleable throw or area carpets so if it gets damp or starts mildewing, or accidentally gets wet, it can easily be taken up and washed out with a hose and aired out and treated as needed.

4) the other long-term solution, though harder to do in existing floor than with new construction, is to saw cut in your after-the-fact case through the floor slab and put in underdrains - at least around the perimeter of the slab, and in areas where the natural groundwater rather than runoff coming in around the foundation is the cause in several rows under the slab field as well. These would all lead to a sump and sump pump to permanently draw down the water level and keep it below the slab - preferably by a foot or two at least.

There are a lot of factors involved in this - natural groundwater elevation, soil permeability, whether your house is on basically high or low land, where the water is coming from, whether you can intercept the water by sloping the area around the house or by channeling or berming off surface drainage, whether some or most of the water is roof runoff that can be trapped by gutters and downspouts and then directed away from the foundation, etc.

You can find a lot of previous questions with answers about basement water issues in the Home > Basement Waterproofing link in Browse Projects, at lower left.

But my recommendation - unless you are prepared to dig quite deep (several feet deeper than normal) with a drainage system and drop a chunk of money into it, if your basement slab is currently commonly damp or wet, I wouldnot recommend any type of flooring that can hold moisture in - so stay with a concrete, tile, or grouted stone floor there. Cheap open weave inorganic throw rugs or area carpets if it will tolerate it - or some people go with porous synthetic outdoor carpet that lets the moisture through and does not mold easily. But nothing permanent, a solid sheet or layer, organic, or expensive - because if you have trouble now you are likely to have at least some moisture problems in the future.

Also, with any drainage system, unless on a hill high and dry (in which case you should not be having this problem anyway), there is always the chance of sporadic basement flooding due to too much inflow for the drainage system to handle - unusually heavy rains in an extreme storm cell or hurricane, extremely rapid melt of thick snowpack, unknown blockage of gutter/downspouts causing a lot of water to pool right by the foundation, etc. So my recommendation with all basements potentially susceptible to wetness is put down a water-impervious flooring or none for at least several years, or only easily removeable and hand washable/dryable (like in garage) throw or area rugs or carpets, until you are sure you do not have water infiltration or permeation issues. WAYYYY too many people fully finish their basement only to tearfully tear it out a year or few later when it all gets wet. Unfortunastely, unless designed and built that way initially (maybe 1% of houses or less), below-ground or daylight basements are NOT designed to be dry.

One caveat on that - with more modern technology - with standard underdrainage or french drains to get the water level at least a foot below the slab - a situation which would normally cause continually damp slab and a lot of moisture transfer, some of the more professional wet-floor (whenn in use, not during application) polyurea or polyaspartic urea floor finishing products, IF applied to a properly ground and dried out slab, can do a pretty dramatic job of keeping the water from coming through - though you also have to waterproof the slab perimeter and at least base of the foundation wall as well - commonly with a wall-to-wall edge-sealed heavy vapor barrier under whatever floor you are putting down.

Answered 3 years ago by LCD



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Answered 3 years ago by Member Services

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