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Question DetailsAsked on 4/27/2016

I just purchased a house built in 1955 on a concrete slab. I removed the carpet in living and dining rooms. the c

the carpet has been off for about a month. I noticed some dark spots in 2 areas like someone dropped stain on subfloor during staining something. The spots are moist but dry up in time. What is causing the spots?

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1 Answer


Below is a similar question with response which might help you - what you are seeing is moisture from the ground below which is wicking up through the concrete to the surface. Just damp ground below can cause this under rugs and flooring, but if it looks and feels damp at times, then in all probability you have actual free water under the slab, not just moist soil with a deeper water table. This could be natural groundwater, surface runoff (commonly from the roof) that is going down through the loose soil around the foundation and getting under the slab, or runoff from watering or such. If these are appearing and then drying, you should be able to correllate them with watering of plants around the house or rainfall or lawnn watering or such - with possibly a day or two lag after the water application till it shows up under the slab and wicks through.

Basically, unless measures are taken to remove the source of the water, any flooring you put down there is likely to start mildewing and molding, eventually possibly even rotting. Plus I would suspect your basement humidity is probably up unless it is the source of air for your furnace (during heating season), because a concrete slab on moist ground which looks dry can put several gallons a day into your basement air - a damplooking slab up to tens of gallons of water, which of course can lead to mildew and mold and a musty smelling basement.

Answered 4 years ago by LCD

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