basement floorI pulled up some old foam-backed carpeting in my basement laundry room to replace. The concrete slab is old, and had crumbling concrete patch work, so I set to scraping that all up, seems the more I scrape, the more comes up. AND, all the scraping has revealed 4 small (golf ball sized) patches of damp concrete. Damp to the touch, but not wet, really. A paper towel comes away dry. A few had a small amount of mold under the broken concrete patch. I cleaned up the floor as best I could and have been running a de-humidifier down there for 4 days, with no change.

I was planning on patching up the broken up areas, enough to level it for some more cheap carpeting. It's the laundry room, I'm not concerned with perfection, but now I have two worries. First, is my house going to float away if I don't spend $20,000 getting my floor ripped out, new drains installed, etc.? Second, if I wait til a dry spell, seal the concrete with something, fill with concrete patch and lay down some carpet tiles, will I go to basement water-proofing hell?

Fact#1: the basement isn't really damp other than the floor patches.

Fact#2:this part of the house is old. Real old. (1880) The floor looks to be block with a thin concrete cap, though, so maybe its been re-done. Fact#3: There is a sump pump, sitting below the floor level, functioning properly, and I can see 2 drain tile openings in the basin. Water looks to be dripping from them, so drainage is working at some level.

The slab in question is smallish, about 100 sq ft, about 2 ft higher than the rest of basement, ( 2 steps up) which is a finished rec room with linoleum finish on floor. The concrete cap is pebbled concrete work, and looks like smooth concrete troweled over to patch in some areas.

Any educated thoughts would be wonderful

  • It's tough to think of how you could get isolated, durable golf-ball-sized wet patches in a large concrete slab. Perhaps the texture of those patches is different, and thus it feels damp? Apr 2, 2016 at 14:22
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    It was probably poured after the house was built, a hand mix is usually not as well mixed as a delivered truck load. Many times old patches do break up hydraulic cement makes for a good seal if you think moisture is coming through. As long as your sump is working I would clean out the old patch and put in a new one and not worry.
    – Ed Beal
    Apr 2, 2016 at 15:33

1 Answer 1


Moisture is coming up through the concrete from below. This is normal in the wet season depending on your area.

NEVER put carpet on a basement floor. It is just a really bad idea. It is a sponge for everything from a leaking water heater, to moisture from humidity in the summer to pet urine, etc. It also hides problems like this until they become moldy and unhealthy.

Clean and patch the floor as best you can and stain it (best) or paint it with epoxy basement paint (better than carpet). If you get moisture in the future it will not damage stain but it may make a small area of paint peel.

Any way you go the moisture is going to continue coming up through the floor unless you hammer up all the concrete and re-pour it with a plastic vapor barrier under it. Modern building codes require a vapor barrier under basement floors to prevent these problems.

Good luck with your project!

  • Around here we have active sumps in many neighborhoods, and we still manage to have full carpeting in basements without the horrors you describe. "Never" is rarely a useful word. :)
    – isherwood
    Apr 6, 2016 at 14:39

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