I live mid-block (houses on both sides), in a 1906 city row house with brick basement walls and a cement floor. We never have water in the basement. We have dark spots that get lighter and darker but never 100% go away. They are clearly moisture that is coming up through the floor at certain places.

Tests have shown the cement is about 33% wet in those dark spots. They are all at the front of the house. Outside, the tiny front yard is field stone and slopes away from the house, so no water is getting down around the front foundation. In the back, the outside level matches the interior and there is no standing water there (and no dark spots at the back of the house anyway).

What is causing the dampness and what can I do to stop it...before I finish the basement as usable space?

  • 2
    That type of moisture isn't always from below. When my basement gets too humid, surface condensation can occur and darken the entire floor. What are the current weather and interior conditions?
    – isherwood
    Oct 11, 2019 at 13:14
  • Hello, and welcome to Home Improvement. You might try putting plastic down over one such area, and the dry area next to it; does the spot get bigger or smaller? And, you should probably take our tour so you'll know how best to participate here. Oct 11, 2019 at 13:18
  • 1
    Its super dry in DC lately with well below normal rain...not humid at all. The dark spots look like what a piece of paper might do if you set it down on 1 drop of water. They are random, but always in the same general area. It looks like water moving across the floor, but its just dark...no water.
    – Patrick M
    Oct 11, 2019 at 13:18
  • I have the same problem and I am cleaning concrete extremely well cause those black spots are a form of mold once it is cleaned I am putting drylock extreme on my concrete 2-3 coats and so far it’s working for me my house is 1902 and moisture seeps through my concrete where no cracks are.
    – user107994
    Oct 11, 2019 at 22:40

2 Answers 2


I have had issues with small springs under slabs, yes the dark spot is moisture. The tough thing if it is always there most sealers need the pad to be dry to seal properly. I have had really good luck on several of my own homes with 2 part epoxy applying in late summer. The worst place I would pull gallons of water out of my dehumidifier, once coated with 2 part epoxy paint their was hardly any moisture in the air being removed. The concrete has a high ph so mold is usually not a problem but in an older home the ph may be low or ~8 (when fresh 12-15). But mold in not usually a problem on a clean pad.


1906 row house will have a combined rain and perimeter drain system probably vitrified clay tile. The pdrain surrounds the footers of the foundation and typically receives the rain water and any ground water that pools near the bottom of the foundation footer. The pdrain then takes this water away typically to the sewer main. Your pdrain is probably clogged preventing any ground water from getting to the perimeter drain.

Modern slabs have a gravel and vapor barrier layer under the concrete slab. Typically before finishing this kind of space you'd get rid of your concrete slab, redo the perimeter drain, add a gravel layer, add an insulation layer, add a radon barrier (poly) and depending on ground water conditions add interior perimeter drain / sump pit, sump pump, rough in for active/passive radon mitigation, and/or redo your exterior perimeter drain. That is going to be a lot of work / pricey.

Cheaper option: add a sump pit / sump pump for the ground water possibly add a trench with gravel from area of high moisture to sump pit. Add a layer of poly on your concrete for the radon. Install drycore floor base.

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