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I observed that my indoor garage floor often exposes damp patches, as on the picture attached. I noticed that it is mostly visible in rainy days. The house is about 25 years old, I'm not sure how old garage floor, but most likely about the same age.

Interesting, it does not happen in entire garage, but specifically in one side of it only, I wonder if floor level matters?

When these patches happen, I power up fans and try to dry the floor. Is there anything I can do about it?

Garage floor

View from far

UPDATE

These are two photos outside of garage. The first one is a small garden (it actually came with the house, which I purchased recently) adjacent to the garage (interesting enough that this wall and floor inside the garage does not have any watery issues at all, very dry).

Garden adjacent to garage

The second one is a path walk on the other side of garage along its wall (this garage wall inside is the one causing problems).

The path walk grade/level visually looks OK and declines away from the house. Path walk

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  • Does this happen after cooler periods of time, where part of the garage floor might be cooler and has humid air condensing on it? Is there anything different about that section such as closer to outside doors? Is this all on grade, or is there living area underneath part of it that might be impacting the concrete temperature? Is this just dampness, or actual liquid water on the surface? Pictures from farther away might help diagnose the issue.
    – Milwrdfan
    Commented Jun 29, 2023 at 15:39
  • @Milwrdfan, I would say this usually happens after heavy rains which we had here recently (I'm in Ottawa/Canada). In daytime I keep garage door and windows opened, so I hope the ventilation process somehow helps to dry up the floor. And this is actually water on the surface.
    – Mark
    Commented Jun 30, 2023 at 2:00
  • I get this because of poor grading outside the garage. My driveway is slightly higher than the garage floor and rain actually runs toward the garage door. I don't have a little step at the garage, so the water just runs under the garage door seal. This will be remedied in the next year or so...
    – FreeMan
    Commented Jun 30, 2023 at 12:51
  • Several of the pavers on the last image appear to be titled such that they would run water towards the house, to my eye, from that picture. Might be worth taking an umbrella out in the next rainstorm to verify how water is flowing.
    – Ecnerwal
    Commented Jun 30, 2023 at 18:38

2 Answers 2

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First, check for any water landing near the edges of the foundation. Gutter problems or water infiltration into the wall can result in a wet slab. Check for these problems first. They're often cheap and easy to fix, and they'll cause you all sorts of ongoing issues with the structure.

It's also quite likely that the slab was installed without a vapor barrier below it. This is fairly common in residential construction. Here is an article about when and where to install vapor barriers below concrete slabs to prevent moisture migration up through the slab. I've included the key flowchart below.

Flowchart for concrete vapor barriers

If the slab has no vapor barrier and the moisture pressure gradient near there puts water below grade on a regular basis, it's going to come up through the slab. You might be able to mitigate this somewhat via adding a nearby drain below grade to remove the water to somewhere more distant from the building.

Now, assuming the gutters are fine, the grading is proper, and there's either already a subsurface drain or it's not possible to add one, if there's no vapor barrier you're going to be stuck with mitigating the moisture as it comes in. The easy options there are ventilation (if it's dry outside) and dehumidification (if it's not dry outside). I've also had decent luck in solving a previous garage by treating the concrete with a densifier to reduce the amount of moisture that migrates through it, then adding a ventilated tile floor over the floor so that I didn't have to worry about items sitting on the damp floor. It didn't solve everything but it did make things work well enough that the issue was mitigated.

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  • thanks for your feedback Hm.. It does not seem that there are problems with gutters. Assuming there are, does it mean that the water always accumulates down the wall and makes it way to the floor slab?
    – Mark
    Commented Jun 30, 2023 at 2:12
  • Water near the foundation will come through the concrete, that's why you check the gutters to make sure everything is okay.
    – KMJ
    Commented Jun 30, 2023 at 5:42
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Vague question yields broad answer...

Stop or remove the water before it gets in. That might mean:

  • fixing the grading around the garage and/or house (unclear if it's attached or detached) so the ground slopes away from the building for at least 8-12 feet to remove surface water.

  • Installing subsurface drains to remove subsurface water outside the building.

  • Sealing exposed exterior concrete to prevent wicking of water through the concrete.

  • fixing any flashing, caulking, weatherstripping, or gutter related issues contributing to water ingress.

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