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Damp garage floor 2 years after construction

Posted by flathead (My Page) on
Wed, Oct 5, 05 at 20:24

I'm looking for information on how to address damp spots on my garage floor that don't appear to be condensation - could it be moisture from ground below? 3 yrs. ago I hired a garage contractor who talked me into using a concrete additive that he claimed would waterproof the concrete - he refused to put in a plastic moisture barrier, for some reason. And this guy was recommended to me by a friend... who also didn't get a moisture barrier. One year ago, beginning in the winter (wet season in Seattle) I began seeing dark, damp streaks and blotches appearing somewhat randomly on the floor. The spots are still there after a dry spring and summer. Where the concrete is not darkened, it appears to be quite dry (majority of the floor area). The garage is closed up almost all the time and is pretty tight. A thermo/hygrometer shows that it's usually 60-64 and high humidity (60+ %) in there. Aggravating factor: 20-ft. concrete driveway slopes downhill from alley to garage, dropping 3 ft., drains into a french drain that empties into a dry well beside the garage. I don't get rain water under the garage door, ever, but with the ground surface's steep grade above and next to the slab, could there be ground water moving under the driveway and then under the slab? Funny thing about the spots is - though they seemed random at first, I noticed they seem to coincide with the marks left behind by the plastic sheeting that the concrete guy put *over* the freshly poured slab to help slow down the drying. What could be the connection? Could the moisture be coming from above (from constant high relative humidity in garage) and the spots coinciding with where the surface texture of the floor is different due to contact with a plastic sheet? That would seem highly unlikely to me, but...

Thanks in advance


Follow-Up Postings:

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RE: Damp garage floor 2 years after construction

If the garage is detached or attached, there is no requirement to have a vapor retarder under the slab.

The exception is if the garage is located under habitable space above, the garage floor needs to be treated like a basement floor and a vapor retarder would be required under the concrete.

At any rate, it does sound as if water is wicking up through the concrete slab. Concrete is porous and is permeable to water and ground water vapor.

Sure sounds like you are developing wet spots because of either water table rises, wicking water, runoff or condensation of vapor within the slab.


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RE: Damp garage floor 2 years after construction

Thank you for the reply, manhattan42. Garage is attached, has living space above and interior stairs. The contractor never cited building code, just said the plastic barrier was not needed and his policy was to not use them in order to minimize wait time for the concrete to dry. I don't know what UBC says (this county goes by it pretty much, I now know) but if it says you must have vapor barrier below living space then this work shouldn't have even passed my city's inspection. And as far as the waterproofing "additive" goes I guess that was a total fraud.

So what are my options for eliminating the wicking water?


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RE: Damp garage floor 2 years after construction

In my attached garage, floor above grade, my humidity meter in the Summer usually reads 70 to 85%. We keep the doors closed most of the time for security reasons. I used 4 layers of 7.5 mil poly under the concrete, so it really has a barrier. A dehumidifier would be the only answer for my situation and probably for yours.


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RE: Damp garage floor 2 years after construction

A minimum of a 6 mil vapor barrier would have been required under this concrete slab and it appears that this code violation slipped past the inspectors.

It is also nonsense that keeping the vapor barrier under the concrete will increase curing times. Concrete cures by chemical reaction, not by evaporation.

Be that as it may, you could use an epoxy based water and vapor proofing coating for your garage floor.

It won't stop water getting into your slab, but it will prevent liquid water and ground water vapor from penetrating up through the slab.


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RE: Damp garage floor 2 years after construction

I eliminated my moist garage by instlling a cheap bedroom type ceiling fan on the ceiling of the garage. I turn it on high, and leave it on all spring, summer and fall, and I have no moisture at all.
It won't solve your problem, but it might help aleviate it.


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RE: Damp garage floor 2 years after construction

Great responses, thanks to you all! I'll do some looking into the code issue, can't understand why this supposedly reputable contractor would have poured a slab without a vapor barrier. The epoxy coating may be the solution.


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RE: Damp garage floor 2 years after construction

Would installing a ventilation fan into the roof of the garage help the problem? I was thinking about one of those solar powered units. any thoughts?


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RE: Damp garage floor 2 years after construction

"Be that as it may, you could use an epoxy based water and vapor proofing coating for your garage floor.

It won't stop water getting into your slab, but it will prevent liquid water and ground water vapor from penetrating up through the slab"

Depending on the vapor drive through the slab you may end up with blisters under the coating as the moisture reaches the top of the slab and debonds the coating locally at first and then growing laterally.

If the moisture is really troublesome then I would suggest the dehumidifier.


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RE: Damp garage floor 2 years after construction

I forgot to mention that the walls and ceiling of my garage are finished with drywall, just like the interior of our living space. The garage is fully insulated, including the doors. The drywall finisher I hired for the garage actually did better work than the one who did the house.


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RE: Damp garage floor 2 years after construction

Did you ever get your damp garage floor fixed?


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RE: Damp garage floor 2 years after construction

There are many products available that would help with your problem. You can get moisture barriers that will withstand 20# of water pressure per sq. ft. per 24 hour period. You can then coat the surface with an epoxy, an overlay, or whatever to get to your desired finish floor.

To simply say an epoxy will fail because of moisture is wrong. An improperly installed epoxy will fail, as will almost any other type of floor covering. If done correctly, you can have a beautiful floor that will remain dry and last indefinitely.

A de-humidifier will only make a moisture problem less visible. It will do very little to fix it.


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