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Tom S. 09-18-2009 01:51 PM

Damp Concrete Garage Floor
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Hi, I'm new to the forum. Looks like a great site.

I put up a shed/garage in my back yard about 4 years ago. The building turned out real nice. Problem is, the floor's been damp since day #1. It's that way year 'round. The building is 20' x 20' on a concrete slab. The slab is 1' thick at the perimeter, 4"-6" thick elsewhere, laid on top of compacted gravel, no vapor barrier. It sits at the highest point in my relatively flat yard. The soil is clay. The building is not plumbed, there are no water lines nearby, and the area did not seem wet before or during excavation. The guy who built it for me is long gone.

The bare concrete is always slightly damp, and efflorescence appears all over. The combination of moisture and alkalinity has caused damage to the bright metal surfaces on the small collection of motorcycles that I store in the shed. Because of this problem, I have not bothered to finish the interior walls or add wiring.

I had hoped to leave the floor in bare concrete, but it was just too much of a problem.

I tried to seal it w/this stuff:

Then I painted it w/this 2-part epoxy:

The paint stuck for about 6 months, and then flaked off.

This all seems like a ground water problem, but here's the really odd part that might lead to a different answer. When the floor was poured, they ran out of concrete about 3/4 of the way through the job. The builder called out for more concrete, which I believe was delivered from another plant, maybe even a different vendor. The area of the floor that was poured in the 2nd batch dried fine, and the paint has stuck to it w/no problems.

Does this point to a problem w/the 1st batch of concrete? Is it possible that it never cured completely?

At this point the building is worse than useless to me. It is actively damaging the toys that I built it to house. I'm going to put everything into a storage unit until I figure this out. It would be nice if I could find a product to seal the floor, but I don't know were to turn. That Moxie stuff shown in the link above seems to be so much snake oil. Does anyone make a surface coating that will stick to wet concrete? I suppose I could lay out some 6 mil plastic and pour another thin slab on top, but that would create a big problem w/the door openings. Raising the building would be a real pain in the rear end.


Here are some photos.

RegeSullivan 09-18-2009 04:09 PM

Before you put a lot of money into fixing the floor you might want to make sure it is not as simple as a cold floor causing the moisture in the air to condense on the floor. The damage to your toys makes me think the moisture level is very high in that garage. Try taping a piece of heavy clear plastic to the floor and make sure to get a good seal. In a few days check it for moisture. If you only find none or just a small amount of moisture between the plastic and the floor You need to ventilate better or insulate the space and maybe even heat it.


Tom S. 09-18-2009 05:18 PM


Originally Posted by RegeSullivan (Post 329375)
Before you put a lot of money into fixing the floor...

Good points, thanks. I just recd a moisture test kit that I sent away for. It involves taping a plastic box to the bare floor, open-side-down, that contains a package of anhydrous calcium chloride. The test material is removed from the sealed box periodically and weighed. In this manner you are supposed to be able to accurately gauge the amount of water coming out of the concrete. It also contains some litmus strips and solution to test pH at the surface. I'll post the results.

I'd be pleased to discover that you have the right answer. I'm not completely optomistic, however. We've just had a record string of 90+ degree days, and even then the surface of the floor was dark and blotchy looking. The building is not heated, but it is well ventilated. It looked dry after I applied the Moxie sealant. But before long, a wet spot would appear beneath anything set directly on the floor. I have a round 5-gallon plastic paint bucket for a wastebasket. There's always a dark circle where it's been sitting, even when you move it around the room.

BTW, calcium chloride is sometimes used to prepare dirt tracks for car and motorcycle racing. It helps the soil retain moisture, and keeps the dust down.

stuart45 09-18-2009 05:35 PM

We always put a plastic vapour check under concrete floors, as without it there is always a good chance of rising damp. This does depend on your ground conditions and climate also. The second mix of concrete may have been a bit stronger and the moisture is taking the easier route to the surface. It does sound like rising damp by your description. Sometimes the salts brought to the surface are hygroscopic and these can attract condensation at an even relatively low R/H of around 50%.

Scuba_Dave 09-18-2009 05:39 PM

There should have been a vapor barrier under the cement
Very inexpensive to put in before pouring the slab :(

Cement does stay fairly cool when in contact with the earth
And an object on top would allow the cement in that area to remain cooler
Enough warm air could still condense under an object against a cold floor I suppose

Gary in WA 09-18-2009 05:51 PM

I second the wet coming from below because of the bucket test. The air cannot dry the moisture under the bucket, hence it stays wet.

"The soil is clay. " ---- there is your problem. Water is traveling sideways on the clay to the interior slab, rising due to the warm garage from the sun on the roof. Divert downspout water 10' away, install drain tile around the perimeter of the whole garage, into a backyard drywell.
Be safe, Gary

Tom S. 09-18-2009 08:07 PM

Thanks, guys. Yeah, twenty bucks worth of plastic might have solved this before it stared. Code in this area doesn't require a vapor barrier in this application, and even before the concrete went down the builder- who was experienced -said he'd never had a problem w/o one. Today I talked to a couple of concrete contractors. Both said that they likely wouldn't have done it either but also agreed that it's cheap insurance. Great. They each recommended saw-cutting around the inside perimeter of the room, removing the floor and pouring a new slab over a barrier. That's what my best friend recommended over a year ago. The contractors also said that I should install a french drain around the building. That would require dry wells like GBR mentioned because the ground is flat.

stadry 09-19-2009 04:54 AM

post the test results of moisture test when rcd,,, there are some coatings avail that may help,,, none're found at apron stores, tho,,, occasionally some floors will transmit wtr vapor even w/barrier - that's usually wicking from ground moisture.

Jack A. Trades 09-19-2009 05:28 PM

If you have access to power, have you considered installing a dehumidifier or a few fans? That would be easy on the budget.

Scuba_Dave 09-19-2009 06:18 PM

Much cheaper then repouring concrete
But running a dehumidifer is like running an AC - they take quite a bit of power

I'm not sure what KWH charge these were based on:


Typical dehumidifiers cost about 6 to
10 per hour of operation. Monthly costs from $15 to $35 are

common but can exceed $50 per month if the dehumidifier runs a lot.


Will cost between $15 - $45 per month depending on usage. If emptied every day, the cost is towards the higher end
It all depends upon how much moisture there is
Constant moisture would mean it would be running most of the time
So it would be on the high end cost-wise
And it would be dumping all the heat into the garage unless you rig it up to exhaust outside

RegeSullivan 09-20-2009 07:28 PM


There is no doubt in my mind that you are getting at least some moisture coming up from the ground. I also believe it would be much more controllable in an insulated space. If the down spouts are not draining far enough away from the building moving them could go a long way in solving your problem.


logan7ms 01-14-2010 10:26 AM

Moisture Stop Epoxy Coatings
Hey Everyone, you might find this helpful.

Just wanted to let people know that there are moisture stop epoxy coatings out there. They penetrate deep in the concrete. For people like myself that had moisture problems in their garages or basements, this option worked the best for me and had lasted 5 years now with no peel-ups or flaking.

beenthere 01-14-2010 12:57 PM

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calconcrete 03-25-2010 01:15 PM

Damp concrete garage floor

You need to get some definitive information about how severe the moisture problem is on your garage floor.

I'd suggest going to and getting one of their moisture test kits. You do the test over 24 hours and mail the materials back to them. They will send you a report for free detailing how much MVE (moisture vapor emissions) are coming off the floor.

We use an epoxy and polyurea sealer from Versatile Building Products: It easily withstands 8 lbs of MVE pressure without peeling up.

Best of luck with your garage. Water problems are never easy to deal with.

Patrick Dowd
California Concrete Restoration

stadry 03-27-2010 05:15 AM

impossible to suggest a mtl that'll work til you post the rising moisture numbers,,, forgot to ask - why'd you neglect the moisture/vapor barrier when originally placing the monopour ? ? ? what indicated your shed would be in a drought ad infinitum ? [ that means ' forever ' ] ? ? ?

every time i think most understand the basics of conc const, another 1 bites himself in the *** :eek:

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