A sweating garage floor can be a real nuisance. It can create a slippery surface and make your garage damp as a result. If you wonder why your garage floor gets wet, particularly in the spring, then your garage may be suffering from this condition.
The quick answer to stop a garage floor from sweating is to do one or more of the following steps:
- Decrease the dampness/humidity of the air
- Increase air circulation across the garage floor
- Increase the air temperature of the garage
- Change the garage floor surface
- Keep the garage door, man doors, and windows closed and sealed
So, before you decide to make any of the recommended changes above, let’s discuss what makes a garage floor sweat to begin with. You will then have a better understanding of what you can do to help stop it.
We will also discuss how to complete each step and which products or methods we can recommend that work best.
Why does my garage floor get damp when it rains?
Wet or damp concrete is a unique garage floor repair. Before you plan to stop it, you need to understand what causes it. There are a couple of reasons why your garage floor may be getting wet. The most common is condensation — AKA sweating garage floor.
When relatively warm, moist air comes into contact with a very cool concrete slab, the air that comes in contact with it cools. If the air cools to or below the dew point, it will begin to condense on the concrete surface. This is what causes the wet and sometimes dewy-looking garage floor.
As a refresher, the dew point is the temperature at which air becomes completely saturated with water vapor. If your concrete surface is cooler than the dew point, the boundary layer of air in contact with it can cool to or below the dew point. This happens if left undisturbed long enough.
Otherwise known in the concrete industry as sweating slab syndrome, it’s the same condition that causes the outside of a cold beverage container to get wet on a warm and humid day. Or the surface of your car to get wet when it’s left outside on a cool, damp night.
A damp garage floor generally takes on a darker look due to the moisture it absorbs.
Sweating garage floors seem to be most prevalent around springtime in areas of the country that experience cold winters. Ground temperatures are still very cool from winter and tend to keep the concrete colder than the air around it.
However, it can also be an issue in areas of the southeast. Warm, moist air from the ocean can come into contact with relatively cooler concrete slabs during winter and spring.
A Wet Garage Floor Is Not Always Caused From Sweating
Another condition that can make your garage floor wet is moisture that rises from beneath the slab. This is different from a sweating garage floor and is often misunderstood as the same thing. It most often occurs with concrete that lacks a moisture vapor barrier beneath it. Basements are a good example.
Though concrete is hard and dense, it contains small capillaries that can act like a sponge. If the ground beneath the concrete is wet or has a high moisture content, the concrete will absorb that moisture.
If the air above the concrete slab is warmer and drier, the moisture in the concrete will seek equilibrium. It does this by rising to the surface and evaporating.
If there is enough moisture for it not to evaporate fast enough, it will create a wet surface. This is the most common cause of efflorescence. And just like a basement, it can make a garage damp as well.
How to test for a sweating garage floor
If you are unsure whether you have a sweating garage floor or a moisture issue beneath your slab, the easiest way to tell is to test. Tape a 16” square piece of heavy plastic down to the concrete when it is dry. Make sure to seal all the edges. Leave it in place until your concrete shows signs of being wet.
Remove the plastic and then look at the concrete. If the concrete under the plastic is dry and the surrounding surface is wet, then you have a sweating garage floor. However, if the underside of the plastic is wet and the surrounding concrete is mostly dry, you have a moisture problem from below the slab.
Another positive indicator of a sweating slab is if you remove something from the wet garage floor that has a flat surface and you find the concrete dry underneath. An example would be a floor mat or a cardboard box.
On the contrary, if your garage floor looks dry and you remove an item to find wet concrete beneath it, you have a moisture problem under the slab.
How to stop a garage floor from sweating
Stopping a garage floor from sweating can be more challenging than you may think. It’s a problem involving the physics of condensation that you are trying to prevent.
For example, you cannot stop your garage floor from sweating simply by applying a sealer or coating. The surface of the garage floor will still get wet (sweat) because it is cooler than the dew point. If the air that comes in contact with the concrete has a chance to settle long enough, it will condense, regardless of whether there is a sealer or coating.
In order to stop a garage floor from sweating, you need to eliminate the conditions that make a garage floor sweat to begin with.
Think of it this way. If you park your car outside on a cool night with damp air, the surface will be wet with dew from condensation the next morning. If you wax your car to seal the paint, it will not prevent it from getting wet. It will just cause the water to bead up nicely. The same principle is applied to your garage floor.
To stop a garage floor from sweating, you need to eliminate the conditions that make a garage floor sweat to begin with. In other words, you need to do at least one, if not more, of these 5 things.
- Decrease the dampness/humidity of the air
- Increase air circulation across the floor
- Increase the air temperature of the garage
- Change the garage floor surface
- Keep the garage door, man door, and windows closed and sealed
So let’s discuss each of these items in more detail, including what may be required.
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Decreasing the dampness of the air
Decreasing the moisture in the air is one of the more effective ways to prevent concrete sweating. If the humidity in the garage is kept at acceptable levels during the times of year when sweating is most prevalent, you will greatly reduce the chance of it happening at all.
The best way to reduce the moisture content in the air of your garage is with a dehumidifier. If set to run during the times of the year when you encounter sweating, it can greatly help to reduce if not prevent it.
The key is to use the right dehumidifier for the job. For a typical 400-500 square foot garage, we recommend using a unit rated for 2500 ft² spaces or larger.
One of our favorites is this 35-Pint Dehumidifier by hOmelabs. It’s rated for 3000 ft² and works well for a standard 2-car garage. For a 3-car garage or larger, the 4500 ft² unit would be better suited.
Price The hOmelabs Dehumidifier Here
The reason for using a higher-rated dehumidifier is that it will remove moisture from the air much more quickly. Plus, garages are far more drafty than the inside of your home. They will continually let in the relatively warm, moist air at a greater pace. Dehumidifiers rated closer to the actual size of your garage will be too inefficient to do the job.
Also, it’s important to use a dehumidifier rated for use in colder temperatures. Many of the in-home units are not rated for temperatures low enough to work in a non-heated space. They can freeze up if the temperature dips into the low 60s.
The hOmelabs units are UL and Energy Star rated. They are also rated by the manufacturer to work in temperatures as low as 35°F.
Though larger units such as these emit dry heat when in operation, it is not enough heat to affect the temperature of a cool garage.
Of course, running a dehumidifier requires that you only open the garage door and man doors when necessary. Leaving doors or windows open defeats the purpose of using one.
Increase air circulation within the garage
Another method that can be effective at reducing garage floor sweating is running a fan to increase air circulation across the floor. If the air near the concrete surface cannot rest long enough to cool, it will not condense. If it is allowed to rest long enough and form a cooler boundary layer of air at the surface, then it will condense.
A larger-sized oscillating fan can move a lot of air. One of our favorite fans for our garages is the Hurricane brand. They come in a variety of sizes and configurations depending on your needs.
Let’s go back to the example of a car sitting outside on a cool, damp night. You may have noticed that dew will not collect on the car while you are driving, but it will begin to collect soon after you park it. This is because the air does not have a chance to settle on the surface and condense while you are driving. It’s the same concept as your garage floor.
A lower floor height is good for moving more air under parked cars if necessary. The wall mount or upright stand will help to circulate more air throughout the entire garage. Just set it up in an area that covers most of the garage floor.
If you have a larger garage or more space, a larger wall-mount fan is a better choice. They will move much more air and can create a circulation effect when directed toward the floor.
Often, combining a fan with a good dehumidifier can help mitigate some of the worst conditions for sweating garage floors.
Heat the garage
One option that is not always practical but worth mentioning is heating the garage. This will bring the concrete surface closer to room temperature. When the conditions for sweating exist (relatively warm, moist air), the concrete will not be cool enough to condense the air that comes in contact with it.
The problem with heating a garage, however, is that it can take a week or more for the concrete at the surface to warm. Plus, running a heater for long periods can become expensive since most garages are not insulated.
Amazon has a compilation of popular garage floor heaters if you like the idea of heating your garage, even temporarily. You can find garage floor heaters here.
If you are in the planning stages for building a home or separate garage in an area of the country where concrete sweating is common, you may want to investigate radiant floor heat for concrete instead.
Change The Garage Flooring Surface
Changing the flooring of your garage is arguably the most effective way to stop garage floor sweating. We don’t mean sealing the garage floor or applying a coating. We are referring to installing a different flooring surface.
Great examples . When such a garage flooring system is installed, the top surface of the flooring will no longer acquire the same temperature as the concrete. Instead, it will acquire the same temperature as the air in the room.
As a result, when warm, moist air is introduced to the garage, it cannot condense on the floor since the floor will closely adopt the same temperature as the air. Another benefit of this type of flooring is that it helps insulate your feet from the cold concrete below.
Seal the garage from drafts
Lastly, you can lessen the effect of garage floor sweating by taking steps to slow the intrusion of air into the garage in the first place. This is particularly helpful when running a dehumidifier and/or fan. You can achieve this by ensuring that your garage door and man door are properly sealed and that windows are kept closed.
For the garage door, a garage floor door seal can be very effective. It works well at preventing the moist air from traveling across the concrete floor and settling to cool.
Side garage door seals are worth considering as well. Not only will they help to seal the garage door better, but they also work well at keeping blowing rain and other critters from entering your garage.
For your man doors, make sure the door seal gaskets are in good shape. Also, don’t forget an under-the-door seal. Like the garage door, this helps prevent outside air from flowing across the garage floor.
Make sure to check for any other areas of your garage that may be drafty or letting in air from the outside. Do not block or seal any venting required by local or state building codes.
We realize that when you open your garage door to park or exit, you introduce large volumes of outside air that may be conducive to sweating. However, this air can often move in overnight or in the early morning when you keep your doors closed. Making sure your garage is sealed properly can go a long way towards reducing this air from getting in.
Final thoughts
As a reminder, sealing your garage floor will not stop it from sweating, but it will prevent some of the adverse effects, such as efflorescence. You will also benefit from easier cleanups and stain resistance. Replacing the flooring in your garage is the most effective way to stop it completely.
You may have been surprised by the methods required to help prevent your garage floor from sweating. However, once you introduce one or more of the procedures we stated, you can significantly reduce or eliminate the problem.
This is a great article, thank you very much for publishing it. I have a question that is more of a puzzler than anything.
I live in the SF Bay Area. 50% humidity all the time and never too cold.
I have an 80+ year old garage that was settling badly in one corner, making a huge crack down 1/3 of the slab. My contractor cut that slab out, hand dug out the foundation and repoured it, then poured new 1/3 slab. Everything’s great.
The new garage door has been on back order, and now that we’re in the rainy season, the floor damp after a rain, but no water is really pouring into the garage. After reading this great article, I’m 99% sure it’s just condesation, and when we get the doors on, it should go away. (I’m planning on putting down carpet squares too, which I had in there before. Only the sunken corner ever got wet underneath the carpet squares.)
Here’s the puzzler: The old 2/3 of the concrete slab is sweating, but the new 1/3 slab does NOT seem to be sweating. At least, it’s not looking damp, dark or misty. How come?? Any thoughts on why?
I believe both slabs are the same thickness. Old 2/3 of slab almost surely would not have a moisture barrier under it, but I don’t recall seeing if the contractor put a barrier under the new 1/3 of the slab before repouring. Would that change the temperature of the slabs relative to one another, affecting the sweating? Or new concrete different density than old concrete?
Quite the puzzler, and mostly idle curiosity, since I’m not longer worried about it, after reading this article.
Thanks
Hi LH. Most likely there is a temperature variation between the new pour and the old slab. The old slab has been subjected to the cooler temps for a much longer time. It takes a while for concrete to change temperature and is why sweating is most common in the spring. If you want, you can test it to make sure. When the old slab is dry, tape a 16″ square sheet of plastic to the surface. Seal it all around the edges with good tape. When the concrete shows signs of being wet, pull up the plastic. If the concrete underneath is dry, then the slab is experiencing condensation. If the plastic is wet underneath, then it’s a possible moisture issue.
Shea,
I’m sure it’s condensation. I move a box today, and the floor under it was dry.
I also found out today that the new 1/3 slab DID have a moisture barrier put underneath, before pouring the slab. That might change the temp of it relative to the old slab.
Once again, great information! Thanks so much.
This article covers sweating concrete but not moisture from below. How can I solve that problem?
Hello Sue. Moisture from below the surface is a much different problem entirely and one that can be difficult to stop. Coatings and penetrating sealers do a good job at stopping moisture that comes in contact with concrete at the surface. Moisture from below, however, does not interact in the same manner. Coatings and sealers are poor performers in most cases when trying to stop moisture from the other side.
Concrete is essentially a hard sponge. It is porous and will absorb moisture in the soil if it’s there. For above grade slabs, atmospheric conditions above the concrete (typically relatively warmer and dryer) will draw the moisture up to the surface that the slab has absorbed . This moisture will either evaporate and leave behind the minerals that traveled with it (efflorescence) or it will be so much moisture that evaporation cannot keep up with it and result in damp or even visibly wet concrete. Sometimes application of densifier will help slow down this moisture and sometimes even stop it all together. This article here explains how a densifier works.
If the concrete slab is below grade, then moisture may be pushed up through the concrete due to hydrostatic pressure. Hydrostatic pressure is very hard to control and sometimes may require grinding of the concrete and special moisture vapor blocking coating applied (expensive). Even that may not be enough and is why it’s not unusual to sump pumps in basements.
Your first course of action is to determine why you have moisture to begin with and see if it’s something you can fix to help mitigate the moisture problem. Is it due to poor drainage around the slab and water is pooling up against it when it rains? Are there planter beds up against one side? A leaky water or drain pipe? Or, is something you can’t control like one or more sides of the slab below grade and up against or near a dirt bank?
I have a garage floor slab that gets very wet, sometimes small puddles. One side of the garage has about 3ft of soil against the wall, the other is at grade. I moved a metal tool box and it was more wet under it. Is any kind of coating going to help this?
Hi Jordon. No, most coatings are just going to peel up with that much moisture coming up through the slab. There are moisture vapor blocking epoxy primers that can be applied, but they are expensive and may not work with that much moisture. Testing would need to be done first to determine how many lbs of moisture flow per 1000 square feet per 24 hours you have. A better alternative would be to install a french drain along that side of the slab to mitigate the moisture. If that is too costly, then we would recommend installing an interlocking tile floor instead. They will allow moisture to collect and evaporate under the tiles.
What if I want to enclose the sweating carpet/garage into a living space? Same principals apply?
Hi Eon. Yes, the same principles apply. However, enclosed spaces are generally heated or air conditioned and not subjected to the immediate outside weather changes that can cause sweating of a slab.
Shea,
My garage floor has been plagued for years with what I think is a combination of sweating and moisture from beneath. I live in north Florida where it is almost always very humid with frequent rain, yet can be cool/cold in the winter months. Your article about sweating was very informative and I will be implementing some of the sweat reduction things you outline as well as possibly sealing the floor. In the past I’ve intentionally left my main garage door up about a foot, day or night, rain or shine, so my cats can have access. That will have to change!
But the reason I write is, I have a curious thing going on at a few spots on my floor for which that I cannot come up with a logical explanation. The long story is that the garage was completely cleaned out for a paint job. The clean-out was done after several days of no rain and low humidity so all the usual wet zones were dry. Two or three days after the clean-out high humidity rolled in (heavy fog at night and morning but no rain) and as expected the usual wet zones became wet again… except for two or three once hidden spots. From the shape and location of the dry spots I can see that they are the outline of where a plastic storage box sat virtually unmoved for months/years. They are completely surrounded by wetness but for some reason water isn’t condensing right where the boxes sat. No special treatment was ever done to those spots or the surrounding wet zones. I’m baffled as to why these spots are exempt from condensation and wish I could extend their magic to the surrounding areas!
Let me add that the entire garage floor usually doesn’t get wet. The wetness happens most frequently (but not exclusively) around the outer walls where I am pretty certain water also comes up from below. Once a wet zone dries there is typically a white residue “ring” left behind. The floor is at ground level and during moderate to heavy rain the ground outside often saturates.
Any theory about the mystery dry spots?
Hello R. Great mystery! Our educated guess has to do with the plastic storage boxes and the fog. Many plastics will slowly release monomers (small particles of polymers) and other substances during their life. Most likely, these monomers have leached over time into the upper surface of the concrete, thus making it much less porous. When the overnight fog rolled in, the more porous concrete was able to absorb the moisture in the air that came in contact with it. That is why it looked darker and damp. The temperature of the concrete was most likely not low enough to cause condensation. This technically isn’t sweating, just a result of the wet foggy air coming in contact with a porous surface ready to absorb moisture. Assuming the concrete where the plastic boxes sat is much less porous, the concrete would not absorb the moisture out of the air and appear dry.
Thanks Shea. You might be onto something. Thinking of all the tendencies that I can remember (hopefully correctly) over time, I would tend to agree that my floor’s not really “sweating”, though it definitely correlates to high moisture in the air. During the recent fog event the floor wasn’t really cold. So the simple absorption of the saturated air (fog) in some zones could be what’s going on. In fact, as I type, the floor is drying with a light continuous rain going on, i.e., humid air but less humid than that foggy night. And I think the floor is cooler now than during the fog — cold front moving through. Seeing this makes me less fearful that the problem is water rising up from below. Especially, since you state above that efflorescence can even occur from condensation and absorption. Seeing efflorescence made me just assume that ground water was rising up.
So if you’re are correct and I’m getting absorption into the porous surface I suppose a penetrating sealer should fix the problem. But before I do anything I will experiment to nail down once and for all if there is water rising up.
Thanks for your insights.
Hello,
This has been a helpful forum – thank you. We have a sweating floor near a lake in Michigan and I fear moisture is coming from below (it’s a poorly built slab-on-grade cottage( as well as humid air above. Wondering if anyone has heard of installing a ventilation system below the floor using fans similar to those used in radon mitigation to keep air flowing through the floor with concrete or tile above? Thank you.
Hi Margo. That’s creative thinking, but moisture vapor from wet soil is not going to be alleviated with such a system for a variety of reasons. The first thing we recommend doing is a moisture test to get a much better idea of the problem you need to address. I’m assuming the floor is bare concrete?
Hello! I have a recurrent water stain on my garage floor that keeps showing up in the same area. However, it never feels wet to the touch. The intensity of the stain darkens and lightens. Sometimes this happens when it rains and sometimes not. I live in Florida so due to the year-round warm weather, this happens all the time. The house is less than 3 years old. Recently, I had left an object on part of the stain for a while. When I lifted it, I used a moisture meter (don’t know if those things are actually useful for concrete) and it read 28% for the area under the object. The surrounding areas which were exposed to air and had the chance to dry off were 12%. If pictures will help, I can send them. Thank you for your help!
Hi Pesi. This is not uncommon in Florida due to the high water table. The dark area being in one spot is an indication that you have moisture traveling up through the concrete from below. When it reaches the surface it evaporates. It would only be wet if the moisture content traveling through was at a rate faster than what could evaporate at the surface. If the area under the object was just as dark or darker, then than confirms it’s moisture from below. Sometimes moisture in one spot may indicate a leaky pipe that is running under the slab. It may be worth checking if you are not sure if there is plumbing or a drain pipe of some sort below the slab.
Thanks for the info. You said the answer to a sweating floor is to add flooring to the garage (but not a sealer) I’m not understanding what the difference is. I added a cheap plastic underlayment and some vinyl flooring and now there is moisture between the underlayment and my vinyl floor seeping up through the cracks of my vinyl. I am not sure what is going on or how to solve it. Thanks
Hi Sarah. Have you first verified that you have a sweating floor and not a floor with moisture issues? We discuss how to determine that in the article.
Finally, somebody actually figured this out! Everyone else claims it is moisture from below and applying a coating of some type is the answer.
A way to prevent this in new construction is to put your slab up high on fill well above grade so it is high and dry and can quickly heat up. Better yet, pour your slab over insulating foam.
Thank you for the information.
You are welcome, Rob.
I live in south Louisiana and just had a polyurea coating put down on my outdoor screen porch. The first two months I didn’t have problem but last week every time it rains the floor sweats. Now last week we added limestone up against (next to screen porch) the house and flagstone next to it. Water pours off the roof into the limestone. Could that be causing the problem?
Hi Sue. Flooring outside and adjacent to the screened porch should not have an effect. The most likely cause is the polyurea coating. If you had bare concrete before, the porosity of the concrete surface more than likely absorbed any moisture from condensation (sweating) that may have been occurring when the conditions existed. The most you would have noticed would have been a slightly darker look to the concrete when this happened. Polyurea is a smooth coating and sealant. When the conditions for sweating are prevalent, moisture will easily collect on the surface since it is smooth and will not absorb moisture. It’s similar to what happens to the surface of paint on a car when dewy conditions exist. We would recommend that you try running a small fan across the surface of the floor next time it rains and see if it prevents the sweating from occurring.
What do you recommend for an outdoor carport with a sweating concrete floor? It’s not easy to dehumidify or control the temperature when one end of the carport is open. Would ceiling fans help? Does an epoxy floor with anti-slip surface still sweat? Thank you.
Hi D. No a coating will not help since it easily takes on the same temperature as the concrete. Ceiling fans would help. Garage floor mats are an example of a type of flooring that would not easily take on the same temperature as the concrete beneath it. The surface temperature is similar to the air temperature and will not cause the air to condense on the mat.
Thank you! This is very helpful.
This is a great article, everything within suggests to me my slab is sweating condensation, and not moisture from below. Have a single car attached garage that was clearly once a carport that was enclosed into a garage. Last July, there was an intense warm and humid front that moved in with some heavy rains. Noticed some thicker condensation than usual, assumed it was a drainage issue…so had drainage installed as appropriate.
Came across this article since then. I have the misty/dark looking concrete on humid days (sometimes a little damp to the touch), and I’ve taken surface and air temp. Sure enough, it seems to always be the case that the air temp is something like 70, the slab upper 40s, and the slab is below the dew point. Cold front comes in later dropping the dew point, and it dries out. It can drop several inches of rain during the day, but if it’s colder out, nothing happens with my slab… no dampness at all. Conversely, it can be hot, humid, and no rain, but if I open that garage door, here it comes! Anyways, I think this article is great and indicates I’m dealing with condensation.
Hi PK. Thanks for the kudos. We are glad it helped to confirm your analysis.
Great information. Here is my puzzler. We have 2 garages. The one attached to the house has finished walls & ceiling (drywall & paint). The floor sweats like crazy when conditions are prime in the spring and summer. The other building has no drywall, open trusses and has wood siding. The cement in this building never sweats. Could it be because of its unfinished state and wood siding? Both buildings have been up for decades. Thanks!
Hi DI. Because the enclosed building is insulated well, the concrete most likely is retaining its cold state for a long time which keeps it below the dew point. The building with the open trusses and wood siding would not. In addition, it may be getting some airflow across the surface. As a result, the concrete slab may be reacting to warmer temperatures much quicker and does a much better job at staying above the dew point.
Thanks for the explanation. It confirms our thoughts but with the ‘whys’ explained 🙂
If the moisture problem is from under the slab, do your recommended solutions still apply?
Hi Pete. No, moisture from under the slab is a completely different problem that can sometimes be difficult and/or costly to fix, if it can be fixed at all. If the slab is above grade, many times the moisture source can be from water pooling up against the slab. This is usually remedied by fixing the drainage to slope away from the slab. Sometimes it can be from a leaky water line or drain pipe under the slab. If the slab is below grade or up against a slope or hill, then a moisture vapor barrier would need to be installed or a french drain if it’s a side of the slab against a slope or hill. This can be costly or sometimes not possible without replacing the slab. Running a fan over the area would help the moisture that collects to evaporate faster, but that is about it.
Hello. Great information here. Thanks.
My problem started around September 2022. It is now December 2022. I live in Lafayette, Indiana. The house is 24 years old. The garage floor is on or above grade, and there is no basement. Wet spots began showing up in three or four places on the garage floor. These wet spots are always in the same places and the garage door bottom seal is always wet as well. I had that seal replaced within the last month. The garage door repair man had no ideas why this is happening. Lately a new wet spot is showing up on the side of my driveway and it is now freezing as the temperature has dropped. The inside wet spots are always under the parked cars in the garage and between them.
And now the oddest thing for me to understand is after I remove the wet spots and dry the floor, it will stay dry for 3-4 days. Then the wet spots will appear again. I patched some of the surface pitting on the floor, but to no avail. I’m thinking, based on your analysis, that the moisture is coming from beneath the slab. Occasionally I will see some of the efflorescence you mentioned where the wet spots emerge.
What are your thoughts and is there anything I can do to relieve this situation?
Thank you.
Hello Bob. Before you do anything you need to run a test to determine if the condition is concrete sweating (condensation) or moisture from below the slab. We suggest taping a couple of 16″ square sheets of plastic to the concrete in the garage where these wet spots occur. Make sure to completely seal all the edges and use tape that will stick to concrete. This needs to be done when the concrete is currently dry. We suggest placing one under the car where it parks. Leave it there until the concrete becomes wet again and then pull up the plastic. If it’s dry underneath, then you have issues with a sweating slab. If it’s wet under the plastic, then it’s moisture from below.
Our guess is that you have a sweating slab. If the concrete cools enough and you then pull a warm car inside the garage, the warm car will warm the air over the cool concrete and create condensation. Our article discusses what can be done to help eliminate issues with a sweating slab. Let us know what happens. If it’s not a sweating slab, then there other steps that need to be taken.
Hi Shea. Ok Yes. I taped down plastic in three places on the garage floor. After the fourth day, as expected, the water appeared. And, there was water under each of the three plastic taped down spots. It seems like it’s getting worse each time (more water), but it is basically in the same spots. So, it looks like moisture from below. Why not everyday? What can I do?
Bob
Hi Bob. You are positive that your slab is above grade, correct? It’s not a few feet or so away from a dirt bank with higher ground or anything like that? Water drains away from the house and does not sit up against it? No planter beds up against it either? If not, then the first thing we would recommend is making sure you don’t have a leaking water line, drain pipe, sewer line, leach line, or anything like that running under the slab. It’s a bit unusual to have moisture under a slab that is above grade. The fact that it seems to be getting worse is concerning as well.
One way to check if you have a water pipe leak is to make sure that all water sources are shut off within the house and not running. This includes outside water as well. Then check your water meter outside. If the little red wheel is not turning, then you don’t have any leaks. If it is turning, then you do have a water leak. Plumbers also have leak detectors for drain lines and sewer lines.
If all checks out OK, then the only thing you can really do at this point is to apply a densifier to the concrete when it is dry. This article here explains how they work and includes a product we like. Many times a densifier treatment will slow down moisture intrusion and sometimes even stop it. However, you can’t completely waterproof concrete at the surface from moisture coming up from below. The only way to do that is from under the slab, which of course is too late to do at this point. It needs to be done before the concrete is placed.
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Thanks. I did have the utilities people come out and check the water meter. They found no problems with that. I’m the original owner of the house which is 24 years old. Never had this problem before.
Hi Bob. A malfunctioning meter will not cause the problem. What we are saying is to check the meter to see if any water is passing through when you know for a fact that there is no water being used for the property. If the little red wheel is moving, then water is leaking from a pipe. We are guessing you have a problem somewhere unless you live in an area where it’s not uncommon for groundwater to come to the surface from time to time. You may want to have a good plumbing company with leak detecting equipment come to check it out. We don’t envy your position. It’s no fun trying to source a water issue.
Thanks. I will look into that.
This article was so helpful! I also appreciated the detailed reply to the question about how to midigate (or at least keep the floor dry on top of a floating surface) a garage floor that is damp because it is below grade on a wet hillside.
Thank you.
We are glad it helped!