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How a Carpet Gets Wet While On A Cement Slab

By March 10, 2021March 26th, 2021Concrete issues
Why is my carpet wet house is on a cement slab

There are many things that can happen when a house is in the building phase.  We choose a cement slab and then think that our house is protected from any water intrusion.  However, sometimes that carpet can be wet inside the house even when it is on a cement slab.  Why?  Many times when the house is in the building process, there is a rush to get it done. 

The concrete doesn’t have time to dry all the way and this allows moisture to be in the concrete.  Other times, it has to do with the wrong vapor retarder being used or it is torn.  Additionally, it can also be the slope of the land and other things that may go wrong inside or outside of the home.  All of these things can cause the carpet to be wet when the house is on a cement slab.

Let’s take a look at why this is important, what causes this, how do you know there is a leak, what to do when the carpet is wet, the types of floors prone to this, MVER vs. RH, how builders can prevent future issues, other areas to check, and when to call a professional.

Why This is Important

This is important because water intrusion in the home causes mold and other issues.  It is not natural for water to come into the house and it needs to be figured out and corrected right away. It can be a variety of things that we will take a look at.

Sources of moisture on concrete

Sources of Moisture on Concrete

  1. The proportion of water mixed with cement and not giving the cement time to dry.
  2. Rain, snow, and sprinkler systems can cause havoc on a worksite and get the concrete moisture higher than it needs to be.  It is even worse if the grading of the ground slopes toward the home instead of away.
  3. Water sources such as pipes can leak or if you have an older home, your pipes may have worn out.
  4.  The environment is another problem.  The RH humidity in the slab is lower than the air’s relative humidity so it causes problems and moisture gets into the slab. 
  5. Inadequate drainage can cause the carpet to be wet in the house on a cement slab.
  6. Using the wrong vapor retarder.  A vapor retarder should have a perm of 0.3 and be installed over granular fill that separates it from the groundwater.

How to Tell if There is a Leak in the Concrete

Wet Carpet

The water soaks for as long as it can in the concrete and then when it is full, the water goes up to the carpet.  Thus leaving the carpet wet in the house and it will have a smell to it even on the cement slab.

Water Bill Increase

An increase in the water bill shows there is a leak somewhere in the home and you need to get it checked out.

Hot Floors

The floor will be hot if there is a pipe from the hot water tank that has ruptured. Copper pipes, for instance, have been known to burst underground.

Decrease in water pressure

Decrease in Water Pressure

A decrease in water pressure happens when there is a leak in your water mainline and you need to contact a professional right away.

Odd Sounds

Odd sounds could be water running in the pipes and you need to have a professional look at it.

Moving or Cracking in the Foundation

This can be due to a slab leak that needs to be repaired right away.

What to do when the carpet is wet

What to Do When the Carpet is Wet

When the carpet is wet, then mold starts to grow. To get rid of it:

  1. Ventilate the room
  2. You may have to cut out part of the carpet and replace it
  3. Try using baking soda and vinegar to remove it
  4. Vacuum if you have a HEPA filter
  5. Use a dehumidifier, fan, or open the windows
  6. Use Anti-fungal spray

4 Types of Floors Are Prone to This

1. Adhesives

Adhesives are used on some flooring, however, if it doesn’t have the moisture tolerance for the concrete subfloor, then there will be problems.

2. Floating Floors

Floating floors are the best type of flooring for use as they lock together and don’t have to be attached to the subfloor.  Installing a moisture barrier is important. Unfortunately, these often become compromised and then don’t do their job.

Wood flooring

3. Wood flooring

Adding in an underlayment between the flooring and concrete slab can help keep these intact with no issues.

4. Grout or Cement Bonds

If there is excessive moisture in grout or cement bonds, it will appear like efflorescence. When the moisture evaporates away, the water-soluble minerals get transported to the grout. 

The minerals are part of the concrete slab mixture.  If the slab did not dry before the tile was installed, the grout can be impacted by the drying concrete.

How to tell there is excess moisture on the concrete

How to Tell There is Excess Moisture on the Concrete

One way to tell there is excess moisture is if there is a white or grey powdery stain on the concrete.  This is called “efflorescence.”   This is because the moisture moves up through the slab and evaporates through the surface.

If there is a white stain on the concrete, that is from the water evaporating. Testing is needed to find out the issue.  You may also notice that mold is growing on the concrete.


MVER is Moisture Vapor Emission Rate and is a calcium chloride test.  It measures the moisture level on the concrete flooring.  However, it takes 72 hours and a salt tab is placed on the concrete slab and under a sealed dish.  Then, the amount of moisture that evaporates is looked at by the weight of the salt tab after the time expires. 

The problem is that the environmental conditions often skew the results and only measure moisture at the surface and not under the slab.

On the other hand, the RH or Relative Humidity test, tests for the moisture below the surface of the slab at a certain depth.  This is done by using a sensor that goes into the slab.  It can read what the moisture condition of the slab will be when the flooring is installed and can be completed in 24 hours, unlike the MVER test.  

What the Homeowner Can Do

Homeowners should look at where the stain is in relation to sources of water in their home. For example, if it is near an exterior wall, it’s more likely to be from a sprinkler, poor siding/paint, or improper grading.

Another example, if it is near a bathroom, it might be a plumbing leak.

Lastly, if it is in the middle of the room, it could be the vapor barrier was improperly installed or a pipe burst under the concrete.

How builders can prevent future issues

How Builders Can Prevent Future Issues

  1. Use low water to cement ratio and give the cement time to dry.
  2. If concrete is poured below grade, take caution to make sure there is no run-off towards the site.  If it does, you may be adding more drain lines or pumps to get the water out of the area.
  3. Using a vapor retarder that is at the appropriate perm level for the size of the space.
  4. Work into the plans enough time to allow the concrete to dry.  Also using a dehumidifier or a fan to dry everything out will help.
Other areas to check

Other Areas to Check

In addition to checking the negative grading as mentioned.

  1. Check the foundation for any cracks and if you find them, contact us to take a look.
  2. If you have a basement, check for mold there.
  3. Use downspouts on your home and make sure they are 10 feet from the home.
  4. Installing a French drain or interior drain may be needed.
  5. Make sure the air is good in the house and use a floor fan.
  6. Add eaves or downspouts to your home to help carry the water farther away.

When to Call a Professional

If there is mold from the wet carpet and it is bigger than 10 square feet, call a professional to diagnose the mold problem and see if you need to replace all the carpet.  Also, the professional can use thermal imaging to look for the source of the water. If you are not sure what is causing it, contact a professional to investigate for you.


Water on the carpet can happen when the concrete has absorbed as much water as possible.  Thus, it then continues to go up into the carpet.  It is difficult to know if this will happen as we are not there when the home is built. 

Other times, it is because of negative grading or broken pipes and these can be fixed.  We can take a look at your walls during our home inspections in Houston.

Questions about possible broken pipes, slab issues, mold removal or in need of ours services?  Comment below.

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