5 Reasons Moisture Is Coming Up Through Your Concrete Floor
When concrete is first poured, it contains a lot of excess water that has to dry out. A lower water-cement ratio means less excess water to remove.
You recently installed a hardwood floor, and you made sure to check the wood’s moisture content many times, allowing the wood to acclimate fully before installation.
Now, the floorboard edges are turning upward, a condition called cupping. You know there must be a moisture issue. But how?
It may be coming up from the concrete slab beneath.
But you’re still confused. After all, you used a concrete moisture meter, and the slab seemed dry. Where is all that moisture coming from? We’ll look at five possibilities:
- Too high of a water-cement ratio
- Not enough drying time before installation
- Vapor retarder problems
- Poor drainage
- Plumbing leaks
Once we’ve covered these potential moisture sources, we’ll provide you with some tips and tools for keeping concrete moisture under control.
1. Too High of a Water-Cement Ratio
Water is a core ingredient of concrete. Concrete is a mixture of water, cement, and aggregate, and the water reacts chemically with the cement so the concrete can harden once it’s been poured.
But just because concrete has hardened doesn’t mean it’s dry.
A full truckload of concrete can easily contain more than 320 gallons of water. That’s a lot of moisture that has to leave the slab!1
The moisture dries from the surface first, creating a gradient: more moisture in deeper parts of the slab and less moisture on the surface. You may not notice the excess moisture.
That’s why the water-cement ratio matters from the start. The ratio is the weight of the water divided by the weight of the cement. For example, a mix with 235 pounds of water and 470 pounds of cement has a 0.50 water-cement ratio.2 The more water in the mix beyond what the cement needs, the more there is to dry out later. Always follow the architect’s or engineer’s specified mix design.
2. Not Enough Drying Time Before Installation
Even with the right water-cement ratio, concrete needs time to dry before a floor can be installed on top. Moisture from deep within the slab slowly moves to the top and evaporates off the surface.
If the slab hasn’t had enough time to release that moisture, it will keep releasing it after the floor has been installed on top.
The best way to know whether the concrete has dried enough is to perform in situ relative humidity testing. More on that test later. But first, let’s cover three more reasons for moisture coming into your floor.
3. Vapor Retarder Problems
A damaged or improper vapor retarder can let ground moisture move up through the concrete.
A vapor retarder goes beneath the concrete to keep moisture from the ground from coming up into the slab. Some problems with vapor retarders are:
- Not having one
- Having the wrong kind
- Having a damaged one
ASTM E1745 is the standard for plastic vapor retarders under concrete slabs. It places vapor retarders into three classes (A, B, and C) based on their tensile strength and puncture resistance.
When it comes to moisture, permeance is one of the most important properties because it determines how much water vapor can pass through the retarder. It’s measured in perms, and the lower the rating, the better. All three classes under ASTM E1745 must have a permeance of 0.1 perms or less.
Unfortunately, vapor retarders can get damaged during construction. If you can, check that the retarder is intact and its seams are sealed before the concrete is placed. Otherwise, holes or tears could let moisture in.
4. Poor Drainage
A good vapor retarder will help keep moisture from coming up from the ground, but poor drainage can still cause problems, especially without one. If the site was graded improperly, rain, snow, and sprinklers can cause water to pool around the structure rather than draining away from it.
If water builds up in the soil around the foundation, the pressure from that water, known as hydrostatic pressure, can force moisture through the concrete and into the building.
This typically happens when the water table is higher than the concrete slab, such as with a below-grade basement (in which case you may not want to install a solid hardwood floor anyway).
5. Plumbing Leaks
Plumbing leaks above, in, or under the concrete can let water soak into it. Sometimes, these leaks in supply or drain lines are hidden, wetting the concrete for weeks or months before anyone notices the problem.
And if left untreated, they could cause significant damage to the home or building.
How to Keep Moisture Under Control
Relative humidity testing of the concrete floor is the first step in keeping moisture under control.
To properly deal with moisture issues in your concrete, you’ll need to measure the amount of moisture in the concrete, assess the situation, and mitigate the problem based on the source of the moisture. Unfortunately, you may have to pull up the hardwood flooring that’s there.
As we mentioned earlier, one of the challenges with concrete slabs is hidden moisture.
A concrete moisture meter can help you find moisture “hotspots,” but it only reads near the surface. The C555, for example, measures to 1/2 inch deep.
ASTM F2170 is the standard for in situ relative humidity testing. For a slab drying from one side, you drill to 40% of the slab’s depth (20% for a slab drying from two sides) and place an RH sensor in the hole to get a better picture of the moisture inside the slab.
ASTM F2170 calls for three tests in the first 1,000 square feet of the slab and one more test for each additional 1,000 square feet.
If the relative humidity of the concrete is higher than the flooring manufacturer’s limit, allow the concrete to dry further before putting any flooring back.
Depending on the moisture source, you may also need to:
- Fix leaks
- Improve drainage around the building
- Waterproof the exterior of the foundation
- Install a sump pump
- Install a moisture mitigation system (as a last resort)
Once the slab is dry and you install a floor over it, how can you know a problem won’t arise again?
The best way to have peace of mind is to monitor the moisture conditions of your hardwood floor. You can do this (or help your customers do it) with a small data logger, such as Floor Sentry®, that fits into the underside of a wood plank.
It records the temperature and relative humidity inside the floor and alerts you if conditions go out of range, so you can fix the problem before your hardwood floor gets damaged.
Say Goodbye to Concrete Moisture Problems
Don’t wait to address a concrete moisture problem. It’ll only get worse and cause more damage to the building.
Diagnosing the problem and measuring the concrete’s moisture levels will give you a clear path forward for controlling the moisture. And monitoring the conditions inside the hardwood floor will help you catch any new moisture problem early.
Learn how Floor Sentry can help you catch hidden moisture in your hardwood floors.
2. “How to Calculate Water Cement Ratio for Concrete,” ConcreteNetwork.com.
Jason has 20+ years’ experience in sales and sales management in a spectrum of industries and has successfully launched a variety of products to the market, including the original Rapid RH® concrete moisture tests. He currently works with Wagner Meters as our Rapid RH® product sales manager.
Last updated on October 7th, 2026
800-634-9961
Hi, I have a concrete floor in my garage and had the floor done with a two part epoxy. I had to have it redone because I was getting water blisters. The same thing happened again and the contractor that did it tells me that the reason it’s happening is that there was no vapour barrier installed. He called it off gassing. I have just done it for the third time at his insistence but I am sure it will happen again. Is there any product that I can use that will seal my concrete and eliminate this issue? Thank you.
There are products. The first place I would start is with the manufacturer of the epoxy being installed. You are looking for a moisture mitigation system that is warranted even without an intact vapor retarded.