Installing hardwood flooring is a significant investment, and one of the most common reasons for installation failure isn’t the wood itself—it’s excess moisture in the subfloor. Whether you’re installing hardwood over concrete or plywood, failing to check moisture levels beforehand can lead to warped boards, cupping, buckling, adhesive failure, and costly repairs.
Fortunately, testing a subfloor for moisture is a straightforward process when you use the right tools and techniques. In this guide, we’ll explain why moisture testing is essential, how to test both concrete and plywood subfloors, and what to do if moisture levels are too high.
Why Moisture Testing Is So Important
Hardwood is a hygroscopic material, meaning it naturally absorbs and releases moisture depending on its environment.
If the subfloor contains excessive moisture, the hardwood will absorb that moisture after installation, causing it to expand. As the wood continues to move, homeowners may experience:
- Gaps between boards
- Cupping or crowning
- Buckling
- Loose planks
- Squeaky floors
- Cracked finishes
- Adhesive failure
Testing moisture before installation helps prevent these problems and extends the lifespan of your hardwood floor.
Understanding the Role of the Subfloor
The subfloor acts as the foundation for your hardwood flooring.
The two most common subfloor materials are:
Concrete
Concrete can retain moisture for months after being poured. Even older slabs may absorb moisture from the ground if they lack an effective vapor barrier.
Plywood
Plywood generally dries faster than concrete, but it can still absorb moisture from crawl spaces, leaks, or high indoor humidity.
Because each material behaves differently, they require different testing methods.
How to Test Moisture in Concrete
Concrete moisture should never be judged by appearance alone. A slab may look completely dry while still containing enough moisture to damage hardwood flooring.
1. Use a Concrete Moisture Meter
A non-destructive concrete moisture meter provides a quick assessment of surface moisture.
While useful for preliminary checks, it’s often considered a screening tool rather than a final measurement.
2. Perform an In-Situ Relative Humidity Test
Many flooring professionals use in-situ RH testing, which involves placing probes inside drilled holes in the concrete slab.
This method measures the slab’s internal moisture and provides a more accurate picture of long-term conditions.
3. Calcium Chloride Test
Another commonly used method is the calcium chloride test, which measures the amount of moisture vapor emitted from the concrete surface over a specified period.
This test is often required by flooring manufacturers before installation.
How to Test Moisture in Plywood
Wood subfloors require a different approach.
Use a Pin Moisture Meter
A pin-type moisture meter measures the moisture content inside the plywood by inserting small probes into the wood.
Professional installers typically take readings in multiple locations throughout the room to identify any hidden moisture issues.
Compare Moisture Levels
The moisture content of the plywood should be reasonably close to that of the hardwood flooring before installation.
Large differences increase the risk of expansion or contraction after the floor is installed.
Where to Take Moisture Readings
Testing only one location isn’t enough.
Take readings:
- Near exterior walls
- Around windows and doors
- Near plumbing fixtures
- In room corners
- Around HVAC vents
- Across the center of the room
- In areas with previous water damage
Testing multiple locations provides a more accurate understanding of the subfloor’s overall condition.
Signs of Excess Moisture
Even before testing, certain warning signs may indicate a moisture problem:
- Musty odors
- Water stains
- Visible mold or mildew
- Damp concrete
- Warped plywood
- Efflorescence (white powder on concrete)
- Persistent condensation
- Previous flooding or leaks
If any of these signs are present, investigate the source before installing hardwood.
What Happens If Moisture Levels Are Too High?
Installing hardwood over a damp subfloor can cause serious problems, including:
Expansion
Boards absorb moisture and swell, placing pressure against neighboring planks.
Cupping
The edges of the boards rise higher than the center as moisture enters from below.
Buckling
Severe moisture can force boards completely off the subfloor.
Adhesive Failure
For glue-down installations, excessive moisture may prevent adhesives from curing properly or cause them to lose their bond over time.
How to Reduce Moisture Before Installation
If testing reveals elevated moisture levels, don’t rush the installation.
Instead:
- Identify and repair leaks.
- Improve ventilation.
- Run the HVAC system under normal living conditions.
- Use dehumidifiers if necessary.
- Allow newly poured concrete to cure fully.
- Install an approved moisture barrier when recommended.
- Retest the subfloor before proceeding.
Patience at this stage can prevent expensive repairs later.
Best Practices for Moisture Testing
To ensure accurate results:
- Test the subfloor before delivering hardwood whenever possible.
- Follow the flooring manufacturer’s moisture guidelines.
- Take readings in multiple areas.
- Keep indoor temperature and humidity stable.
- Record moisture readings for future reference.
- Never assume a subfloor is dry based on appearance alone.
Professional installers often document moisture readings as part of the installation process to verify that site conditions met manufacturer requirements.
Final Thoughts
Moisture testing is one of the most important steps in a successful hardwood flooring installation. Whether you’re working with concrete or plywood, understanding the moisture conditions beneath the floor can help prevent warping, gaps, squeaks, adhesive failures, and other costly issues.
By using the proper testing methods, allowing the subfloor to dry when necessary, and following manufacturer recommendations, homeowners can create a stable foundation that supports the beauty, durability, and long-term performance of their hardwood floors.
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