WHY MOLD GROWS UNDER FLOOR
As a Mold Inspector in West Palm Beach I travel all over Palm Beach, Dade, Broward and Monroe Counties. My job is to figure out the cause of, and solution to mold problems. One of the most difficult to inspect and understand is mold under floors.
Clients often ask about mold under floors, so I decided to write about it.
Common questions are can mold grow under floors? How does mold grow under floors? How does it get under there in the first place?
So let’s discuss it below.
A FOOD SOURCE
When floor coverings are installed microscopic particles of dust, dirt, bacteria, and mold spores are always trapped under the floor covering. It is common for there to be dozens or hundreds of mold spores per each cubic meter of air in a healthy building. Thousands of bacterial cells and dust particles are likely in that same cubic meter of air. Any dead bacterial cells, dust, and dirt trapped under the floor can serve as a food source for the mold spores trapped under the floor during instillation. Thus, while ceramic tile, tile mortar, and tile grout are not a food source for mold the particles trapped during instillation are a food source.
Of course, if it is a carpet or wood floor, the floor covering itself is a perfect and abundant food source, no dust needed. If it is linoleum floor, the linoleum glue is a likely food source for mold growth under the floor.
WATER – A LIMITING FACTOR
But there is a problem for the millions of mold spores commonly under all our floors. There is not enough moisture to let mold grow under floors. Even when wet mortar is used, the mortar dries before mold can grow. As a result, you will not have mold growing under most floors, just normal harmless dormant spores. Do not worry about them, they are trapped and not growing. But….
If a leak occurs, such as from a broken plumbing fixture, a hurricane, or a flood, water can and often does end up under floor coverings. Water always finds a way. Another water source is moisture vapor from a floor slab with no vapor barrier.
When water gets under the floor, the dormant mold spores that may have been waiting for decades are awaken and start to grow. Feeding off the dust, dirt, and if present the wood or carpet. The mold growth does not stop until the food is exhausted or the water dries.
A PROBLEM WITH TESTING FLOORS FOR MOISTURE DURING FLOOR MOLD INSPECTIONS
During mold inspections of floors, non wet floors often read as if they are wet even when perfectly dry because moisture meters pick up signals from concrete under the floor. Concrete, tile, and other masonry materials will cause false high moisture readings even if they are dry.! Even if the material is buried under 1/4 or 1/2 inch of wood the meter will pick up false readings from the masonry materials even when they are dry. During mold inspections of floors testing for moisture can be tricky.
Professional Tip:
When using a moisture meter test always compare readings from the suspected wet floors to readings from the suspected dry floors. If moisture levels in the suspected wet floor areas are significantly higher than suspected dry floors, then you know that suspected wet floors are very likely wet. If you do not do this, instead you just rely on the meter’s scale to tell you if a floor is wet, most floors will seem wet. This is because of the false positive reading from the concrete slab under the floor covering.
PICTURES OF MOLDY FLOORS
FROM ACTUAL MOLD INSPECTIONS
In this West Palm Beach condo a pipe hidden inside the wall of the closet seen here leaked. The water flooded the entire property. Now much mold is under the floors. All 5 of these images are from the same property.
This is the living room of the above referenced property. The wooden Parque type floor looks fine from a distance. But in a few areas mold is coming up from between the floorboards.
Parque type wooden floor near a brown entertainment center and near a white section of wall. On the floor in this area some mold is coming up from between the floorboards.
Closer view of mold coming up between the floorboards.
Closer view of mold coming up between the floorboards.
Moisture meters showing 99.9. This is as high as this meter can read. Mold grew under the floor and popped up between the floorboards due to the abundant moisture. I think this was due to a foundation problem if I remember correctly.
Mold under wood floor. White Pen Asp mold coming up from between floor boards.
Mold was under this wet wood floor. Closer view of white Pen Asp mold coming up from between floorboards.
SOME EXAMPLES FROM MIAMI AND WEST PALM BEACH MOLD INSPECTIONS
The above photos are from the below examples for moldy floors encountered during mold inspections.
In 2019 I did a mold inspection in West Palm Beach for a personal trainer of the rich and famous.
He had mold under his floors, but the water source was not your typical plumbing leak. My clients’ mold resulted from moisture on the interlocking rubber floor mats that were used in his gym. The mats were already wet when installed by people in charge before he came onto the scene.
Also, in 2019 I did another mold inspection in West Palm Beach, and the clients had mold and moisture under their master bedroom wood floors. Sliding glass doors were leaking and the water went under the wood floors. Mold could be seen coming up through seams between floorboards in this nice home.
And in 2025 I did an inspection in a condominium unit where a pipe inside a bedroom closet wall leaked. The entire condominium flooded, and the fancy wooden Parque style floors and the walls became wet and moldy. The sad part of this story is that the client had just purchased the condominium unit two months earlier.
Years ago I did a commercial building mold inspection of an entire nursing home in Homestead Near Miami.
All the nursing home’s bedrooms had linoleum floor coverings. Mold growth and bacteria was under the linoleum tile in all the bedrooms. The reason was because when the cleaning crews would mop, they would use lots of water. That water was getting under the floor coverings and causing smelly mold under the floor coverings.
TEST YOURSELF
Question: You should now be able to figure out which properties floor had more mold, the gym floor with the rubber mats? or the home with wood floors?
Answer: The gym mats did a better job of trapping moisture because they were rubber and did not let water evaporate. Thus, it would seem that the gym floors would be moldier. However, the home with wood floor coverings had more mold. The reason is wooden boards will provide hundreds of pounds of molds natural and favorite food source (wood). Rubber mats with tiny amounts of bacteria, dust, and dirt, will provide only grams of poor-quality mold food. The wooden floors were much more moldy.
