Moldy subfloor and joists may suggest additional hidden trapped mold contamination - © Daniel FriedmanMoldy Subflooring in Buildings
What to Do About Trapped Mold Contamination in Building Flooring Layers

  • HIDDEN MOLD in FLOOR / SUB-FLOOR - CONTENTS: procedures to test for mold between floor finish flooring subfloor surfaces - Mold suspected between layers of subfloor and finished flooring, OPM: use of other people's money to reduce the consultant's risk? Where may mold be hidden in buildings
  • POST a QUESTION or READ FAQs abuot mold trapped between layers of building flooring

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What to do about possible mold hidden between layers of building flooring:

This article discusses the decision to use test cuts or demolition to search for problematic mold reservoirs between building finish flooring and the subfloor, giving examples of when such investigation is warranted and when it is not.

Our page top photograph shows heavy mold contamination on floor joists and on the visible under-surface of subflooring? What about mold that may be present between the subfloor and the finish flooring above? Is that a problem that justifies demolition of the subfloor?

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A Photo Guide to Inspecting Multiple Layers of Flooring & Subfloors or Wall Studs & Sheathing to Inspect for & Remove Hidden Mold

Test cuts for mold between flooring layers (C) Daniel Friedman

Our two photographs just above demonstrate a procedure we use to inspect for evidence of high levels of mold trapped between layers of flooring and subflooring in a building.

No invasive mold inspection: If inspection of the finish flooring from the room side (above) shows no evidence of water damage to the flooring itself, the probability that a large amount of water entered the space between flooring layers and thus that there is a significant mold reservoir located there is low and further investigation including subfloor test cuts are probably not justified.

Invasive inspection is justified: Where a history of extensive building leaks, water damage, floor damage, and visible mold suggest to the inspector that further investigation is appropriate, inspecting from the area below the suspected floor, select the highest-risk most-likely moldy location by tracing the water leak path, stains, and or visible mold in the building. (Photograph, above left).

  1. Using a hole saw (see more photos below), cut up through the subflooring to expose the under-side of the finish flooring above, in the area of highest risk.
  2. Inspect for mold, studying all of the surfaces exposed by the test cut: the originally-exposed underside of the subflooring where the cut was made, the upper (previously hidden) side of the subflooring now visible on the upper side of the round plug cut out using the hole saw, both sides of kraft paper or building felt used between the subfloor and finish floor, and finally the exposed underside of the finish flooring (photograph, above right).
  3. If mold is visible in this worst-case area, and presuming that tests indicate that the mold is not simply a cosmetic species, additional test cuts to determine the extent or size of the mold reservoir might be justified.

When cutting a test opening to explore a wood floor installed on sleepers over a concrete slab, we use two diameters of hole saws (see photos)

  1. First we cut a larger-diameter round hole (say 2.5") through the finish flooring using our hole saw's larger cutting blade. We remove and save the wooden plug created by this step.
  2. Second we cut a smaller-diameter round hole (say 2") through the subflooring to expose the cavity below.
  3. We examine the exposed and inner or hidden surfaces of each plug which is cut, as well as any rosin paper or other material that may be installed between layers of flooring.
  4. Finally, using a flashlight and often a borescope as well, we explore the floor cavity itself for evidence of visible mold, organic mold-friendly debris, insect activity, or evidence of leaks or flooding.

Test cuts for mold between flooring layers (C) Daniel Friedman

Removing finish flooring, leaving subflooring in place: in cases where the finish flooring was water damaged from building leaks, the flooring will need to be removed as it is nearly impossible to flatten and secure buckled wood flooring in place. Removing the finish flooring exposes the upper side of the building subfloor for inspection.

Of course the under-side of the building subfloor can usually also be inspected from below. If the subflooring is plywood and has become water damaged such that it has delaminated, those areas need to be removed and replaced. If the subflooring is intact and sound, it can be left in place, followed by cleaning or mold removal from all exposed surfaces and if appropriate, treatment with a fungicidal sealant.

Leaving subflooring and flooring in place:

In cases where there is no other justification for removing the finish flooring (such as water damaged, buckled finish flooring), even if small areas of mold are suspected or detected between the finish floor and subfloor, or between the subfloor and the upper side of floor joists, the combination of limited size of the mold reservoir, the low probability of air and particle movement from between flooring layers into the occupied space, the effects of building dryout, cleaning, and sealing of potential openings and pathways, in most cases will add up to justification for leaving these materials in place.

Photograph of inspecting for hidden mold with a larger test cut.

Wall, floor, or subfloor test cuts for mold

The photo at right shows us cutting a round hole in a wall to expose the wall cavity.

We cut high enough above the floor to avoid the sill plate and to give access to the wall cavity itself.

We cut close to but not on top of a wall stud, so that when examining the wall cavity we can examine for hidden mold.

Details of using wall test cuts to inspect for hidden mold in wall cavities can be read at Wall test cuts to spot hidden mold.

How to Make (or not make) Random Test Cuts to Check for Mold in Building Cavities

Random wall test openings: We have little confidence in and are reluctant to simply make random test cuts in buildings. Since water can take peculiar paths through hidden openings, such as wall plate holes drilled for pipes or electrical wiring or between single pairs of studs or ceiling or floor joists, cutting a hole that does not reveal a problem is no assurance at all that no hidden mold problems exist.

When to Make Strip Cuts to Look for Hidden Mold in Building Ceilings or Walls

When there is an ongoing building complaint that makes us suspect hidden leaks or mold, if we strongly suspect a hidden mold problem but have not found its precise location, on occasion it is justified to make a "strip cut" across multiple wall studs or ceiling joists, exposing multiple wall cavities.

What makes a lot of sense sense is to study the building carefully to decide on the building points at most risk of having been wet from leaks due to construction details or other site observations. That's where one would make a test cut.

We frequently add text and photos to this series of articles. (See "What MOld Looks Like" and "Stuff that is Not Mold" links ). Readers should also review Mold in Fiberglass Insulation in Buildings at our The Mold Information Center - What to Do About Mold in Buildings

Small amounts of mold can be removed simply by cleaning or removing infected materials, something most homeowners can handle -- but see the Warning Notice at the end of this article. Some mold species can make you sick.

Secondary Risks Associated with Mold Contamination in a Floor Structure

Reader question: mold growth & floor cupping in a hardwood floor in a previously-wet basement

I had about 1200 sq. ft. of hard wood floor installed by a floor installer back on Dec. 2013. The installer did not do the moisture reading of the existing concrete slab prior to floor installation.

When the existing laminate wood floor and tiles were removed there was no sign of mold on the slab and slab looked dried to naked eye. We notice old water stains on the walls and all were documented and reported to the owners (have many pictures)

After the wood floor was installed due to the moist slab (caused by drainage problem around the house) mold grew under the plywood. After the floor was installed and we told the owners about our findings, they admitted that the house had previous mold problems but it was remediated.

The floor was installed for over two months and there was no sign or smell of mold. Due to cupping that occurred the day after the floor was finally stained after two months, opened the avenue to our battle. To make the long story short, I hired a reputable company and got a mold test done. The reading were elevated in two areas, one a place we did no touch or installed any wood floor but had prior mold issue and is also adjacent to an exterior wall. There was no mold in mid section of the house that was away from the moisture and most of the wood floor was installed in that area. The second high mold reading was in a room adjacent to another exterior wall (drainage issue) with prior mold presence. This is the room that the owners had also removed two sections of the wood floor to look underneath!

I want to know if the mold grew under the wood floor after we installed it (Slab got moist due to heavy rain-drainage problem) is it likely (possible) for mold to be airborne when contained under the wood floor? I'm sure cutting the floor exposed some into the air. I

The other question is to know if the floor was not disturbed (cut open) and the owners had fixed the drainage problem and eliminated the source of moisture, could we expect for mold under the wood floor to die? The lab result indicated a moderate mold presence under the plywood and early stages of mold growth.

Your insight and opinion on this issue is highly appreciated. I can imagine how busy you can be answering questions like mine. However, your prompt reply is greatly appreciated and needed! Please also let me know if there are sources (reliable and credential) where I can find answers to my questions in addition to your input. - Anon 5/1/2014


This is more than I can answer in a sentence or two, pro-bono (no fee) with confidence that I' dbe giving entirely reliable advice. I'd need more information, clarity about the history and building construction and who saw or did what when.

But in general,

In my OPINION I often think from inspection, testing, and experience that that is in at least some cases, limited problematic mold growth (that means it's not just cosmetic) trapped between tight-fitting wood surfaces such as between a stud and sheathing or joist and subflooring is unlikely to be a detectable problem in a building provided that

And remaining as well is the complex issue that in a building where there are particularly at-risk occupants (elderly, infant, asthmatic, immune impaired), there are extra reasons to be extra careful.

Finally, in concern for both building occupants and the contractors involved, I worry (based on some cases I've handled) that should there later be an illness or injury in the home that appears even remotely possibly related to the original "left in place" mold contamination, upset or angry parties looking for something or someone to blame may raise the issue again, at great cost and distress to everyone, regardless of the actual merits.
What this sometimes leads to is more thorough cleaning and remediation that in fact were justified.

What this also sometimes leads to is the OPM problem: spending other people's money to reduce your risk by taking extreme steps that are not obviously cost-justified.


Reader Follow-Up:

Excerpts from private communication:

The wood floor was not installed in a basement, but the concrete floor of a one story building. The house has drainage problem and had previous mold issues. The house was built in 1989 and is of concrete block structure.

The previous mold issues were found in the rooms that had one or more exterior concrete block wall. During my investigation, I was told that the mold spores are more likely to get pulled down by gravity than going up in the air. If this is true then the mold growth under the wood floor /subfloor can be explained. The mold spores trapped under the sub-floor started to grow when the slab got wet during the rainy season. The re-growth of mold happened in the areas they previously had mold and was remediated. However, the source of water intrusion has been poor drainage and lack of sufficient down-spouts around the house. A/C drain pipe close to the foundation and roof leak through a chimney have been noticed as well.

The lack of moisture reading of the slab did not cause the mold to grow. Though, having done one and documenting the results would have cleared us from all allegations. As I said before, I have no doubt that the slab was reasonably dried (it was not a new slab) and there was no sign of fresh stains (water intrusions) when the wood floor was installed. Unfortunately, the installer did not follow the protocol of the industry and relied on his visual inspection. And because of his failure I willingly accepted the fault and paid the owners back the contract price plus additional charges they requested. However, when their mold issue resurfaced, they backed up and blamed me for the mold growth.

Just for your information, there had been a change in the temperature and humidity inside due to inadequate A/C running after the floor installation.

I’m trying to gather as much information as I can in order to fight them. I have an attorney and we have strong supporting evidence and witnesses for our case.

The reason I wanted to know what is the possibility of mold being airborne from under the wood floor (contained mold spores) is to have an expert opinion in hand. I did not see any comment/opinion to this question. Nevertheless, you said that if the area dries out, further mold growth would diminish or maybe even stop in that particular area. You also said if the source of mold growth is not fixed or if an extensive mold contamination reservoir is left in place, depending on mold species, it may resume activity more quickly and more severely than if it had been removed previously. The identified mold was Aspergillus/Penicillium in this case. Can you please give me your opinion on the first question?

I welcome any offer/comments coming from other readers on this matter and thank them in advance.


OK so we're talking about a slab on grade? in any event the moisture concerns remain about the same, except that without a basement or crawl area the water intrusion risk into the floor or subfloor structure would be reduced. Not eliminated, reduced.

Concrete block can be itself a source of condensation, moisture, leaks, depending on how the walls were constructed, flashed, sealed, coated.


The mold spores trapped under the sub-floor started to grow when the slab got wet during the rainy season. The re-growth of mold happened in the areas they previously had mold and was remediated.

Even a perfect remediation job (of which I'm doubtful in this case if moldy materials were left in place), cannot make a building proof against future mold growth unless the job includes addressing the causes of mold growth in the building -leaks and moisture.

What I say to remediators is that for them, a lot depends on the expertise and scope and documentation of the final clearance inspection. If the work was properly performed then if there is a mold problem later in the building one ought to be able to assert that it's a new issue, not a left-over from the previous one. But in my own inspections, if I saw even a very good cleanup but if at the same time was in doubt about the adequacy of steps to address leaks, condensation, water entry, high humidity, I'd issue a very strong warning that indoor mold problems are likely to recur.


The lack of moisture reading of the slab did not cause the mold to grow.

I think you may have mis-typed. Of course "lack of moisture" does not cause mold to grow. It's the presence of moisture not its lack that is a gating factor in indoor mold contamination issues.

But if someone built a floor over a slab without considering moisture movement up through the slab OR moisture from other sources that floor is at risk of future mold and moisture troubles.


However, when their mold issue resurfaced, they backed up and blamed me for the mold growth.

IF the mold growth in the building is traced to work performed by a contractor AND if that job should have included measures to prevent leaks or condensation problems then that contractor is likely to be held accountable.

What is more difficult is the argument about who is responsible for building leaks. If you are building a floor and the floor is damaged (buckling, cupping, mold contamination) from building leaks that occur from other building features outside the scope of your job ( a burst pipe, a roof leak, a wall or window leak) then it's a bit of a reach to blame the floor contractor.

BUT even a floor contractor (if that were the limit of your scope of work) who recognized moisture and leak problems at a building would be smart to warn the owner in writing of those issues and to explain how they would jeapordize the work being performed.


I was told that the mold spores are more likely to get pulled down by gravity than going up in the air.

that's correct for heavy spores that settle out of air and incorrect for light small spores that can remain airborne for a long time and follow rising air convection currents. I would be careful about assuming the accuracy of statements about mold movement, growth, or een remedy by anyone who lacks expertise in mycology and IAQ.


The reason I wanted to know what is the possibility of mold being airborne from under the wood floor (contained mold spores) is to have an expert opinion in hand.

Sorry if I was unclear. Mold spores might enter a building area from below a floor space - say plywood on sleepers on concrete, depending on a number of variables that I listed.

However proving that mold spores detected in air came from a specific indoor source would require more steps than just supposition. I could outline those in more detail but a general explanation is that we'd need to identify the genera and species detected in the building, to be able to argue that it's occurring at a higher level than incidental (that could have come in through a window), to match it to an existing, found, proven, documented mold reservoir in the building - thus to support a proximate cause argument.

And even if the floor structure is proven to be the source of an indoor mold contamination issue, which is possible, that does not adequately address the causation issue.

Put another way, the drywall contractor is not going to be succesfully sued in a claim that her drywall installed on a ceiling or wall was moldy if one shows that the mold occurred because of leaks into the building or its wall or ceiling cavities. The leaks caused the mold, not the drywall.

Building materials are for the most part not designed to be mold-proof. In fact at MOLD APPEARANCE on VARIOUS SURFACES - I have demonstrated that we can find mold growth in buildings on just about every imaginable surface.

OPM: Some Consultants Spend Other People's Money to Reduce Risk For the Consultants, not for the Clients

Extensive demolition for mold cleanup (C) Daniel FriedmanWith this as many topics, even smart people but with no actual field test data to support a view, construct mental models of what they think is happening (yet without a single fact nor shred of evidence) and give aggressive cleanup advice accordingly.

This approach is especially dangerous when the advice suffers what we call the "OPM" problem: that is, Ms. X, the consultant give advice that is safest for her (because the client cannot possibly complain in the future that her advice was not aggressive enough), because the safest (for her) advice does not cost her a dime - the client is the one who pays for the sometimes extreme measures that are suggested.

Consultants who offer OPM-based advice simply playing it safe and they are not earning their fee.

Our photograph (above left) shows extensive demolition performed to remove problem mold on floor framing and subflooring in an older home. In this case the combination of multiple layers of moldy subflooring and flooring combined with a cost comparison of alternative approaches indicated that it was actually less costly to remove the subfloor and clean the joists than to leave the subfloor in place. In other buildings and circumstances it is reasonable and more economical to leave the subflooring in place.

Readers should also see the general advice at WOOD FLOOR DAMAGE.


Continue reading at HIDDEN MOLD in OTHER PLACES or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.





Or see TRAPPED MOLD BETWEEN WOOD SURFACES for a discussion of the immobility of mold between wood framing and other surfaces.


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