Pre-Cleanup moldy basement framing How to Clean Mold off of Lumber or Plywood
Effective Cleaning of Mold-Contaminated Wood Building Materials

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Best procedures for cleaning mold-contaminated wood:

This article offers advice on cleaning mold found on surfaces of un-finished wooden building materials such as framing lumber (rafters, floor joists, wall studs), and building roof, wall, and floor sheathing such as plywood, tongue-and groove pine boards, and other structural wood surfaces in buildings.

We discuss the pros and cons of using fungicidal sealants and bleach on wood surfaces and give sources and list types of those products. We also discuss common errors made when cleaning wood surfaces, such as relying on bleach or performing expensive and unnecessary cleaning on cosmetic black mold on wood surfaces.

We include photographs of surfaces which have been cleaned during a good mold remediation project, and we provide photos of the effects of use of fungicidal sealants as encapsulants, particle immobilizers, and clear (or pigmented) surface sealants.

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How to remove mold from wood framing, plywood sheathing, tongue and groove subfloor or roof decking

Post-Cleanup using a baking soda spray processThe object in cleaning mold from a wood surface is to remove all fungal material from the surface of the wood: fungal spores, hyphae, and conidiophores (the spore producing structures of mold). It is not necessary to remove all stains from wood.

Here we provide a guide to cleaning mold from smooth wood surfaces. We describe how to remove mold from irregular or hard-to-reach wood surfaces - media blasting and how to clean mold from wood trusses and cross-bracing. We discuss when to try sanding wood surfaces to clean or remove mold contamination. And we answer these questions:

How clean do moldy surfaces need to be? How do I clean mold off of wood flooring, subflooring, joists, cross bracing, or trusses?

Article Contents

moldy framing exposed in living spaceOnce we have physically removed mold from a building surface by wiping or vacuuming, stains may remain. Don't panic about those marks. Microscopic examination of stained wood fragments will generally show that what remains in these wood cells are sterile fungal hyphae.

If the wood surface is cleaned thoroughly and building leaks, high moisture, or other water sources are prevented, keeping the humidity at the proper level, growth of problem mold on the cleaned surfaces is quite unlikely.

Our first photograph of mold on wood framing under a floor (above / left) as well as our page top photograph both show mold growing on wood framing lumber, subflooring, and even on electrical wiring insulating jackets in a wet basement before these surfaces had been cleaned by the mold remediation contractor.

Our second photo (below) shows wood framing and subfloor that have been adequately cleaned, without any application of a fungicidal sealant.

At this inspection we found that although the mold cleanup looked complete to the naked eye, the remediation containment barriers had been removed before a successful mold remediation clearance inspection and test.

Testing in the crawlspace below the opened floor we found over 120,000 P/A spores/M3 - a rather high level of airborne mold that was identified as principally Penicillium or Aspergillus mold spores.

Technical note on Pen/Asp: When we have just the mold spores alone, as is common in a "mold air sample test", it can be difficult to determine which of these two mold genera is present so they're reported together as Pen/Asp or P/A.

The reasonableness of this reporting approach stems from the observation that both of these mold families produce many individual mold species that are particularly-harmful to humans (and other animals) because of their very small size - they're breathed deeply into the lungs and because the spores are often allergenic, pathogenic, toxic.

In the adjoining basement we found 7000/Pen/Asp spores per M3 of air. The rest of the house interior was low when tested immediately after containment removed. Our opinion was while cleaning had been well done, poor containment management meant that there was a high risk of recontamination.

Additional airing-out, air scrubbing, surface vacuuming, and re-testing in the adjoining basement were needed because of the mishandling of containment, despite good cleaning work.

Guide to Cleaning Mold from Smooth Wood Surfaces

Photograph of an extensive mold remediation project after demolition and cleaning of subflooring and floor joistsPerfectly adequate moldy-wood-surface cleaning may be accomplished by wiping or (where feasible) power-washing or media blasting. 

Where wiping a moldy surface, take care not to spread moldy debris from a moldy surface onto a previously uncontaminated surface by making the mistake of re-using the same moldy rag over and over on all surfaces. Professionals use "steri-wiping" which takes care to avoid spreading moldy debris by always folding and using a clean side of the wipe when moving to a new spot.

Where the framing lumber is indoors or otherwise in a location where water spillage is a concern, wipe the areas of heaviest mold to remove any loose mold from the surface of the lumber.

Unless professional area-containment has been set up (barriers, negative air), do not use violent cleaning methods such as power-washing or sandblasting indoors, as you will spread moldy debris throughout the building and you'll increase the ultimate project cleanup cost.

Where the framing lumber is outdoors where water spillage and the creation of aerosolized mold spores is not an issue, pressure wash the infected lumber to remove surface mold.

"Cleaning" in this case can be simply wiping with a sponge wet with water or detergent. 

The object of cleaning is to remove most of the loose moldy particles.

The object (except in medical facilities) is not to produce a particle-free sterile surface. However beware of cross-contamination. Wetting a rag and wiping a very moldy surface off is fine but if you then use the same dirty rag to wipe another fairly clean surface you may be in fact spreading moldy debris around.

A professional uses sterile wipes and folds to a clean side of the wipe for each wiping stroke. For a small homeowner non-critical project this may be overkill but think about and avoid spreading moldy debris by your cleaning procedure.

Watch out: An attempt to to "kill mold" on moldy surfaces is not normally a proper nor sufficient approach to a moldy building. You need to remove the mold (cleaning) and correct the cause of its occurrence (fix leaks or moisture). Even if you could "kill" all of the mold organisms (which is doubtful) you may be leaving dead but still toxic or allergenic particles in the building.

See our detailed warnings about relying on bleach for "mold remediation" in MOLD CLEANUP, BLEACH

Cleaning mold from irregular or hard-to-reach wood surfaces - media blasting

If mold needs to be removed from a roof deck through which roofing nails penetrate, hand wiping is not feasible. Power cleaning using spray equipment such as media blasting using baking soda or dry ice is very effective for these surfaces. See our media blasting article at MOLD CLEANUP - MEDIA BLASTING - live link is given just below.

However spraying anything in an attic creates a secondary problem: contamination of building insulation. Our experience is that if attic conditions have been wet enough to produce problem mold justifying a professional cleaning of those surfaces, the insulation is probably contaminated and needs to be replaced.

Unless a building is being totally gutted to its framing, power-washing with water or any other liquid is problematic in a building attic where the procedure risks leaks into and wetting contents of the building below the work area. Dry-process spray cleaning works well in these areas. Typical high pressure sprays use baking soda or frozen C02, followed by HEPA vacuuming of the work area and probably of other building areas.

More Reading on methods for cleaning mold from difficult to access areas or removing mold from irregular building surfaces:

MOLD REMOVAL, MEDIA BLASTING (complete article, with illustrations) on the effectiveness of baking soda media blasting for cleaning fungal contamination in buildings, Daniel Friedman, Dennis Melandro, originally published in Indoor Environment Connections, Rockville MD, June 2003

Cleaning mold from wood trusses and cross-bracing

In an area of high levels of mold growth or moldy dust and debris, the irregular surfaces formed by wood trusses and also by older wood cross bracing between floor joists prevents thorough cleaning of surfaces and creates many dust collection points.

A similar construction detail where significant moldy dust reservoirs may be left in place is the upper surface of wood furring which has been nailed across the under-side of floor joists to support (now removed) ceiling tiles.

Because the surfaces formed by trusses and cross bracing can form a significant dust and debris reservoir, I always check these areas during a mold remediation clearance inspection. If work has been hasty or incomplete, these are among the first areas to be under-cleaned.

Spray process cleaning, media blasting (live link given above), and HEPA vacuuming are effective for these areas.

Sanding wood surfaces to "clean" mold

We often see remediators attempting to clean up mold by sanding surfaces, by hand or by power sander.

Sanding wood surfaces to remove mold is physically possible for smooth surfaces but in our opinion this is a slow, labor intensive procedure which is impractical for any large area cleanup. Spray processes are significantly faster and more thorough.

Sanding wood building surfaces by hand is

  1. probably completely unnecessary and
  2. may indicate inexperience or a response to an improperly informed and frightened building owner.

Clean the surface mold, dry the building, and if you like, use a sealant as discussed below. The fungal material left inside of wood framing or sheathing and which forms visible stains is not going to affect building occupants provided the building is kept properly dry and free of leaks. Even if you removed all of the stain by deep sanding, future building leaks will still \ produce new mold growth, so sanding is in most cases a wasted effort.

An exception we make to this general advice is where exposed beams are cleaned or sanded for cosmetic reasons. Still in that case media blasting cleans better and faster.

Just How Clean do Moldy Surfaces Need to Be?

Does the building need to be sterile? Is the object to reach a mold spore count level of zero?

No, the building does not need to be sterile, nor should you seek a "zero mold count". We  have worked on cases where sterility was a necessary cleanup goal: medical facilities such as in operating rooms and treatment rooms. But in a normal office or home there is always some airborne mold along with lots of other airborne particles in building dust.

No washing, sanding, scraping, or other surface cleaning will remove all mold spores from wood where mold was previously found. It is unlikely that most construction materials, even when new, are free of mold spores, nor is "zero mold" a reasonable nor possible objective.

Cleaning moldy framing lumber followed by application of a sealant may be the most cost effective alternative (where removal of the lumber is cost-prohibitive or otherwise not possible). While lumber replacement with apparently "clean" new lumber may sound appealing, it is likely to be cost prohibitive and in fact may include its own mold when it is unloaded at the work site.

How Much Mold Must Be Removed - How Clean Do Surfaces Need to Be?

Following a properly executed mold cleanup, if there is a future mold problem in a building it is unlikely to be due to having left behind an "inoculation" of problem mold, and more likely to be due to a new building leak that was left unattended.

Good practice for the extent of and means of physical removal of moldy debris varies by material.

For drywall we remove all visibly moldy material and continue removing drywall to no less than the next adjacent stud, rafter, or joist.

For fiberglass or other porous building insulation which has been wet, we remove all suspect insulation and all insulation within 24" of the suspect material. However if moldy and wet conditions were long-standing in a building, removal of all of the insulation may be necessary.

For building insulation that has been exposed to dusty conditions or high levels of airborne mold it is often more cost effective to simply remove the material, clean the surfaces, and re insulate than to spend that same money on testing the insulation for mold.

Generally it is less costly to remove and discard more material than to "finish" the job and then discover that it needs to be done over again because the initial work was insufficient.

Framing lumber, or roof or wall sheathing that is not rotted does not need to be replaced. Unless framing lumber has been actually damaged, such as by rot, replacing it due to mold contamination is not justified and would be improper.

Physically clean moldy surfaces of the framing lumber and exposed roof or wall sheathing. No you do not normally need to demolish the roof or wall to treat the small remaining areas between the narrow edge of a rafter or wall stud and the roof or wall sheathing that is nailed against it.

Rotted moldy wood (C) Daniel Friedman

Our photo above shows wood supporting a basement stair that is surely rotted. We would remove and replace material like this rather than trying to clean it.

As with this stairway, there are other cases where it is less costly to replace a building material than to clean it.

Depending on the materials of which they were constructed, the cost to clean and re-seal the shelves in this moldy kitchen pantry may be greater than the cost of discarding and replacing the shelving.

Kitchen pantry mold (C) Daniel Friedman

When we notice that the drywall on the pantry walls is also moldy we understand that the shelving has to be removed in order to remove the moldy drywall. We do not clean moldy drywall. It should be removed and the exposed framing surfaces cleaned.

Pantry gutting and reconstruction are a a more cost-effective approach to this particular mold cleanup project than any surface cleaning attempt.

Cleaning Mold off of Wood Flooring

Details about cleaning moldy wood flooring both when installed in buildings and when the flooring product has not yet been installed are discussed at MOLD CLEANUP - WOOD FLOORING. Excerpts are just below.

Physically Clean Moldy Wood Surfaces, Dry The Wood Flooring Before Installation

Here we describe cleaning mold off of wood flooring products that have not yet been installed in a building.

Except where major costs are at issue that would be effected by a determination of the type of material or mold present, or where there are other reasons to test for mold, in our opinion testing is not necessary for small mold cleanup jobs (less than 30 sq. ft. of contiguous mold on a building surface).

In any case you can physically clean the surfaces to remove the mold. Physically cleaning means wiping, scrubbing with a scrubby sponge and any household cleaner. Don't waste money or time with mold killing washes, it's not necessary, and using bleach or similar agents can create a cosmetic problem or a problem with future adhesion of finish coatings on the flooring upper surface after installation.

Watch out: be sure that your wood flooring has dried properly before it is installed or flooring shrinkage, gaps, or even more serious problems may occur. The wood should be below 18% moisture before any coatings are applied, and it should be thoroughly acclimated to the building interior where it is to be installed before it is secured in place. This can mean storing the wood in the destination building for days or longer before it is installed.

If an exposed flooring surface remains stained even after surface mold has been removed, you will need to sand that surface - a step typically performed after the flooring has been installed.

Stains that might remain on the flooring underside will be of no cosmetic import and as long as the floor is installed indoors and not exposed to water or high moisture, mold growth should not be a problem.

If nonetheless you want to take steps for extra "mold proofing" you can, after cleaning and drying the wood, coat the underside with a fungicidal sealant, or even with simple quick dry shellac or a lacquer primer-sealer paint. The top flooring surface will be finished and sealed after installation unless you are dealing with a pre-finished flooring product.

If the exposed (upper) surface of the flooring material is moldy and if mold stains have penetrated the actual coating, for cosmetic reasons you'd need to sand through the coating and through the stain until the wood appearance is satisfactory.

Watch out: often mold-stains penetrate rather deeply into wood materials. While the stain does not itself signify an increased risk of future mold re-growth, its appearance may be unacceptable. But deep stains can require removal of quite a bit of wood surface - something that can be a problem in wood flooring, and in particular if the flooring is a Vee-grooved pre-finished product.


Reader Question: how should we clean moldy wood framing in our new home

We are building a new home, the wood framing has mold, the walls are still open. How should the builder clean or remove the mold? - Linda Lewis 4/7/2013


Thanks for the question Linda.

If the mold is ONLY the cosmetic black mold that we describe at BLACK COSMETIC MOLD then it really is only cosmetic and could be left in place.

There are two difficulties with this:

So we might want to clean the lumber.

I tend to stay away from labor intensive approaches (hand sanding) and from incomplete approaches (wiping off). Keep us posted and send along some photos if you can (by email to the page top or bottom CONTACT link) as what you learn may help others.

This article series includes advice on cleaning mold found on surfaces of un-finished wooden building materials such as framing lumber (rafters, floor joists, wall studs), and building roof, wall, and floor sheathing such as plywood, tongue-and groove pine boards, and other structural wood surfaces in buildings.

We discuss the pros and cons of using fungicidal sealants and bleach on wood surfaces and give sources and list types of those products. We also discuss common errors made when cleaning wood surfaces, such as relying on bleach or performing expensive and unnecessary cleaning on cosmetic black mold on wood surfaces.

Reader Question: is it OK to apply a sealer or wood preservative over black stains on plywood roof sheathing?

Last September I had a new garage roof fitted and within 7 months the plywood used inside developed black mould spots. The company that fitted the roof said it was caused by condensation. Having wiped the roof with a mild detergent I was left with black stains on the plywood. I would like to know if it would be ok to put a wood preservative on it despite the stains - don't want it to come back.

Would appreciate your advise? - [Anon by private email 23 July 2015]


Yes you can apply a wood preservative, leaving cosmetic black stains under the preservative.

But if you do not find and fix the cause of condensation (removing a water source and/or improving ventilation) I'd still expect future trouble. You may find stains or mold appearing on other wood or mold-friendly surfaces in the same area if indoor moisture levels are high, or if there is building insulation present that may become mold contaminated.




Continue reading at MOLD CLEANUP - WOOD FLOORING or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see MOLD CLEANUP of WOOD, FAQs - questions about how to clean off moldy wood, posted originally at this article

Or see BLACK COSMETIC MOLD - not all black mold is harmful

Or see FUNGICIDAL SPRAY & SEALANT USE GUIDE - sprays and sealants used after proper cleaning reduce further risks

Or see MOLD CLEANUP, BLEACH - not an adequate mold remediation approach

Or see MOLD CLEANUP - WOOD FLOORING - how to remove mold from wood flooring surfaces

Or see MOLD REMOVAL, MEDIA BLASTING - good approach for irregular or hard-to-clean surfaces

Or see SIDING, WOOD CLEANERS, STAINS, PAINTS how to remove stains on siding & how to choose the proper paint or stain coating

Suggested citation for this web page

MOLD CLEANUP - WOOD FRAMING & PLYWOOD at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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