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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.

Cleaning mold from wood framing, or wood building sheathing and similar wood surfaces is discussed at MOLD CLEANUP - WOOD FRAMING & PLYWOOD. Also see MOLD CLEANUP - WOOD FLOORING where we describe removing mold from wood flooring surfaces. Readers should also see the general advice at MOLD CLEANUP GUIDE- HOW TO GET RID OF MOLD and finally, see Wood Floor Damage. Also see Hidden Mold in Flooring & Subflooring.

Also see TRAPPED MOLD BETWEEN WOOD SURFACES for a discussion of the immobility of mold between wood framing and other surfaces, and see MOLD GROWTH on SURFACES for an index of what mold genera/species are frequently found on various building surfaces and materials.

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1. CLEAN MOLDY SURFACE - How to remove mold from wood framing, plywood sheathing, tongue and groove subfloor or roof decking

The 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. 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. The first photograph above shows mold growing on wood framing lumber and even on electrical wiring insulating jackets in a wet basement before these surfaces had been cleaned by the mold remediation contractor. The second photo (above right) 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. 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 were needed because of the mishandling of containment, despite good cleaning work.

Guide to Cleaning Mold from Smooth Wood Surfaces

Perfectly adequate 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. See our warnings below about using bleach. 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.

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).

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 by Media Blasting - A test report (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 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.

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

Just below,and in more detail at MOLD CLEANUP - WOOD FLOORING we describe cleaning mold off of wood flooring products that have not yet been installed in a building. Also see MOLD CLEANUP - WOOD FRAMING & PLYWOOD where we describe methods for cleaning wood surfaces in general.

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.

MOLD SPRAY SOURCES & TYPES lists some sealant products and FUNGICIDAL SPRAY & SEALANT USE GUIDE discusses warnings about using sealants on mold cleanup jobs (don't use a sealant as a substitute for actual cleaning).

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.

More Reading:

MOLD CLEANUP - WOOD FLOORING - details about cleaning moldy wood flooring
Mold Levels: allergenic or toxic mold: how much means a problem
Mold Removal by Media Blasting - A test report (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

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MOLD CLEANUP - WOOD FRAMING & PLYWOOD
  CLEAN MOLDY SURFACES
  CLEANING SMOOTH WOOD
  CLEANING IRREGULAR SURFACES - MEDIA BLASTING
  CLEANING MOLDY WOOD TRUSSES
  SANDING WOOD TO REMOVE MOLD
  DRY THE MOLD-CLEANED SURFACE
MOLD CLEARANCE INSPECTIONS