Lumber Mold - Types, Causes, and Concerns

Written By Brantley May

Discolorations on Lumber: A Science-Based Guide for New Construction

Wood is a natural material with a long life before it reaches the job site. Along the way, it can develop many different types of discoloration—some biological, some chemical, some purely cosmetic. In new construction, these marks are often labeled as “lumber mold,” but not all discoloration is mold, not all fungi behave the same way, and not all growth represents an exposure risk. This guide explains the science behind these discolorations and how to interpret them during new construction.

1. Non-Biological Discolorations

Not every dark spot on lumber is biological. Several types of non-fungal discoloration are frequently mistaken for mold.

Dirt and Handling Marks

  • Dark or smudgy surface contamination.

  • Does not smear like mold when rubbed.

  • Often removable with light cleaning.

Iron Stain

  • Blue-black or grey discoloration.

  • Caused when iron particles react with tannins in wood.

  • Can originate from nails, saw blades, metal contact, or contaminated water.

  • Lightens with oxalic acid, confirming non-biological origin.

Sunlight and Weathering

  • UV exposure darkens, yellows, or greys the surface.

  • Does not indicate fungal activity.

Natural Heartwood/Sapwood Variation

  • Natural color differences within the tree can look like staining but are not biological.

Some discolorations cannot be perfectly diagnosed visually. Even experts sometimes require microscopic examination, but pattern recognition and texture differences are often sufficient to a professional eye.

2. Fungi and Wood: What Can Grow on Lumber?

Fungi require moisture, a food source, and suitable temperatures. And here’s the key point many people misunderstand:

Believe it or not, some lumber companies will tell you that “wood is inedible for most fungi,” and that’s actually true.. but this is an oversimplification.

A more accurate description is:

Wood is more difficult for fungi to structurally decompose than paper-based products, but many fungal species can still colonize and grow on wood surfaces under sufficiently moist conditions.

This is because:

  • Wood contains cellulose, hemicellulose, extractives, and other organic compounds fungi can use.

  • Only specialized decay fungi can break down lignin and cause structural deterioration, but many molds and staining fungi can grow on wood without degrading its structural fibers.

  • Surface growth does not require breakdown of lignin.

In practice, mold growth on damp framing lumber is common, especially in humid climates or during construction delays.

3. What People Call “Lumber Mold”

“Lumber mold” is an informal term that can refer to several distinct biological conditions.

Surface Molds

Examples: Cladosporium, Penicillium, Aspergillus, Trichoderma, Aureobasidium

  • Grow on the surface of lumber.

  • Require moisture levels typically above 16–20% or humidity levels consistently above 75%.

  • Appear fuzzy, powdery, smudgy, or spotty.

  • Colors include black, green, white, grey, or brown.

  • Do not structurally damage wood but indicate moisture exposure.

Stain Fungi (Bluestain/Sapstain)

Examples: Ophiostoma, Ceratocystis

  • Cause blue, grey, or black streaks inside the wood.

  • Often originate in the forest or sawmill, not on the job site.

  • Do not weaken structural fibers.

  • Typically inactive and not an exposure risk after kiln drying.

Decay Fungi

Examples: Serpula, brown-rot and white-rot fungi. These are often in the phylum Basidiomycota.

  • Require long-term saturation.

  • Produce soft, crumbly, or cubically cracking wood.

  • Often create white, cottony or fan-like growth. Often seen looking like long skinny roots branching across the surface of the wood

  • Represent a true structural issue and must be addressed immediately.

4. Is “Lumber Mold” Dangerous?

Stain Fungi (Bluestain/Sapstain)

  • Not active mold.

  • Not airborne in meaningful quantities.

  • Not associated with health effects.

  • Do not indicate current moisture problems.

  • Do not require remediation.

Surface Mold

  • Indicates past or current unusual moisture conditions.

  • Can release spores or fragments, especially when disturbed.

  • Should be cleaned before walls are closed for building hygiene

  • Tiny amounts likely indicate minimal exposure. However, unusual amounts should not be ignored.

Decay Fungi

  • Structural concern rather than an indoor air concern.

  • Requires immediate assessment and replacement of affected wood.

  • Note: Decay fungi are a structural issue, not primarily an exposure risk. However, keep in mind that if the moisture levels were bad enough to cause decaying fungi, mold and bacteria are also likely present

5. Should Lumber Be Cleaned Before Enclosure?

Visible surface growth on framing should be cleaned before continuing phase of construction, especially drywall installation. This is because:

  • It greatly reduces the risk of exponential mold issues in the wall if incidental water were ever a factor down the line (leaking pipes, etc.)

  • It prevents sealing mold into wall cavities that can become an exposure risk.

  • It avoids odors or future questions about fungal contamination.

  • It acts as a touch point to ensure moisture levels are below 16-17% so that moisture is not trapped in walls, worsening issues

What about mold on the attic framing? Does it matter?

Yes. As buildings becoming tighter, we are seeing air exchange between the home and attic more than ever. This sounds counterintuitive. However, this is because the building envelope continues to increase in air tightness but the ceiling of the home that separates the attic from the living space tends to be the least-sealed area.

There are many recessed lights, fixtures, top plates, etc. that are poorly sealed. In addition, mechanical systems in the home (such as exhaust fans, range hoods, and return vents) can put the home under a negative pressure, causing air to infiltrate from the attic spaces, all leading to a higher exposure risk for the occupants in the home.

Issues with mold growth on framing are especially concerning when the attic is a vent-less system, such as spray foam attics. This is because whatever is inside the attic is inside the boundary between indoors and outdoors and technically ‘inside’ of the home, increasing air exchange with the breathing space.

6. How to Tell the Difference Between Lumber Fungi and Mold of Concern

Bluestain/Sapstain (Not a Concern)

  • Inside the wood, not on the surface.

  • Streaky, follows the grain.

  • Not fuzzy or powdery.

  • Does not smear when rubbed.

Surface Mold (Should Be Cleaned)

  • On the surface only.

  • Fuzzy, spotty, or smudgy.

  • Can smear when wiped.

  • Indicates moisture conditions during construction.

Decay Fungi (Structural Concern)

  • Soft, spongy, or crumbly wood.

  • Cottony or fan-like mycelium.

  • Requires evaluation and replacement.

When in doubt, surface testing should be performed in order to verify the type of fungi that exists.

7. Is “Lumber Mold” an Exposure Risk If It Came From the Tree?

Stain fungi originating in the tree:

  • Are not active indoors.

  • Do not release problematic spores in living spaces.

  • Are not linked to indoor air quality concerns.

  • Are simply natural discolorations.

Surface mold that grew during storage or construction:

  • Could contribute to exposure, especially if disturbed.

  • Is simple to clean and manage before enclosure.

  • Should not be sealed into walls while active.

8. Moisture: The Most Important Factor

Moisture—not the wood, not the species, not the job site—determines whether fungi can grow. Dry wood prevents:

  • Surface mold from growing

  • Decay fungi from initiating

  • IAQ concerns related to mold

Moisture management is the foundation of mold prevention in new construction.

10. Resources

Building Science & Moisture Control

  • Building Science Corporation: https://buildingscience.com

  • ASHRAE Moisture Management Guidelines

Mycology & Fungal Behavior

  • Flannigan, B., Samson, R.A., Miller, J.D. Microorganisms in Home and Indoor Work Environments.

  • Viitanen, H. (1997). Mold Growth on Wood in Relation to Humidity and Temperature.

Public Health

  • World Health Organization (WHO). Guidelines for Indoor Air Quality: Dampness and Mould.

  • U.S. Environmental Protection Agency (EPA). Mold Resources: https://www.epa.gov/mold

Wood-Specific Biology

  • Canadian Wood Council – Bluestain Information

  • Zabel & Morrell. Wood Microbiology: Decay and Its Prevention.

Brantley May

Brantley is a Building Scientist and Council-Certified Indoor Environmental Consultant with a specialty in Mycology and certified by the McCrone Research Institute and the Oshner Medical Center for the examination and identification of fungal spores and pollen via microscopy.

Brantley is certified by the Building Performance Institute as a Building Analyst, Building Envelope Professional, and Infiltration and Duct Leakage Expert. Brantley also holds several certifications related to the identification of Mycotoxins, Water Damage, Odor Control, Structural Drying, Infectious Disease Control, and Energy efficiency issues and is a student of Mechanical Engineering.

He is also a state-licensed mold assessor who carries the credentials for teaching and proctoring exams for other professionals to receive their state accreditations and certifications in mold and environmental-related services.

Next

Pressure Imbalance: The One Thing Your Mold Inspector Might Be Missing