Mold on Wood: Surface Mold, Stain or Rot, and What to Do

The pages ranking for this search are forums, a franchise blog and furniture sellers, and most start with vinegar. That skips the real question: is the fungus on the wood, staining it, or eating it? This page answers it from the USDA Forest Products Laboratory (FPL) Wood Handbook, EPA remediation guidance and an EPA registered product label. More is on our mold hub.

Five things that look like mold on wood

The FPL Wood Handbook chapter 14 sorts the fungi on wood into molds, stains and decay, and adds chemical stains because they look alike. Its 2010 edition calls fungi on wood and paint surfaces mildew. Only one of the five weakens the wood.

What you see	How it looks	Depth	Strength effect	Usual answer
Mold	Fuzzy or powdery growth, pale to black; green and yellow common	Surface on softwood; deeper in oak	Slight	Dry, HEPA vacuum, clean, sand
Mildew	Black deposits on wood or paint, often with green algae	Surface, or under a clear finish	None; cannot decay wood	Clean, refinish with a mildewcide
Sap stain	Blue, grey or brown streaks; radial wedges on the end grain	Into the sapwood	Slight	Keep where looks do not matter, or replace
Decay (rot)	Brown and cubed, or bleached and spongy; white or brown fans	Through the wood	Serious	Fix the water, replace the wood
Chemical or iron stain	Grey, brown or pink, or iron stain where iron meets tannins	In the wood	None	Oxalic acid lifts iron stain

Two traps sit in this table. White growth is not always mold: the FPL says fresh decay fungi can show as fan shaped patches or strands, usually white or brown. And colour is not a species guide; EPA notes molds come in many colours. See our black mold guide and mold vs mildew for those questions.

Depth is the test

The FPL Wood Handbook chapter 14 draws the line between mold and stain on the depth of the discolouration. On softwoods, surface growth often brushes or surfaces off. On large pored hardwoods such as oak, the wood beneath is commonly stained too deeply to surface off, and typical sap stain cannot be removed by surfacing at all.

Check	What you see	What it points to
Brush a small patch	Growth lifts, wood beneath looks normal	Surface mold on softwood
Plane or sand a test spot	Colour stays below the surface	Sap stain, or mold in oak
A drop or two of household bleach	Dark colour fades in 1 to 2 minutes, per the FPL	Mildew; no change suggests dirt or iron stain
Look with a 10x loupe	Threads when damp, egg shaped spores when dry	Mildew, not dirt
Probe with an awl or ice pick	Point sinks in easily	Decay, even under a sound surface
Look across the grain	Brown and cubed, or pale and spongy	Brown rot, or white rot

The probe matters most: the 2010 Wood Handbook says decay often occurs in the centre while the surface appears sound.

The moisture numbers

Moisture content is the weight of water as a percentage of the dry weight of the wood, per EPA moisture control guidance. The FPL and two lumber associations set the lines, and they agree closely.

Why humid air grows mold but rarely rot

Per the FPL Wood Handbook chapter 14, water vapour in humid air will not wet wood enough for significant decay, but it will permit some mold. Wood in still air settles at an equilibrium moisture content, and according to Table 4-2 of the 2010 handbook, wood at 70 F reaches 13.1% at 70% relative humidity and passes 20% only at about 90%. According to the Southern Forest Products Association, a humidity of 70 to 90% supports mold.

So the water source predicts the fungus: damp air grows mold, while a leak or ground contact pushes wood toward rot.

When it is rot

According to the FPL Wood Handbook chapter 14, decay cuts toughness first, then bending strength, then every strength property. In its laboratory tests toughness fell 6% to more than 50% by 1% weight loss, and at 10% weight loss, when decay is visible only under a microscope, most strength losses can be expected to exceed 50%, per the same chapter. Its rule: wood with visible decay may be assumed greatly reduced in all strength values.

Houses sit in the fast band all year, so moisture is the lever. The FPL notes that so called dry rot fungi carry water into wood through strands; they still need water.

Mold on new framing lumber

The FPL Wood Handbook chapter 14 explains the new construction problem: unseasoned stock piled in warm, humid weather may be visibly discoloured within 5 or 6 days, and lumber can be infected at any stage if it gets moist enough.

The Southern Forest Products Association says that, discolouration aside, mold has little effect on the structural integrity of framing lumber. The FPL adds the catch: mold and stain make wood more absorbent and wettable, and early decay can hide in stained areas. That is a reason to dry it, not to panic.

Step	What to do	Source
Read the grade stamp	S-DRY or KD is 19% maximum, per the WWPA; S-GRN is unseasoned	WWPA
Store it off the ground, covered	Vapour permeable cover, short site storage	SFPA
Clean what molded	Clean it and let it dry before use	SFPA
Meter before enclosure	Framing at 19% or less, per the SFPA; never enclose wet wood	SFPA; FPL
Sill plates on slabs	Pressure treated, with a membrane under the slab	FPL

According to the WWPA fact sheet on cleaning mold (2002), mold cannot grow on wood dried below 20%. What fails is the meter reading nobody takes.

Furniture against structural wood

A dresser and a floor joist can grow the same mold and need different answers: furniture needs conservation, framing needs drying, cleaning and proof that it is sound.

Question	Furniture and finished wood	Framing and subfloor
Moisture in service	6 to 10%, per the FPL	19% or less at enclosure, per the SFPA
Surface	Finished; mildew can grow under a clear coat (FPL)	Bare, so stain and mold reach the wood
Cleaning	Brush into a HEPA vacuum, then a wood cleaner	HEPA vacuum, damp wipe, sand or plane
Replace when	Decayed, or not worth restoring	Soft, crumbly or cubed
Who to call	Furniture restorer or conservator (EPA)	Structural engineer if integrity is in doubt (EPA)

The National Park Service describes mold cleaning on collections as drying, dislodging, lifting and vacuuming spores with a HEPA filter, brushing mold off wooden furniture while vacuuming, and per the NPS, collections should be kept below 68 F and 55% relative humidity. Our hardwood floor water damage guide covers floors.

Vacuum, wipe, sand or replace

EPA 402-K-01-001 lists wood surfaces as cleanable by wet vacuum, damp wipe and HEPA vacuum at every size, adding removal only above 100 square feet. It says to clean mold up, not just kill it, and not to paint or caulk moldy surfaces.

Method	Use it on	How	Source
Dry first	All wood	Dehumidifiers, gentle heat, fans; pry wet paneling off the wall	EPA Table 1
HEPA vacuum	Loose growth; final cleanup once dry	Bag contents and filter in sealed plastic	EPA Method 3
Damp wipe	Finished or sealed wood	Water and detergent; EPA specifies wood floor cleaner on wood	EPA Method 2
Sand or plane	Bare framing with colour left	Removes surface growth, not sap stain	FPL
Replace	Soft or cubed wood	Fix the moisture; dry lumber, or treated wood if wetting continues	FPL

Sanding makes dust, so pair it with HEPA vacuuming. EPA's Brief Guide to Mold, Moisture and Your Home puts the do it yourself line at about 10 square feet; our DIY mold remediation guide covers that size, and the mold remediation process covers containment above it.

What borates are for

The Bora-Care label, EPA Reg. No. 64405-1, as EPA accepted it on 3 June 2010, calls the product a termiticide, insecticide and fungicide concentrate, and its target list names brown rot, white rot and wood decay fungi. For mold, it tells users to mix in Mold-Care, a separately registered moldicide.

The label explains the chemistry: an inorganic borate salt, soluble in water, that lasts only if the wood is kept from rain, continuous water and ground contact. The 2010 Wood Handbook agrees that borates are highly leachable, belong above ground, and diffuse into wet wood, which suits remedial treatment.

Treatment	Labelled or advised for	Conditions	Source
Borate, 40% disodium octaborate tetrahydrate	Termites, beetles, carpenter ants, decay fungi, algae	Bare wood; lasts if kept from rain, standing water and soil	Bora-Care label, EPA Reg. No. 64405-1
Mold-Care moldicide	Mold, mixed with Bora-Care	Per its own label	EPA Reg. No. 6836-212-64405
Household bleach and powdered detergent	Mildew on finished exterior wood	1 part bleach to 3 to 5 parts water, per the FPL; never with ammonia	FPL chapter 16
Detergent and water	Mold on wood surfaces	Scrub, rinse, dry	EPA; SFPA

None of these replaces cleaning. EPA 402-K-01-001 says a biocide such as bleach is not recommended as a routine practice, and that background spores will not grow once the moisture problem is solved. Our bleach guide covers bleach on porous wood.

The decision path

Work down the list and stop at the first step that settles it.

1. Find the water. Leak, rain, condensation, ground contact or humid air. Nothing below lasts until it stops.
2. Meter the wood. Above 19 percent, dry it first. Near 30 percent, decay is possible, so probe.
3. Probe it. If an awl sinks in, or the wood is cubed, spongy or crumbly, it is rot: replace it, and call an engineer if it carries load. Skip to step 7.
4. Brush a patch. If growth lifts and the wood beneath is clean, it is surface mold: HEPA vacuum, damp wipe, dry.
5. Still coloured? Sand or plane a test spot on bare framing. Deep colour is stain, which barely touches strength: leave it where looks do not matter, or replace it.
6. Black on paint or varnish? Try the bleach drop. If it fades in a minute or two, it is mildew: clean and refinish with a mildewcide.
7. Verify and close up. Final HEPA vacuum, readings at 19 percent or less, then enclose or refinish. On bare wood out of the weather that has had rot, consider a borate, used as its label directs.

For contractors

Scope on wood turns on one distinction. Cleaning is a surface line item: HEPA vacuum, damp wipe, sand. Decay is carpentry: remove and replace, with the moisture repair named. The FPL Wood Handbook chapter 14 says that once the moisture source is gone, only weakened wood needs replacing with dry lumber; if it cannot be removed, use preservative treated or durable heartwood.

Record on site	Why it matters
Moisture readings by member, with a dry reference	Per the FPL Table 13-2 note, test at least 10% of each item, such as six of 60 joists
Probe results and photos of each decayed member	Decay turns cleaning into replacement
Square feet of affected surface	Sets the EPA Table 2 size band, PPE and containment
Moisture source and its repair	FPL: correct the condition that caused the damage
Product and EPA registration number	The label is the legal use direction

Two duties sit outside the cleaning. The Bora-Care label tells applicators to ask the state regulatory agency whether extra qualifications are required, and EPA notes some states restrict who applies biocides in schools. For roof sheathing, our attic mold removal cost guide covers how that scope is priced.

Questions people actually ask

Is mold on wood harmful to the wood?

Usually not by much. The USDA Forest Products Laboratory says mold and stain fungi affect strength only slightly, mainly toughness and appearance, though they make wood more absorbent. Decay fungi are different: they consume the wood, and visible decay means large strength loss. Health questions are outside this page; see the note near the top.

Does mold on wood ever go away?

Not by itself. Drying stops growth, and the FPL says decay will not progress in wood kept air dry, but the growth and staining stay until they are cleaned off. EPA adds that if you clean up mold without fixing the water problem, it will most likely come back.

What kills mold on wood?

EPA’s aim is removal, not killing: dry the wood, HEPA vacuum it, damp wipe it with detergent or a wood cleaner, and sand bare framing if needed. EPA does not recommend biocides such as bleach as a routine practice. Borates are labelled for decay fungi; the Bora-Care label sends mold to a separate registered product.

What does toxic mold look like on wood?

You cannot tell by looking. EPA notes molds come in many colours, including white and black, and its cleanup methods are set by material and area, not colour. What you can judge on wood is depth: growth that brushes off is surface mold, deep streaks are stain, and soft or cubed wood is decay.

What is the white mold on wood?

It may be mold, or it may be decay. The FPL says fresh decay fungi can appear as fan shaped patches, strands or root like structures, usually white or brown. Probe the wood beneath with an awl: firm wood means surface mold, while a point that sinks in or spongy wood means rot.