How do you tell mold from mildew on wood?

Sand a small test patch with fine paper. Mildew is a surface film — it sits on the finish or the top fibers and sands away in a few strokes, leaving clean wood. Mold has grown into the grain, so light sanding removes the fuzz but leaves a dark stain following the grain lines. If the discoloration goes deeper than about a millimetre, you are past mildew.

Is black staining on wood always mold?

No, and on wood there are three common alternatives worth ruling out. Iron staining (tannin reacting with a wet fastener or fitting) makes blue-black marks radiating from a nail or screw. Weathering greys exposed timber evenly across the whole surface rather than in patches. And wood rot is a fungus but a different kind — it consumes the structure of the timber rather than living on it, and it matters far more than mold does.

How do I know if it is mold or wood rot?

Push a screwdriver into the timber with moderate pressure. Sound wood resists. Rotten wood gives way, feels spongy or crumbles, and the blade can often go in half an inch or more. Rot also changes the wood itself — it darkens and cracks in a cube pattern (brown rot) or turns pale, soft and stringy (white rot). Mold discolours the surface; rot destroys the material. If a screwdriver sinks in, stop worrying about species and get the structure looked at.

Does mold on wood framing mean structural damage?

Not by itself. Mold feeds on surface residues and the wood's outer fibers, not on its structural fiber, so a mold-stained joist can still be perfectly sound. What it does tell you is that the timber has been wet for a long time — and prolonged wetness is what causes rot. So treat mold on framing as a reliable indicator that you should test for rot nearby, rather than as damage in itself.

Can you clean mold off wood, or does the wood need replacing?

Finished wood — sealed trim, painted sills, varnished furniture — usually cleans up, because growth cannot get past the finish. Bare wood is porous, so surface growth sands off but staining that has entered the grain often will not come out completely, and that residual stain is cosmetic rather than dangerous once the wood is dry. Replacement is about structure, not staining: you replace timber that has lost strength, not timber that looks marked.

What is the white fuzzy growth on wood in my crawl space?

Most often a Penicillium or Aspergillus species, which look cottony or powdery white and thrive in the still, humid air under a floor. Two things to rule out first. If it crumbles to a chalky powder between your fingers and dissolves in water, it is efflorescence — mineral salt, not growth. And if the wood beneath is soft, white stringy growth can be white rot, which is a structural matter. Persistent crawl space growth is usually a missing or torn ground vapor barrier.

Black Mold vs Mildew on Wood: How to Tell Them Apart (and When It Is Rot)

Wood is the surface where getting this right matters most, because it is the only common one where a dark patch might mean the material itself is failing rather than needing a clean.

There are actually three things that look broadly similar on timber, and they carry completely different consequences: surface mildew, mold that has stained the grain, and wood rot. Sorting out which you have takes about two minutes and one screwdriver.

Start Here: Rule Out Rot First

Do this before worrying about species, because it is the only one of the three that affects whether the building is safe.

The screwdriver test. Press a screwdriver into the timber with moderate pressure. Sound wood resists and the tip barely marks it. Rotten wood gives way — spongy, crumbly, or the blade sinks half an inch with little effort.

Rot also changes the wood rather than just its color:

Brown rot darkens the timber and cracks it into a cube-like pattern across and along the grain. Dried out, it crumbles to powder.
White rot bleaches the wood pale and leaves it soft, fibrous and stringy.

Both mean the timber has lost strength. If a screwdriver sinks in, the species of anything growing on the surface has stopped being the interesting question — that is a structural conversation.

Mold and mildew, by contrast, do not consume structural fiber. They live on the surface and in the outermost layer. A mold-stained joist can be entirely sound.

Mildew vs Mold on Wood

Once rot is ruled out, the distinction is about depth.

	Mildew	Mold in the grain
Appearance	Flat powdery film, gray or white	Darker patches, fuzzy or slimy, irregular edges
Where it sits	On the finish or the top fibers	Into the grain, following the grain lines
Sanding test	Sands off in a few strokes, wood clean underneath	Fuzz comes off, dark stain remains
Typical location	Finished trim, sills, furniture, sheds	Bare framing, subfloor, sheathing, sill plates
Moisture behind it	Humidity or condensation	Sustained wetting — days or longer
After cleaning	Stays gone once dry	Stain persists; growth returns if still damp

The sanding test is the wood equivalent of the wipe test. Fine paper, a small inconspicuous patch, a few strokes. Clean wood underneath means you were dealing with a surface film. A dark stain that follows the grain means growth got into the material — which is not dangerous in itself once dry, but does tell you the wood was wet for a while.

Finished Wood vs Bare Wood

This matters more than most guides admit, and it explains most of the confusion.

Finished wood — painted sills, varnished furniture, sealed trim — has a barrier. Growth cannot get into the fiber, so almost anything on it is surface mildew and almost all of it cleans off. The exception is where the finish has cracked or worn through, which is usually at the joints and the lower edges of window sills.

Bare wood is porous and absorbent. Framing, subfloor, roof sheathing, sill plates, fence timber and the undersides of decks have no barrier at all, so growth establishes in the fiber and staining is often permanent. This is where the diagnosis is worth taking seriously — not because the stain is harmful, but because bare wood that stayed wet long enough to grow something is bare wood that may be starting to rot.

Where It Shows Up on Wood, and What It Means

Window sills and trim. Condensation running off the glass pools at the sill and soaks in at the joint. Usually mildew on the paint, sometimes staining in the wood beneath a failed finish. A humidity and ventilation problem.

Sill plates, where the framing meets the foundation. One of the most consequential locations in a house and one of the least often inspected. Growth here means groundwater or a foundation moisture problem, and this is where rot most commonly starts. Always do the screwdriver test.

Subfloor. Usually the aftermath of a spill, an appliance leak or a bathroom failure that was dried on top but not underneath. Check from below where you can, because the top surface can look fine.

Roof sheathing in the attic. Dark patches on the underside of the roof deck are usually a ventilation failure rather than a roof leak — most often a bathroom fan venting into the attic instead of outside. Warm wet air hits cold sheathing every winter and condenses. Cleaning the sheathing achieves nothing; redirecting the duct does.

Crawl space joists. Still, humid air and often bare soil below. White cottony growth here is typically Penicillium or Aspergillus, and the usual root cause is a missing or torn ground vapor barrier rather than any leak.

Furniture and stored items. Almost always surface mildew from humidity and poor airflow. Cleans up, and stays clean if the room dries out.

Three Things Mistaken for Mold on Wood

Iron staining. Wood tannins react with wet iron to produce blue-black marks radiating out from a nail, screw or bracket. Distinctive because the pattern centres on a fastener and does not spread beyond it.
Weathering. Exposed timber greys as UV breaks down the surface lignin. Even across the whole exposed face rather than in patches, and it does not smell.
Efflorescence, on wood adjacent to masonry. Chalky white deposits that crumble between the fingers and dissolve in water. It is mineral salt carried out of concrete or brick, not growth.

Cleaning Wood

For surface growth on wood you have identified as sound:

Gloves and an N95, and ventilate the space.
Vacuum loose growth first with a HEPA vacuum if you have one, before introducing any liquid. Wetting a dry colony spreads it.
Clean with detergent and water, or a dedicated mold cleaner. Avoid bleach on bare wood — it is mostly water, it does not penetrate the grain where the growth actually is, and it adds moisture to the material you are trying to dry.
Sand lightly if surface staining remains and it bothers you cosmetically.
Dry thoroughly. This is the step that decides whether it comes back. Airflow and dehumidification, not just wiping.
Residual grain staining on structurally sound, dry timber is cosmetic. You do not need to sand until it is gone.

When to Escalate

A screwdriver sinks into the wood — structural, get it assessed
Growth on framing, sill plates, joists or subfloor rather than on trim or furniture
More than roughly 10 square feet affected
Growth that returns after cleaning, meaning the wood is still getting wet
You find it in a crawl space or attic and cannot identify the moisture source
Wood that is wet to the touch with no recent obvious cause

What Photos Can and Cannot Settle

Depth is the whole question on wood, and depth is exactly what a photograph flattens. A picture can show you color, pattern and texture — enough to distinguish weathering from patchy growth, or to spot the fastener at the center of an iron stain. It cannot tell you whether a stain is one fiber deep or whether the joist behind it has gone soft.

So photograph it for the record and for comparison over time, but settle wood with your hands: the sanding test for depth, the screwdriver test for strength. The general color and texture comparison is in black mold vs mildew, the genus-level breakdown is in the mold identification chart, and if the wood got wet recently, how fast mold grows after water damage covers the window you are working in.