Mold on Wood

Urgency: Moderate Last updated September 27, 2026 · Monitor My Air

Mold on wood is a surface problem when the wood is sound and a structural one when it is soft. Wood that stays above roughly 16 to 20 percent moisture will grow mold on its surface; wood held wetter than that for months starts to rot. A moisture meter ($25 to $40) tells you which you have. Clean surface mold after the wood is dry, and treat soft wood as a repair, not a cleaning job.

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Most likely causes, in order

Crawlspace or basement framing kept damp by ground moisture and humid air, growing surface mold on joists and the subfloor.
Attic sheathing and rafters wetted by condensation from house air leaking up in winter.
Framing wetted by a leak, a flood, or a plumbing failure that was closed up before it dried.
Furniture or stored wood in a humid room, garage or storage unit, growing surface mildew.
Wood that has been wet for months and has begun to rot, where the mold is the least of the problem.

Fuzzy white, green or black growth on joists, studs, sheathing, a subfloor or a piece of furniture is one of the most searched mold questions, and the answer depends on one thing: is the wood still sound?

Why wood grows mold

Mold lives on the surface and feeds on the sugars and dust on it, not on the wood itself. It needs the wood to stay damp: above roughly 16 to 20 percent moisture content for days or weeks. Crawlspaces, basements and attics reach that from humid air and condensation alone, without any leak. Wood held wetter than that for months is where decay fungi take over, and that is rot, which is a different problem with a different fix.

Surface mold or rot, telling them apart

Surface mold wipes or brushes off and the wood under it is hard. Rot is dark, cracked in a checkerboard pattern, spongy, or crumbles when a screwdriver is pushed into it. A moisture meter settles the rest: sound wood that reads under 15 percent is dry and the mold is a leftover; wood reading above 20 percent is still being fed and the source has to be found before any cleaning holds.

Approximate costs

Moisture meter: $25 to $40
Dehumidifier: $200 to $350
Professional cleaning of framing in a crawlspace or attic: see the crawlspace and attic guides on this site for the typical ranges

When to call a professional

Any wood that is soft, spongy or crumbling. That is a structural repair, and a contractor needs to see it.
Growth across more than about 10 square feet, or across a whole crawlspace or attic.
Wood that stays wet after the obvious source is fixed. There is another source, and finding it is the job.

What not to do

Do not spray bleach on framing. It adds water to wood you are trying to dry and does not reach the roots of growth in the grain.
Do not paint, seal or encapsulate wood that is still above 15 percent moisture. Sealed damp wood rots from the inside.
Do not sand mold off dry without protection. It puts the growth into the air you are breathing.

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