How to Remove Mold From Wood: Surface Growth, Stained Wood, and When It's Structural (2026)

Three Kinds of Mold on Wood

Wood is porous, and that single fact decides everything about cleaning mold off it. Growth that sits on a finish — paint, varnish, a sealed deck — is on a hard surface and cleans off. Growth on bare wood sends hyphae into the grain, and what looks like a stain after cleaning is the part you did not reach. And growth on structural wood — the joists over a crawl space, the studs in a wet wall, the sheathing under a leaking roof — is a symptom of a moisture problem that will keep feeding it until the water is fixed.

The good news is that mold discolours wood long before it weakens it. The fungi that soften and destroy wood — rot — need the wood wet for months. Most moldy wood is sound, and most of it can be kept.

First: Firm or Soft?

Push a screwdriver into the worst-looking area. Firm wood is a cleaning or sanding job. Wood that is soft, spongy or crumbles has rot, and rot is replacement — no cleaner reverses it. Do this test before buying anything.

Tier 1 — Surface Growth on Finished Wood

Painted trim, varnished furniture, a sealed deck, a stained fence: the mold is on the finish, not in the wood. EPA's guidance for hard surfaces applies — scrub with detergent and water, then dry. A soft brush with the grain, warm soapy water, wipe, and let it dry in moving air. White vinegar or 3% hydrogen peroxide, sprayed and left ten minutes before scrubbing, add a mild antifungal effect and are safe on most finishes (test a hidden spot; peroxide can lighten some stains).

Bleach is the tool most people reach for and the wrong one for wood: its water soaks in while the chlorine stays on top, so the surface looks clean and the colony under it survives. EPA does not recommend bleach as routine mold practice for exactly this reason — the full explanation is worth two minutes before you pour it.

N95, gloves and eye protection whenever you brush dry growth. Better still, HEPA-vacuum the loose growth off first so the brushing releases less.

Tier 2 — Mold in the Grain of Bare Wood

Bare wood — unfinished furniture, the underside of a deck board, a workbench, framing lumber — takes mold into the grain. After a proper cleaning a grey or black shadow remains. That is not a stain to be scrubbed harder; it is growth below the surface.

Two answers:

• Sand it. 100–120 grit, with the grain, back to clean wood, N95 on and a vacuum running. This is the method for anything you will refinish — furniture, decking, trim. Seal or refinish once it is dry, because bare wood that stays bare re-molds in a damp room.
• Treat it with borate. A borate solution — disodium octaborate tetrahydrate, sold under several trade names and applied with a garden sprayer — penetrates bare wood, kills mold and wood-boring insects, and stays in the wood as long as liquid water does not rinse it out. This is what remediation contractors put on framing after cleaning, and it is the right tool for wood you cannot sand: joists, studs, subfloor, the back of sheathing. It goes on dry, bare wood; it does not work through paint.

Sand for the wood you look at; borate for the wood that holds the house up.

Tier 3 — Structural Wood: Framing, Subfloor, Crawl Space, Attic

Mold on the joists above a crawl space, the studs inside a wall, or the sheathing under a roof is different in kind, because it tells you those materials have been wet long enough to grow it. Cleaning it without finding the water is a subscription.

• Crawl space joists and subfloor — almost always ground moisture and humidity, sometimes a plumbing leak. The fix is a vapour barrier, drainage and a dehumidifier before any wood treatment; the crawl-space guide walks through it. Then HEPA-vacuum, clean, dry to under 16% moisture content, and borate.
• Attic sheathing and rafters — a roof leak or, more often, bathroom and kitchen exhaust venting into the attic instead of outdoors. The attic mold guide covers the ventilation fix.
• Wall studs — mold inside a wall cavity means a leak or condensation inside it, and the drywall usually comes out to reach it. The drywall guide covers the cut-and-dry sequence; mold in walls covers the source hunt.

EPA's threshold for professional remediation is about 10 square feet of growth, and structural mold crosses it quickly. Over that line, or where the wood is soft, or where the source is unknown, a remediation contractor working to the IICRC S520 standard — containment, negative air, HEPA, source correction, verification — is the job, with an independent inspector's scope before the bid. Remediation pricing depends mainly on square footage and access.

Drying: The Number That Decides It

Wood below about 15–16% moisture content does not support mold; wood above 20% does. A pin moisture meter reads it in seconds — the homeowner moisture meter review covers the ones worth owning — and it is the difference between "it looks dry" and "it is dry." Fans and a dehumidifier bring cleaned wood down over two to four days; the room stays under 50% humidity afterwards or the whole exercise repeats.

Bottom Line

Firm or soft first. Finished wood: detergent and water. Bare wood with a shadow left: sand it if you can refinish it, borate it if it holds the house up. Structural wood: find the water, dry to under 16%, then treat — and over 10 square feet, or soft, or with no source in sight, hand it to a remediation crew. Bleach stays in the laundry room.