Mold on wood: framing, joists, subfloors and furniture

Mold, stain or rot?

Three different things happen to damp wood, and they are often confused.

Surface mold. Fuzzy, powdery or spotty growth in white, grey, green or black. It lives on the surface, feeding on sugars in the wood and on dust. It wipes or brushes off, and the wood beneath is hard.

Staining. Blue-grey or black discolouration that goes into the wood. Sapstain fungi cause it in lumber, often before the wood ever reached the house, and it is common on new framing. Water and iron stains from nails look similar. Stain is cosmetic. It does not wipe off, and it does not weaken the wood.

Rot. Decay fungi that digest the wood itself. Rotted wood is soft, spongy, stringy or cracked into small cubes, and it has lost strength. Rot needs wood to stay very wet for a long time.

The screwdriver test: press the tip into the wood. Sound wood resists. Rotted wood lets it sink in.

Mold tells you the wood has been damp. Rot tells you it has been wet for a long time, and it is a structural matter.

Where it turns up

• Attic roof sheathing and rafters, from condensation and bathroom fans venting into the attic
• Basement floor joists and the rim joist, from high humidity or a past flood
• Crawl space framing, over damp soil
• Wall studs and bottom plates, behind drywall that was wet
• Subfloors, around toilets, tubs, dishwashers and patio doors
• New construction lumber, which got wet during building
• Furniture, window frames and trim, in damp rooms

Why wood can usually be saved

Drywall and insulation are thrown out when moldy because growth goes right through them. Solid lumber is different. It is dense, and mold stays at or near the surface. Once the growth is removed and the wood is dry, it is sound.

The usual approach follows from this: clean surface mold from the wood, sand it if cleaning does not remove it, and replace wood that is rotten.

How it is removed

For anything more than a small area, the work area is isolated first with plastic sheeting, kept under negative pressure with a HEPA-filtered air machine, and the heating and cooling system serving it is shut off. Workers wear respirators and protective clothing.

Then:

1. HEPA vacuuming of the wood and surrounding surfaces to take off loose growth and dust.
2. Physical removal of what remains: damp wiping with detergent, scrubbing with brushes, sanding, or wire brushing. On large areas such as an attic deck or a basement full of joists, contractors use abrasive blasting with dry ice or another medium, which is fast and reaches into corners.
3. A second HEPA vacuuming to collect the dust the cleaning produced.
4. Drying, with dehumidifiers and air movers, until the wood is at a normal moisture content. Framing lumber is generally considered dry at around 15 percent or below, and mold growth is unlikely under that.
5. Checking the result. The wood should be free of visible growth and dust. Some dark staining may remain. That is discolouration in the wood, not live growth.

Sealers and encapsulants

After cleaning, some contractors apply a coating to the wood, clear or white. It can lock down traces that remain and make later inspection easier. It has limits:

• It is not a substitute for removal. Coating over active growth or damp wood fails.
• It does nothing about the moisture.
• Painted framing can worry a future buyer, so keep the paperwork describing what was done and why.

Plywood and OSB

Sheet materials are layered or made of strands bonded with resin. They hold water at the edges and between layers.

• Surface mold on sound plywood or OSB can be cleaned as above.
• Panels that are swollen, delaminating, soft or crumbling are replaced.
• Subfloor around a toilet or under a long-term leak often needs a section cut out.

Particleboard, MDF and furniture

Particleboard and MDF swell and lose strength when wet and cannot be restored. Cabinet bases, shelving and flat-pack furniture made of them are usually replaced.

Solid wood furniture can be cleaned: take it outdoors if possible, vacuum with a HEPA vacuum, wipe with a cloth dampened with detergent solution, dry quickly, and refinish if the finish is damaged. Check the unfinished back and underside, where growth usually starts.

Small areas you can handle

For a patch under about one square metre on accessible, sound wood:

• Wear gloves, eye protection and a respirator rated for fine particles
• Dampen the surface lightly to keep dust down
• Scrub with detergent and water
• Wipe clean and dry the wood quickly with a fan and dehumidifier
• Sand stubborn spots, and vacuum the dust with a HEPA vacuum
• Bag and discard cloths and sandpaper

Do not dry-brush or sweep moldy wood. It puts spores into the air.

Mold on new lumber

Dark patches on framing in a new house or renovation are common. Often it is sapstain, which is harmless. Where it is surface mold from lumber that sat wet, it should be cleaned and the framing allowed to dry before insulation and drywall go on. Closing wet framing into a wall is how a cosmetic issue becomes a real one.

Fix the moisture

Mold on wood always has a cause: attic air leaks and poor ventilation, a damp basement or crawl space, a plumbing leak, or a flood that was not dried. Cleaning without correcting it buys a few months. See attic mold removal and mold in the basement.

More: mold removal and remediation.