Does Bleach Kill Mold? Why It Fails on Wood and Drywall

Does bleach kill mold? On hard, non-porous surfaces like tile, glass and tubs, yes. On porous materials like wood, drywall and ceiling tile, it kills what is on the surface, takes the colour out, and leaves the growth inside the material alive. Health Canada says bleach is not needed for mold cleanup and recommends water and dish detergent.

Does bleach kill mold on hard surfaces?

It does. Household bleach is a diluted solution of sodium hypochlorite, and it is an effective disinfectant on surfaces it can fully contact. On glazed tile, porcelain, glass, sealed countertops and metal, the mold is sitting on top with nowhere to hide. Bleach kills it and removes the stain.

Even there, the scrubbing does most of the work. Mold on a hard surface comes off with detergent and a brush, and physically removing it is the goal. Killing mold and leaving it in place is not the same as cleaning it, because dead spores and fragments can still irritate people who are sensitive to them.

Why bleach fails on wood and drywall

Mold does not sit on top of a porous material. It sends thread-like structures called hyphae down into the paper face of drywall, into wood grain, and into the fibres of ceiling tile and carpet backing. The visible fuzz is only the part you can see.

Bleach has two problems reaching the rest. First, the active ingredient reacts quickly with whatever organic matter it touches first, so it is largely used up at the surface. Second, household bleach is mostly water. The water does soak in. So the top layer gets bleached white, the growth below the surface survives, and you have just added moisture to a material that was moldy because it was damp.

The result looks like a success for a week or two. The stain is gone, so the mold appears to be gone. Then it grows back in the same spot, and people conclude the mold is unusually aggressive. It was never removed.

Other problems with using bleach on mold

• Fumes. Bleach in a closed bathroom or basement irritates the eyes and airways, which is the last thing someone with asthma needs during a cleanup.
• Dangerous mixtures. Bleach combined with ammonia or with acids such as vinegar or some toilet bowl cleaners releases toxic gases. Never mix it with other cleaners.
• Damage. It corrodes metal fasteners and fixtures over time, discolours fabric and carpet, and can break down some finishes.
• False confidence. Because it removes the colour, it hides the evidence. That makes it harder to tell whether a problem is solved, and harder for an inspector to see what was there.
• It does nothing about the moisture, which is the reason the mold grew.

What to use instead on a small area

Health Canada treats one to three patches, each smaller than a square metre, as a small job that most healthy adults can do themselves. Its method is simple.

1. Put on rubber gloves, safety glasses or goggles, and a properly fitted N95 mask.
2. Scrub hard surfaces with water and unscented dish detergent, then wipe with a clean damp rag.
3. On painted drywall with light surface growth, wipe with a damp rag and a little detergent. Do not soak it.
4. On bare wood, vacuum with a HEPA vacuum if you have one, then wipe with a damp detergent rag. Some people use white vinegar on small patches as well.
5. Dry everything quickly with a fan and open windows, or a dehumidifier.
6. Bag the rags and take them straight out to the garbage.

If the mold comes back after that, the material is still getting wet or the growth is deeper than the surface.

When cleaning is the wrong approach

Some materials cannot be cleaned, with bleach or anything else. Drywall that is moldy through the paper, insulation, carpet underpad, ceiling tile and particleboard are removed and bagged in professional remediation, because there is no reliable way to get growth out of them. Solid wood framing and roof sheathing are usually saved. The growth is taken off mechanically, by HEPA vacuuming, wire brushing or sanding, and the wood is dried until it reads under 16% moisture content.

That work is done inside containment with negative air through HEPA filtration, because sanding and cutting put a lot of spores in the air. It is the part of the job that should not be improvised with a spray bottle. Health Canada's guidance is to bring in professional help when the total area is more than about three square metres, and we would add any mold that follows a sewage backup or sits inside a wall or ceiling cavity.

What to do next

If you have already bleached a patch, there is no harm done beyond the delay. Let the area dry fully, then check it over the next few weeks. Growth returning in the same place means the moisture is still there. Press on the drywall or probe the wood with a screwdriver. If it is soft, crumbly or stays damp, cleaning has reached its limit. Find the leak or the condensation source first, since no cleaner outlasts a wet wall.

Common questions