Does Bleach Kill Mould? The Honest Answer
Quick answer: Bleach kills surface mould on hard, non-porous surfaces — but it does NOT kill mould in porous materials like grout, silicone, plaster or timber. Bleach strips the colour so mould looks gone, while the spores and roots survive beneath the surface and regrow. To stop mould coming back you need a treatment that penetrates the surface and removes the organic film mould feeds on.
It's the question almost everyone asks when they spot black mould in the bathroom: does bleach kill mould? The honest answer is — partly, and not in the way you need. Here's what bleach actually does, and why your mould keeps coming back.
The short answer
Bleach kills mould on hard, non-porous surfaces like glass, tiles and sealed benchtops. But on the porous surfaces where mould loves to grow — grout, silicone, plaster, timber, ceiling paint — bleach can't penetrate deep enough to kill the roots and spores. It only removes the colour.
Why bleach fails on bathroom mould
Two reasons:
1. It can't penetrate. Bleach molecules sit on the surface. The mould's root structure (and the spores) live inside porous grout and silicone, safe from the bleach.
2. It's mostly water. Household bleach is around 90%+ water. Once the chlorine evaporates, the water left behind can actually feed the surviving mould — making the problem worse.
So you bleach, the black vanishes, and you think it's solved. Then days later it's back in the same spot, because it never actually left. Here's the deeper science on mould spores.
What actually works
To help stop mould coming back, you need to:
- Penetrate the porous surface to reach the spores
- Remove the food source — the organic film of soap, skin cells and oils mould feeds on
- Fix the moisture that lets mould grow in the first place
A clinging, bleach-free mould remover penetrates grout and silicone and breaks down the mould at its source — which helps prevent regrowth.
The bleach-free alternative
At Thrive we match the science to the problem: probiotic biotechnology in MULTIZAP for organic residue and odour, and advanced clean chemistry in MOULDZAP and DRAINZAP for mould, scale and drain build-up.
Frequently asked questions
Does bleach kill mould?
Bleach kills surface mould on non-porous surfaces, but it does not kill mould in porous materials like grout, silicone, plaster or timber. It strips the colour so the mould looks gone, while the roots and spores survive beneath the surface and regrow.
Why does mould come back after bleaching?
Because bleach only removes the visible pigment, not the spores embedded in porous surfaces or the organic film mould feeds on. Bleach is also mostly water, which can actually feed remaining mould once the chlorine evaporates.
What works better than bleach on mould?
Treatments that penetrate porous surfaces and remove mould's food source work better than bleach. A clinging, bleach-free mould remover breaks down the mould and the film it grows in, helping prevent regrowth.
Is it safe to use bleach on mould?
Bleach produces fumes that irritate eyes, skin and airways, and is dangerous if mixed with other cleaners. For indoor mould, a bleach-free remover avoids chlorine fumes and suits porous surfaces better.
Does vinegar kill mould better than bleach?
Vinegar can penetrate porous surfaces better than bleach and kills some mould species, but results vary. A purpose-made mould remover is more reliable because it is formulated to penetrate porous surfaces and cling long enough to work.