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 stop mould for good, you need to:

1. Penetrate the porous surface to reach the spores
2. Remove the food source — the organic film of soap, skin cells and oils mould feeds on
3. Fix the moisture that lets mould grow in the first place

Probiotic mould removers penetrate grout and silicone, digest the organic film, and leave beneficial microbes behind that outcompete new spores — so the mould can't simply regrow.

The safer, more effective alternative

At Thrive, we unite probiotic science with Clean Chemistry innovation to deliver high-performance cleaning that's safe for kids, pets and the planet.

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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 kills mould better than bleach?

Treatments that penetrate porous surfaces and remove mould's food source work better than bleach. Probiotic mould removers digest the organic film mould feeds on and leave beneficial microbes that outcompete returning spores, stopping 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 non-toxic probiotic remover is a safer choice that also works better long-term.

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 probiotic mould remover is more reliable because it both penetrates and removes the food source mould needs to regrow.