Reaching for bleach is most people’s first instinct when they spot mould at home, but it is often not the right tool for the job. Bleach can be effective against mould on hard, non-porous surfaces like tiles and glass, but on porous materials such as wood, plaster, and grout, it frequently makes the underlying problem worse rather than better.
Why Bleach Struggles Against Household Mould
Bleach is mostly water, and that water content is the root of the problem on porous surfaces. Mould feeds on moisture, so applying a watery solution to a porous material such as wood or plasterboard can actually encourage mould to regrow beneath the surface, even as the visible patch appears to disappear.
Bleach’s chlorine content stays largely on the surface, while the water soaks into the material underneath, creating exactly the damp conditions mould needs to re-establish itself. This is why homeowners often find mould returning to the same spot within weeks of bleaching it.
Where Bleach Can and Cannot Reach
On hard, non-porous surfaces, bleach can kill mould effectively because it stays on the surface where the mould is actually growing. On porous materials, mould roots grow beneath the surface, out of reach of the bleach itself. The result is often a surface that looks clean but still harbours mould underneath, ready to reappear once conditions are damp enough again.
Other Downsides of Using Bleach
Beyond its limited effectiveness on porous surfaces, bleach has a few other drawbacks worth knowing about:
- It can damage materials over time. Bleach is corrosive and can weaken fabric, discolour surfaces, and degrade some materials with repeated use.
- Fumes can irritate airways. Chlorine fumes can cause respiratory irritation, particularly for anyone with asthma or existing sensitivity, so good ventilation and a mask are sensible precautions when using it.
- Its strength fades over time. Chlorine gradually degrades once bleach is opened, meaning older bottles are noticeably less effective than fresh ones.
- It does not address the underlying moisture problem. Killing visible mould does nothing to fix the damp or condensation that allowed it to grow in the first place, so the problem is likely to return.
What to Use Instead
For porous surfaces, purpose-made mould removal products are generally more effective than diluted household bleach, since they are formulated to penetrate the material rather than simply sit on top of it. On hard, non-porous surfaces such as tiles and glass, a standard detergent and water, followed by thorough drying, is often enough for light mould, following guidance similar to that recommended by the UK Health and Safety Executive for everyday household mould.
Whatever product you use, the most important step is tackling the source of the moisture, whether that is poor ventilation, a leak, or condensation, since mould will keep returning until the underlying damp problem is resolved.
Frequently Asked Questions
Does bleach kill mould permanently?
On non-porous surfaces, bleach can kill visible mould effectively. On porous surfaces, it often only removes the surface colour while mould continues growing underneath, meaning it is rarely a permanent fix on its own.
Is it safe to use bleach on mould?
Bleach can be used safely with proper ventilation and protective measures such as gloves and, where fumes are strong, a mask. It should never be mixed with other cleaning products, particularly anything containing ammonia.
What is the real cause of mould coming back after cleaning?
Mould returns when the underlying moisture source, such as condensation, a leak, or poor ventilation, is not addressed. Cleaning the visible mould without fixing the moisture problem is only ever a temporary solution.
Related Reading
- Toilet Cleaning Tips for a Sparkling Bathroom
- Effects of Water Damage in Your Home from Plumbing Leaks
About the author: This guide was written by the Routen House editorial team.
