The Truth About Bleach on Mould (and What Professionals Use Instead)

Ask most people how they deal with black mould on a bathroom ceiling or bedroom wall and you’ll hear the same answer:

“I just spray a bit of bleach on it.”

It feels satisfying. The black spots fade, the surface looks “clean”, and for a few weeks you can pretend the problem is sorted.

But if the mould keeps coming back – often worse than before – it’s not your imagination. Bleach is rarely the right tool for the job, especially on the porous building materials you find in real homes.

In this guide, we’ll look at what bleach actually does to mould, why professionals are so lukewarm about it, and which products they reach for instead.

Why mould is more than just an eyesore

Before we talk chemistry, it’s worth reminding ourselves why this matters.

UK government guidance now makes it very clear: damp and mould are not just cosmetic issues – prolonged exposure is linked to serious respiratory problems and, in extreme cases, can contribute to life-threatening illness.

Mould problems are particularly common in:

Condensation-prone homes (cold walls, steamy bathrooms, bedrooms with poor ventilation)
Rented properties where occupants have less control over the building fabric
Winter, when windows are closed and moisture has nowhere to go

So this isn’t just about making a ceiling look better for a house viewing. If you’ve got recurring mould, you need a treatment that deals with both the living growth on the surface and the moisture conditions that allowed it to take hold in the first place.

What bleach actually does to mould

Household bleach (usually sodium hypochlorite in water) is a strong oxidising agent. On hard, non-porous surfaces – glazed tiles, glass, some plastics – a properly diluted bleach solution can:

Discolour and break down the pigment in mould, so the black staining fades
Kill a proportion of the fungal material sitting on the surface

That’s why it looks so dramatic when you first apply it. The mould stain vanishes before your eyes.

The trouble is that most building surfaces – plaster, emulsion paint, grout, timber, wallpaper, masonry – aren’t hard, non-porous materials. They’re full of tiny pores and capillaries. That’s where mould likes to anchor itself and send roots (hyphae) deeper into the substrate.

Multiple housing and local authority guides now warn that simply bleaching mouldy walls is not an effective removal method. It may fade the stain, but it doesn’t reliably remove or neutralise the growth within those pores.

So you’re left with a surface that looks better – but still contains viable mould and the same underlying moisture problem.

Five big problems with using bleach on mould

Let’s break down why professionals are wary of relying on bleach as the main treatment.

1. It often only removes the stain

On porous materials like plaster, grout and painted walls, bleach tends to work on the top layer. It oxidises the colour and gives the illusion that everything has gone. But the mould can continue to live deeper inside the surface.

That’s why you see classic “ghosting”:

Week 1: mould treated with bleach looks better
Week 4–8: faint grey shadow begins to re-appear
A damp, cold spell later: the full black spotting is back

2. Poor penetration into building materials

Bleach solutions don’t penetrate deeply into porous materials. Several specialist damp and mould contractors specifically highlight that bleach is ineffective on substrate such as wood, plasterboard and masonry, where the mould has grown into the material itself rather than just on the surface.

Biocidal mould washes, by comparison, are formulated to soak into the substrate and stay active long enough to kill mould spores and associated growth within the surface layer.

3. It doesn’t fix the cause (and can mask it)

Bleach has no effect on:

Condensation caused by warm moist air hitting a cold surface
Penetrating damp from defective gutters, render or pointing
Rising damp or plumbing leaks

At best, it gives you a cleaner-looking wall while the fabric of the building continues to be wet. At worst, it encourages you to keep ignoring the underlying issue until the problem is bigger, more expensive and more dangerous.

4. Fumes, splashes and indoor air quality

Bleach is a harsh chemical. Even when diluted correctly, it releases fumes that can irritate eyes and airways – a real concern in homes where people already have asthma or respiratory conditions, the very people mould affects most.

Used carelessly, it can:

Discolour paint, carpets and fabrics
Corrode some metals
React dangerously if mixed with acidic cleaners (e.g. certain descalers or vinegar), producing chlorine gas

5. False sense of security

Because bleach gives instant cosmetic results, it can create a dangerous “out of sight, out of mind” mindset. The room looks fresh, so the windows stay shut, the extractor fan stays unused, and the same mould growth pattern returns every winter.

Professionals, by contrast, think in systems:

Diagnose moisture → Kill and clean growth → Protect the surface → Improve ventilation and heat patterns.

Bleach at best is a small part of one step, and for many substrates it’s not very good at that step either.

Is there any place for bleach on mould?

For balance, it’s worth saying: bleach is not completely banned from the conversation.

Some health and fungal-safety guidance notes do recommend a diluted bleach solution as one option for cleaning small areas of mould on non-porous surfaces – things like bathroom glass, some tiles and sanitaryware – providing you ventilate well and wear gloves.

But there are clear limits:

The affected area should be small (typically under 1 m²)
Surfaces should be hard and non-absorbent
You still need to tackle the underlying cause of damp or condensation

In other words, bleach might have a role in small, hard-surface cleaning, but it is not a professional solution for mould on walls, ceilings, timber, wallpaper or plasterboard.

What professionals use instead of bleach

So if not bleach, what do professionals and specialist contractors use when they’re tackling mould on real building surfaces?

1. Specialist biocidal mould washes

The industry standard starting point is a biocidal mould wash or multi-surface biocide. These are liquid treatments formulated specifically to:

Soak into porous surfaces
Kill mould, mildew, algae and other biological growth
Help loosen residues from the surface so they can be safely removed

Examples on the UK market include dedicated mould wash concentrates and multi-surface biocides used on walls, ceilings, render, timber and masonry.

In your case, Biokil Crown ACS Mould Wash Concentrate is a good illustration of what a professional-grade product looks like:

Concentrated biocidal cleanser for interior and exterior surfaces
Designed to remove mould, mildew, algae and moss from masonry, render, tiles, wood and painted surfaces
Intended specifically as pre-treatment before painting or waterproofing so regrowth is minimised

For larger or more stubborn areas, contractors may step up to higher-strength, multi-surface biocides that can be used indoors and outdoors to clean biological growth ahead of masonry protection creams and other systems.

The key difference vs bleach is that these products are:

Formulated to penetrate and remain active for long enough to deal with spores
Designed as part of a system that includes later protective coatings

2. Careful cleaning and preparation

After the biocidal wash has been applied and allowed to work for the specified dwell time, professionals will:

Gently scrub and wipe away loosened mould and staining
Avoid dry brushing or hoovering which can release spores into the air
Rinse (if the product requires it) and allow the surface to dry thoroughly
Deal with any loose, flaking paint or damaged plaster so that the surface is sound and ready to accept a protective coating

3. Anti-mould paints, additives and systems

Once the substrate is clean, treated and dry, the next line of defence is a coating that resists mould growth.

Professional anti-mould paints and fungicidal emulsions contain built-in biocides that inhibit mould and mildew on the paint film, even in humid conditions. They’re widely used in kitchens, bathrooms and other high-condensation rooms.

There are two common approaches:

Dedicated anti-mould emulsions – ready-mixed paints with a fungicidal package built into the formula
Fungicidal paint additives – concentrated additives mixed into standard emulsion, gloss or varnish to give them anti-mould properties

Either way, the idea is the same: even if the surface becomes damp from condensation, the paint film itself is hostile to mould spores trying to re-colonise.

In more severe condensation situations, contractors may combine anti-mould paints with anti-condensation or thermal coatings. These include microsphere-based paints and specialist products that slightly warm up the internal surface, reducing the amount of condensation that forms in the first place.

4. Ventilation and moisture control

Finally, no product can compensate for a house that’s permanently wet.

Professionals will always look at:

Extractor fans in kitchens and bathrooms
Trickle vents and background ventilation
Drying clothes indoors
Heating patterns and cold external walls

Guides for landlords and housing providers repeatedly stress that tackling condensation, improving ventilation and dealing with fabric defects (leaks, bridging, failed gutters etc.) is critical if you want a long-term fix.

Feature	Household Bleach	Specialist Biocidal Mould Wash	Anti-Mould Paint / Additive
Main purpose	General disinfectant / stain remover	Kill mould, mildew and algae on building surfaces	Prevent mould from growing back on painted surfaces
Typical use in homes	Sprayed or wiped on visible black mould to “clean” it	Pre-treatment on walls, ceilings, tiles, masonry and timber before repainting	Finishing coats on walls and ceilings in bathrooms, kitchens, bedrooms etc.
Effect on mould staining	Quickly fades or removes surface staining (especially on hard, non-porous surfaces)	Helps break down staining as growth is killed and loosened	Does not remove existing stains – applied after cleaning / stain removal
Effect on mould spores	May kill some surface spores, but often poor penetration into porous materials	Formulated to penetrate and kill mould and spores within the surface layer	Inhibits new mould growth on the paint film; assumes spores have been treated first
Penetration into porous surfaces (plaster, grout, timber, masonry)	Limited – mainly acts on the surface, so mould often regrows from within	Good – designed to soak into porous substrates for deeper treatment	N/A – forms a protective film on top of a prepared surface
Long-term protection	None – once rinsed or evaporated, there’s no ongoing resistance to mould	Short-term – cleans and sterilises but does not prevent future condensation	Medium to long-term – fungicidal film resists mould in normal/high humidity conditions
Effect on underlying cause (condensation / damp)	No effect at all	No effect – purely a treatment / cleaning step	No direct effect, but helps surfaces cope better with occasional condensation
Best suited surfaces	Small areas of mould on hard, non-porous surfaces (e.g. some tiles, glass)	Most interior and exterior building surfaces (check product data for suitability)	Interior walls and ceilings, especially in kitchens, bathrooms and bedrooms
Risk of masking the problem	High – removes the stain so the issue looks “fixed” while damp conditions continue	Low – part of a wider mould treatment system, not a cosmetic quick fix	Medium – will still fail if applied straight over active mould or ongoing damp
Health & safety	Strong fumes, can irritate skin and airways, can damage finishes, dangerous if mis-mixed	Needs PPE and ventilation, but formulated specifically for mould remediation	Handled like normal trade paint; still follow label guidance and ventilation advice
Role in a professional system	At most, a minor hard-surface cleaner – rarely recommended on its own for walls/ceilings	Primary cleaning and sterilising step before any repair or repainting	Protective finishing step after proper cleaning, treatment and drying
When it’s appropriate	Small, hard, non-porous areas where no better product is available and ventilation is good	Any mould-affected surfaces being prepared for painting or waterproofing	Any room with a history of condensation mould, once surfaces are correctly treated
When to avoid	Porous walls, ceilings, timber, large areas of mould, or where occupants have respiratory issues	Situations where underlying structural damp/leaks haven’t been investigated	Directly over active mould, damp, flaking paint or unprepared surfaces

What professionals use instead of bleach

If you’re a homeowner or landlord wanting to follow a more professional approach, the overall process looks like this.

Step 1 – Diagnose the moisture source

Before picking up any chemical:

Check for obvious leaks from roofs, gutters, downpipes and plumbing
Look for signs of rising damp (salty tide marks, damaged skirtings)
Note where mould is appearing – cold corners, external walls, behind furniture and around windows usually point to condensation

If the mould is widespread, if anyone in the home has serious health issues, or if you suspect structural damp, get professional advice rather than treating it all as a DIY job.

Step 2 – Apply a proper mould wash or biocide

Use a dedicated mould wash concentrate or biocidal surface cleanser rather than household bleach. For example, a product like Biokil Crown ACS Mould Wash Concentrate or Biokil Crown Biocidal Cleanser Concentrate is designed to:

Kill mould and associated biological growth on contact
Soak into masonry, render and other surfaces
Prepare the area for subsequent coatings or waterproofing

Follow the manufacturer’s instructions on:

Dilution rates
Dwell time (how long it should remain wet on the surface)
Suitable substrates and coverage rates

Wear gloves, eye protection and a mask if recommended, and ventilate the area during treatment.

Step 3 – Clean, rinse and let it dry

Once the biocidal wash has done its work:

Wipe or gently scrub off the dead mould and residues
Rinse if the product specifies it (some modern biocides are “no-rinse”)
Allow the surface to dry thoroughly – rushing this step is a common reason coatings fail prematurely

At this point, carry out any filling, sanding and basic surface preparation you would normally do before painting.

Step 4 – Overcoat with anti-mould or anti-condensation products

Once clean, dry and sound, protect the area with:

An anti-mould emulsion or fungicidal paint system for kitchens, bathrooms and bedrooms
An anti-condensation or thermal coating on particularly cold external walls or ceilings, possibly over-coated with a decorative finish

Modern anti-mould paints from trade brands use advanced fungicidal technology specifically to stop mould spores anchoring to the film, even in high-humidity rooms.

If you prefer to use a standard emulsion colour, a fungicidal paint additive mixed into your chosen paint can be a neat compromise.

Anti Mould Paint & Anti Mould Paint Additive - Available here.

Step 5 – Change the conditions that created the mould

Finally, adjust the way the room is used so you’re not constantly recreating the same problem:

Use extractor fans religiously when cooking or showering
Keep a small gap behind large furniture on cold external walls
Avoid drying clothes on radiators in poorly ventilated rooms
Maintain a background level of heat during cold snaps

These simple changes, combined with proper biocidal treatment and protective coatings, are the real “secret” professionals rely on – not a bottle of bleach.

FAQs: common bleach and mould myths

“Bleach worked for me – the mould disappeared!”

In most cases, bleach has faded the stain, not eliminated the cause. If the wall is still cold and damp, new mould spores will land and grow again. When people say “it worked”, they’re often within the first few weeks of treatment.

“Can I just paint over mould if I use a good anti-mould paint?”

No. Even the best anti-mould emulsions expect a clean, sound, mould-free surface to bond to. Manufacturer guidance and independent reviews alike stress that mould-killing washes or sterilising solutions should be used first, before applying the paint system.

“What about vinegar – isn’t that better than bleach?”

White vinegar is popular online as a gentler, less toxic mould cleaner and does have some effectiveness against superficial growth on certain surfaces.

However, like bleach, it:

Doesn’t fix condensation or damp
Isn’t a substitute for professional biocidal products where mould is extensive or on porous building materials
Still needs to be followed by proper drying and, ideally, a protective coating

Think of it as a light-duty option for very minor surface staining, not a full remediation strategy.

Recommended Products

Product	For	Link
Mould Wash	Cleaning Off Mould	Link
Anti Mould Paint	Stops Mould Growth	Link
Anti Mould Paint Additive	To Add Into Paint	Link
Dehumidifiers	Moisture Control	Link
Respiratory PPE	Health & Safety	Link
Hand Protection PPE	Health & Safety	Link

The bottom line: ditch the bleach as your main mould strategy

Bleach is great at one thing: making things look clean quickly.

But when it comes to mould on walls, ceilings, timber and plasterboard, professionals know that looks can be deceiving. It doesn’t reliably penetrate porous substrates, it doesn’t solve the moisture problem, and it can give a false sense of security while the building continues to suffer.

A more professional, longer-lasting approach is: