Bleach or washing soda on a facade: why we advise against it

Bleach and washing soda sit in most cupboards, and at first sight they look like a cheap way to brighten a green or grimy facade. We still advise against it. Below you will find what these products actually are, what they do to stone, joints, metal and the planting below, and what to use instead.

What does bleach do on a facade?

Household bleach is a solution of sodium hypochlorite in water. It is a bleaching agent that oxidises organic growth such as algae and moss, which makes it lighter.

The Belgian Poison Centre explains that sodium hydroxide is added to the solution to stop chlorine gas from escaping, which makes it strongly alkaline (Belgian Poison Centre). On a facade the effect shows quickly: the green colour fades. A lighter facade does not yet mean the problem is solved.

Why do we advise against bleach on a facade?

Because bleach leaves traces you cannot see. It can leave salts in the masonry, attack metal and harm the planting and water below, while the growth often returns quickly.

Salts in porous materials. Brick, natural stone, render and mortar joints are porous and absorb liquid. Historic Environment Scotland advises against treatments that may leave damaging salt residues, naming chloride-containing bleaches as an example (Historic Environment Scotland). As masonry dries, salts can be drawn to the surface and appear as a white powder, known as efflorescence (National Park Service).

Discolouration. Bleach is a bleaching agent. On coloured render, paint or coatings it can leave patches or colour differences that are hard to put right.

Metal. The safety data sheet for an industrial sodium hypochlorite solution from BASF states that it may be corrosive to metals and lists metal among the materials to avoid (BASF). Household bleach is weaker, but it is the same substance. The National Park Service notes that hidden parts, such as iron cramps or ties that hold the masonry to the structure, can also corrode from cleaning chemicals.

Plants and run-off. The same data sheet classifies sodium hypochlorite as very toxic to aquatic life. Whatever runs off the facade ends up in the borders, on the terrace or in the drain. According to the National Park Service, chemical cleaners may damage trees, shrubs, grass and plants.

Fast regrowth. Historic Environment Scotland writes that some organisms produce spores that resist biocides and recolonise the stone soon after treatment. It also points out that excessive growth often signals another problem, such as faulty rainwater goods or damaged pointing. As long as that cause remains, the growth comes back.

Is cleaning render with washing soda or caustic soda any better?

Not really. Washing soda is alkaline, and alkaline cleaners, like bleach, can leave salts in the masonry.

Mind the name, because there are two kinds of soda. Washing soda, or soda crystals, is sodium carbonate. Pure sodium carbonate can cause irritation, so avoid contact with skin and eyes (Waarzitwatin). Caustic soda is something else: sodium hydroxide, also called lye, which is far more aggressive. The National Park Service writes that sodium hydroxide can damage masonry or leave disfiguring brownish-yellow stains, and in most cases should not be used on historic masonry. Alkaline cleaners can also deposit salts in the masonry that are difficult to rinse out thoroughly. Historic Environment Scotland adds that strongly alkaline treatments may cause iron staining on some types of stone.

Can you mix bleach with a descaler or ammonia?

No, never. With an acid, chlorine gas is released; with ammonia, chloramines form. Both gases are harmful.

The Belgian Poison Centre explains that in contact with an acid, for example from a descaler, the hypochlorite breaks down and releases chlorine gas. A single exposure can irritate the upper airways, with watery eyes, a runny nose, an irritated throat and coughing. The Washington State Department of Health warns that bleach mixed with ammonia produces chloramines, which can cause coughing, shortness of breath and chest pain, among other symptoms (Washington State Department of Health). On a facade the risk is real: applying an acidic cleaner or cement residue remover first and bleach afterwards mixes the two on the wall.

Can you use a product that kills moss and algae yourself?

Only if it is authorised for that use. A product used to kill algae or moss is a biocide, and biocides fall under the EU Biocidal Products Regulation (EU) 528/2012.

According to the European Commission, a biocidal product cannot be placed on the market or used unless it contains approved active substances and has itself been authorised (European Commission). So check the label to see whether the product is authorised for that use, and follow the instructions. Historic Environment Scotland recommends testing any product on a small, inconspicuous area first, and notes that products relatively safe for people can still be toxic to birds, fish and aquatic invertebrates.

What should you use to clean a facade instead?

A product suited to the surface, preferably pH-neutral, applied at low pressure and followed by a thorough rinse. Historic Environment Scotland considers pH-neutral treatments generally the best choice.

The National Park Service advises testing every method on a small test area first, covering windows and other vulnerable features, and collecting the rinse water. It also helps to tackle the cause: a facade that stays damp because of a leaking gutter will turn green again. To learn how to clean a facade by letting the product work and rinsing gently, read what is soft washing. Why high pressure can damage joints and render is explained in facade cleaning without high pressure.

Frequently asked questions