Due to recent regulatory changes in the UK, chemicals that were previously marketed as a “moss killer” or “moss treatment” are now no longer allowed to target moss specifically.

In reality, the same chemicals are sold but are marketed as “roof cleaner”, “kills green growth” or “algae and lichen killer”. Note that the word “moss” is no longer included in the manufacturer’s literature.

The best way to control “green growth” on roofs is to have it removed manually and the tiles treated with a suitable “green growth” chemical to prevent it from growing back for several years.

Products that contain a quarternary ammonium are best. You may see these labelled as Benzalkonium Chloride, benzyl alkyldimethyl chloride, BAC or ADBAC.

GK Pro is for professional use only (they won’t sell directly to the public) but it contains a quaternary ammonium at 50% concentration, allowing for dilution at an impressive 1 part chemical to 25-50 parts water.

SmartSeal sells a consumer-friendly roof cleaner with a quarternary ammonium at about 10% concentration with a dilution at 1 part chemical to 5 parts water.

There are many other products on sale that contain the same chemical at varying degrees of concentrated strength. Assuming you dilute as per the instructions, the final product going onto the surface will have the same strength.

Quaternary ammonium products will not stain any part of the roof and are biodegradable but are very strong in their wet state until they have dried.

If you see a product containing Sodium Hypochlorite, be advised that this is bleach and may stain leadwork as it’s very caustic, much more so than quarternary ammonium products. Sodium Hypochlorite should not be used on roofs with aluminium roof window frames or aluminium roof gutters.

Mister Moss recommends the professional application of chemicals but DIYers should thoroughly check the datasheet for the products they wish to buy. In general, most DIY products are far more diluted than the pro versions so aren’t practical or cost-effective for very large areas such as car parks or huge roofs.

Steps should be taken to prevent the chemicals from entering drains and soakaways:

• Block drains.
• Disconnect roof gutters.
• Do not use when rain is imminent.
• Avoid excess runoff.

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