What Is the Best Moss Killer for a Roof?

Moss killers sold for roofs are biocides: chemicals that kill living growth such as moss, algae and lichen. In Great Britain, the biocides regulation says a product must be used as its authorisation allows, and as the label instructs. That is the real test of any product. If you cannot say what it is approved for, or the label says nothing about roofs, it is the wrong product.

The Health and Safety Executive tells householders to steer clear of biocidal products meant only for professional users, and adds that professionals must hold particular certificates for some of those. That is why some of the strongest roof treatments are not sold in garden centres. A roofer who uses them should be able to tell you which one, and show you the label.

Be careful with adverts. The HSE says it is an offence to advertise, sell, supply, store or use a non-approved product, and that adverts must not claim a product is "HSE approved". So a product promising the best result in the shortest time should prompt questions rather than trust.

A reasonable first step is to read how biocide roof treatment works before you choose.

Bleach is a household cleaner. It was made to disinfect a sink, not to treat tiles, and nobody has written roof instructions for it. That is the heart of the problem: with a roof product the label tells you the dilution, the contact time and what to protect. With bleach there is nothing to follow, so every decision is a guess.

Anything sprayed onto a roof runs off. It collects in the gutters, goes down the downpipes and can reach gullies, lawns, borders and the soil around the house. A product that is authorised for roof use comes with guidance about run-off. Bleach does not. Strong splashes can also mark paint, plastic fascias and lead flashings, and they harm planting at the base of the wall.

Bleach also does nothing about the moss that is already there. Dead growth stays in the tile surface, holds moisture and needs to be brushed or washed away, in the way described in our answer on removing moss from a roof yourself. Climbing up to apply it adds the biggest risk of all, a fall from height. If you suspect a neighbour or a previous owner has used it, tell the roofer at the survey.

Home remedies are not the only shortcut. A high-powered jet wash looks like the obvious answer to moss, and it is how many roofs get cleaned. Marley advises against high-powered jetting, warning that it harms the surface and cuts the expected life of slate or tile. A gentler wash that leaves the killing to a treatment is kinder to the roof.

The same caution applies to salt, vinegar, washing-up liquid and the other recipes that circulate online. None of them is made or authorised for roof use. Zinc and copper strips fitted below the ridge are a different idea, a slow preventive measure rather than a clean; see our answer on zinc strips for roof moss.

When you ask a roof cleaner about their method, the right answers are specific. They should say whether they brush by hand, soft wash or use a treatment, which product it is, and what they do about run-off. Vague answers are a signal to ask for another quote. Our write-up of roof soft washing explains what a low-pressure clean looks like in practice.

Four questions cover most of it. Which product is being used, and is it authorised for roofs? Who is applying it, and are they trained for it? What happens to run-off from the gutters and downpipes? And how will the dead moss be cleared and taken away?

Cost is the next question. As a national guide, Checkatrade's April 2026 roof cleaning table gives £8.50 to £14 per m², with hand removal plus a biocide around £11.50 per m², and scaffold, if needed, adding around £18 per m². Treat those as a yardstick only. Check the VAT position on any quote, and whether access equipment is covered. Our figure arrives in writing after we have looked at the roof, and the photos from that visit show what we will treat and why.

Ask too about aftercare. A treatment keeps working for some time after the visit as the growth dies and weathers off, and our answer on how long roof biocide lasts covers the timing. For the wider job, see our roof cleaning and moss removal page. Send us a photo of the moss and a note of any past treatments, and we begin there.