What Is Roof Biocide Treatment? And How Long It Lasts

It is a solution applied to the roof that kills moss, algae and lichen, including the spores too small to see, and stops them re-establishing. It is what makes a clean last for years rather than months. It is a treatment, not a pressure wash.

No. A proper roof biocide is a dedicated treatment that kills moss, algae and lichen at the root and keeps working for weeks after it is applied. The strong household bleach some cheap operators spray can lighten or patch the colour of the tiles and does little to stop the growth coming back.

Yes. The dark streaks on a roof are usually algae and trapped grime rather than moss. A biocide kills the algae at the root, and the rain carries the dead growth and staining off over the following weeks, with no pressure washing needed.

It soaks into the surface and kills the growth at the root. It then keeps working over the weeks after it is applied, as rainfall carries it across the roof and the dead growth weathers away, so the result builds rather than appearing instantly.

Two to five years on most roofs, and longer on a sunny, well-drained one. A shaded, north-facing roof under trees needs treating more often, so we usually suggest a light re-treatment every two to three years.

Yes. Moss and algae trap moisture against the tiles, and over the years that can shorten their life and dull their colour, so keeping the roof clear with a biocide helps protect both. And because it is a gentle treatment rather than a jet wash, it keeps your tile guarantee intact, where pressure washing can void it.

Yes, but lichen is the stubborn one. Moss and algae usually clear within weeks, while the crusty grey lichen can take a few months to weather off and the odd patch needs a second light treatment. That is normal rather than a sign it has not worked.

Yes, when applied correctly. We protect plants and surfaces, keep it out of watercourses and drains, and ask that pets are kept off the treated area until it has dried. Tell us if you have a pond and we take extra care around it.

No, and it is the opposite approach. Pressure washing blasts the roof and can strip the tile surface and force water inside, which is why the NFRC and tile manufacturers advise against it, and several manufacturers warn it can void the tile guarantee. A biocide treats the roof gently and reaches the same clear result without the damage.

Yes, in approach. Soft washing means cleaning a roof at low pressure and letting the treatment and the weather do the work, rather than blasting it. A biocide treatment is exactly that: no high pressure, just the treatment killing the growth and the rain carrying it away over the following weeks.

It depends on how much growth there is. Thick, established moss has to be removed by hand first, then treated. On light growth, or a roof we have already cleaned, a biocide on its own is enough.

We would not recommend it. Beyond the risk of working at height, a biocide has to be applied at the right strength and kept out of watercourses, and the trade bodies advise homeowners not to get up on the roof at all. The safest check you can do yourself is a look from the ground.

Usually not the same day. The treatment works over the following weeks as rain carries it across the roof and the dead growth weathers off, so the roof keeps getting clearer for a month or more after we have been.

A biocide-only treatment is £5 per square metre, and a full clean of hand removal plus biocide is £10 per square metre. Rates include materials and labour and are shown before VAT. See our roof cleaning page for the full rate card and worked examples.

Spring and early autumn are ideal, when the milder, drier weather gives it the best start. It can be applied at most times of year though, and it carries on working over the weeks afterwards whatever the season.