What the treatment does
A biocide treatment is applied to the roof surface after the bulk of the moss has been removed, to kill off remaining spores and lichen that a wash alone won't fully remove. It's typically left to work over the following weeks rather than rinsed off immediately.
Why moss keeps coming back
Shaded, north-facing roofs, and any roof under or near trees, hold moisture for longer after rain, which is exactly the condition moss needs to establish itself. No treatment removes that shade or damp, so some regrowth over time is normal rather than a sign anything's gone wrong.
Lichen versus moss
Lichen shows up as flat, often orange, grey or yellow patches that bond tightly to the tile surface, rather than the soft green cushions typical of moss. It's generally slower-growing but can be harder to remove fully, and some installers use a different approach for heavy lichen than for moss.
Choosing between a one-off clean and ongoing treatment
If your roof is heavily shaded or backs onto trees, ask the installer whether a repeat treatment on a rolling basis makes more sense than a single clean. If it's mostly open to the sky, a single treatment may be enough for some time.
Frequently asked questions
Is the treatment safe for plants and pets?+
Ask the installer directly what product they use and how to keep pets and plants clear while it's being applied and while it dries.
Does treated moss need to be removed afterwards?+
Usually the dead moss is left to wash away naturally with rain over the following weeks, though some installers remove the bulk of it first. Ask what to expect.
How long before I'd need it treating again?+
It varies with how shaded and damp your roof stays. The installer who looks at your roof can give you a more specific idea.
Useful local links
- Portsmouth City Council – Planning and Building Control — Planning and building control guidance for Portsmouth, including Tipner and the rest of Portsea Island.
- Havant Borough Council – Planning Services — Planning services covering Havant, Waterlooville, Emsworth, Purbrook, Langstone, Warblington and Hayling Island.
- Fareham Borough Council – Planning — Planning information for Fareham, Stubbington and Funtley.
- Winchester City Council – Do I Need Planning Permission — Guidance for Southwick, Denmead and North Boarhunt, which fall within the City of Winchester district.
- Gosport Borough Council – Planning — Planning information for Gosport.
- Health and Safety Executive – Working at Height — Official guidance on the safety rules that apply to any work at height, including roof work.