After the clean
How long does roof cleaning last before moss returns?
No single period fits every roof, because the return of growth follows the roof rather than the calendar. Below: what decides it and what you can do to slow it.
The short answer
There is no fixed period for how long a roof clean lasts, and any firm that quotes one for every roof is guessing. Growth comes back when the surface is damp and shaded, so a roof under trees or in deep shade will turn green again faster than a sunny, open one. A clean removes what is there; it does not change the conditions that grew it, and we do not promise a time.
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What decides when growth returns
Moss, algae and lichen all need a damp surface. Anything that keeps a roof wet for longer shortens the gap before the growth is back, and anything that dries it quickly lengthens it. The same treatment on two roofs on one street can give two very different results.
| Factor | Effect on the roof |
|---|---|
| Overhanging trees | Shade and leaf litter keep it damp |
| Slope facing away from the sun | Dries slowly |
| Shallow pitch | Water drains slowly off the surface |
| Gutters that spill | Wet the lower courses |
Nobody can read all of those from the pavement, so the survey takes in the roof and everything around it. A roof under a mature tree has a different outlook from one that has nothing above it.
Look at how the roof sits in its setting. A roof close to a hedge or a wall may dry slowly on one side only, while a roof in open ground dries evenly. That is why what suits next door may not suit yours, and why a general promise from a cleaner is worth little.
The part treatment plays
A biocide is applied so that growth left on the surface is killed rather than just scraped. That gives the roof a clean start, but it does not make the surface unwelcome for ever. The product label sets out how it should be applied and in what weather, and we follow it.
The treatment is one part of the job. Clearing the gutters, cutting back branches and fixing a spill from a leaking joint all deal with the cause of the damp, which a spray cannot do. We are open about this because a quote that suggests a clean is permanent is making a promise nobody can keep.
Our answer on the roof biocide treatment sets out what it does and what it cannot do. A cleaned roof is also easier to inspect afterwards, so a small patch of returning growth can be caught early.
A good treatment also comes with instructions on the label: the maker states how to apply it and what to expect. We read that for each job, and we tell you what the product is meant to do on your roof, and what it is not.
What you can do to slow it
A few jobs around the house help a cleaned roof stay clean for longer. None needs a ladder on the roof itself.
- Cut back branches that shade or drop leaves onto the slope
- Keep gutters and downpipes flowing so the lower courses are not wet
- Check the slopes once a year from the ground with binoculars
- Share a photo when a new patch appears, rather than waiting for it to spread
Keeping watch is the cheap part. A small patch caught early can be dealt with in a short visit, and a roof checked regularly gives us a record of how fast growth returns on your house in particular, which is more useful than any general figure.
It also helps to know your own roof. A note of the date of the clean, and a photo of each slope afterwards, gives you a starting point. When growth appears again, a second photo shows how far it has come and how fast, and that is the information needed to judge whether another visit is worth it or can wait. A gutter check in autumn and spring keeps the lower slope drier as well.
Is it worth cleaning again?
That depends on what the growth is doing. If it is only a thin film on a sound roof, a second clean is a choice about appearance. If it is back in the valleys or the gutters, it is working against the roof again and is worth attention sooner.
Our post on whether roof cleaning is worth it sets out how to weigh that. Where a roof keeps growing moss quickly in spite of treatment, the real question is whether something else, such as shade or a leaking gutter, is the cause.
The quote for a visit is one total in writing, set after we have looked at the roof, along with what it needs and why. Our guide to roof cleaning cost explains how the price is built. The main roof cleaning and moss removal page covers the full job.
One more point: a clean repeated again and again on a roof shaded by a tree may be the wrong fix. Cutting the tree back, or moving the gutter, may do more for the roof and cost less over the years. We will tell you if that looks like your case.
More questions
Why has moss come back on my cleaned roof so soon?
Because the conditions that grew it are still there: shade, trees dropping leaves, a slope that stays damp, or a gutter spilling onto the tiles. The clean removed the growth but not the cause. A photo helps, and we can tell you what looks to be keeping it damp.
Should I have the roof cleaned on a regular schedule?
There is no set schedule we can give you. It depends on how quickly growth returns on your roof and what it is doing. A yearly look from the ground tells you more than a calendar date, and we can survey the roof whenever the growth starts to cause concern.
Tell us what's wrong with the roof.
You'll get one written total price.