Problem guide

Why the moss came back, and how to stop it happening again

Few things annoy a homeowner more than paying for a roof clean and watching the green return within a year or two. It is a common complaint and it usually has a specific, identifiable cause rather than bad luck.

This guide runs through the reasons moss re-establishes quickly, how to tell which one applies to your roof, and what to do differently next time.

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Reason one: no treatment was applied

Scraping moss off removes what you can see. It does not remove the spores and root residue embedded in the pores of the tile surface, and those regrow quickly with the surface now clear and open to rain.

This is the single most common reason a clean does not last. A mechanical clean with no biocide stage is a cosmetic job with a short shelf life, and it is often why one quote was cheaper than the others.

Reason two: the roof was pressure washed

A pressure lance removes the weathered surface layer of a concrete tile along with the moss. What is left is more porous and holds more water than the tile did before, which makes it a better host for the next colony.

Roofs that have been jet washed repeatedly tend to green up faster after each clean. That is not a coincidence, and it is why we clean by hand and treat instead.

Reason three: the conditions were never changed

Overhanging trees keeping the slope permanently damp and dropping organic matter onto it.
Gutters still packed, holding water against the bottom courses.
Valleys and areas behind the chimney stack left full of debris that composts into a growing medium.
A north-facing slope that never sees direct sun and takes days to dry after rain.
Neighbouring conifers or hedging within a couple of metres of the eaves line.

Reason four: only part of the roof was done

It is surprisingly common for a rear slope, a garage roof or an extension to be skipped because it was harder to reach. Untreated areas act as a reservoir: spores wash down and across onto the cleaned sections with every rainfall.

The same applies where a mossed main roof sits above a cleaned conservatory or porch. Clean from the top down, or you clean the lower parts twice.

How long a clean should actually last

A hand clean followed by a proper biocide treatment, on a roof with clear gutters and reasonable light, should hold its appearance for several years rather than months. Our full roof cleans carry a written 24-month biocide guarantee, which is the period we will stand behind rather than an open-ended promise.

Heavily shaded roofs under mature trees are the honest exception. There, the sensible plan is a full clean followed by a maintenance treatment every two to three years, which is much cheaper than repeating the full job.

What to do next

Work out which of the reasons above applies to your roof before spending anything. If it was never treated, a treatment on the existing light regrowth may be all that is needed and it will cost far less than another full clean.

If the gutters are full or the trees are overhanging, deal with those first. Cleaning again without changing the conditions simply buys the same short period a second time.

We are happy to look and tell you which category you are in, including telling you that you do not need a full clean yet.

Our own photographs

Images from Sky Scrapers Ltd work showing the methods described on this page.

a roof slope part cleaned, showing the untouched moss alongside, relevant to why the moss came back, and how to stop it happening again

a moss-covered pitch photographed part way through hand cleaning, relevant to why the moss came back, and how to stop it happening again

a roof section showing the difference between treated and untreated tiles, relevant to why the moss came back, and how to stop it happening again

Where this applies locally

Towns where this comes up most often on our patch:

Questions we get asked

How soon should moss return after a proper clean?

Not for several years on a treated roof with clear gutters and reasonable light. Regrowth within a year usually means no treatment was applied.

Can you treat a roof someone else cleaned badly?

Yes. If the growth is still light we can often treat rather than clean again, which costs considerably less.

Does pressure washing really make it worse?

Over time, yes. It opens the tile surface, increasing porosity and giving the next colony a better foothold.

Will cutting back trees fix it on its own?

It will slow the growth significantly by letting the roof dry faster, but existing growth still needs removing and treating.

Is a maintenance treatment cheaper than a full clean?

Considerably, because there is no scraping labour or waste removal involved. It is the sensible option on shaded roofs.

Get it done so it lasts

Hand cleaning, gutters cleared and a written 24-month biocide guarantee on full roof cleans.