Green roofs look tired, so most people assume moss must be doing harm. The truth is more mixed than the sales pitches suggest. Some moss is harmless, yet some of it does real damage over time.
What the roofing industry says
The roofing trade is more relaxed about moss than many cleaning firms, although that rarely makes headlines. Marley, a major UK tile maker, says low to moderate growth is a natural part of ageing on many pitched roofs. It adds that under normal circumstances this is not harmful to tiles or slates.
The NFRC, the trade body for roofing contractors, agrees. It says moss is rarely a problem and only needs to come off if it blocks gutters, outlets or other drainage points. Since we clean roofs for a living, we think you should hear that straight from the sources.
When moss starts to cause harm
Thick moss is a different story. Marley explains that heavy build-up slows the flow of water off the tiles, so the roof stays wetter for longer. As a result, leaks become more likely and the tiles can wear out faster.
Wienerberger, which makes clay tiles, describes the same mechanism. Moss absorbs moisture and holds it, so clay tiles stay damp and can start to delaminate. Thick layers also add weight to the roof, although the company gives no figure for it.
Three problems to watch for
Not every mossy roof has a problem, although some do. These three signs tell you that growth has gone too far:
- Moss packed into the laps, where tiles overlap, holding water in place.
- Thick growth along the lower edge of a slope, right above the gutter.
- Moss and debris clogging the gutter or the downpipe.
Gutters deserve special attention, because they carry the water away. Marley’s maintenance advice says blocked guttering and drainage pipes can cause flooding, which leads to damp and damage to the roof. Moss that sheds from the slope often ends up in the gutter, so the two problems feed each other.
Moss on the roof versus moss in the gutter
These are separate jobs, and they have different fixes. For example, clearing a gutter is routine work, whereas treating a whole roof is a bigger step. Our gutter clearance service deals with the first one on its own.
If the roof is only lightly mossy, clear gutters may be all you need. However, if moss is thick and the tiles stay wet for days, treatment starts to make sense. That is where a proper plan beats guesswork.
Why moss grows in the first place
Moss grows where it can hold on and stay damp. Wienerberger says moss and lichen thrive when dirt and debris settle on tiles, for example leaf litter and dust. The NFRC adds that rough tile surfaces trap dirt and spores, and that concrete tiles are particularly prone to it.
Shade slows drying, so a roof under trees tends to green faster. That said, no source gives a fixed timescale, because every site differs. Our post on how often a roof needs cleaning explains how to judge it for your own house. Cutting back overhanging branches still helps, and you can do it from the ground. Our post on DIY roof moss removal lists other safe jobs for homeowners.
What not to do about it
Never reach for a pressure washer, because it can do more harm than the moss. Marley says most roofing professionals do not recommend it, and the NFRC warns that power washing can damage a roof, especially with foot traffic on top. Wienerberger adds that the spray can strip colour from concrete tiles.
Nor should you climb up to scrape it off. Marley says homeowners should not access the roof themselves because of the inherent risks. We explain why in whether roof cleaning needs scaffolding, which covers HSE guidance on working at height.
How we treat moss
Treatment kills the growth rather than scraping it off, because spores left behind can regrow. Our moss removal service combines manual removal where needed with a low-pressure biocide treatment. Wienerberger cautions that anti-moss products are not permanent, so we back our work with a 5-year regrowth protection guarantee and advise regular checks.
Since many readers ask, we offer roof moss removal in Dorking, roof moss removal in Farnham and roof moss removal in Leatherhead. We also provide roof cleaning in Crowborough and roof cleaning in Lewes. Our services page shows the full list of what we do.
If your home is old or protected
Some roofs need extra care, because older tiles are fragile and the building may carry legal protection. Read our post on roof cleaning on listed buildings and in conservation areas before you book any work.
Common questions
Should I remove all the moss from my roof?
Not necessarily, because Marley advises leaving light growth alone unless the roof leaks or the gutters block. Heavy growth that holds water is the case where removal pays off.
Does moss cause leaks?
On its own it rarely does. However, a thick mat can keep tiles wet and hide cracked ones, which makes a leak more likely. A roofer can tell you whether a leak comes from the tiles or elsewhere.
How do I know how thick is too thick?
Look at the laps and the lower edge of each slope. While light growth can stay, if moss is deep enough to hold water, or you see it spilling into the gutter, it has passed the point of leaving it alone.
If you are unsure, you can request a free quote. We will look at the roof and tell you honestly whether it needs treatment yet.