Moss on a roof: what it means and what to do about it
Usually, no
Most moss on a UK roof is doing no structural harm and can be left alone. It matters when it is thick enough to hold water against the tiles, when it is blocking gutters or valleys, or when it is on a slate roof where it lifts the slates. What causes real damage is the removal: a pressure washer strips the surface off concrete tiles and drives water under slate, and that turns a cosmetic problem into a leak.
Roof moss removal is sold as a job that needs doing every few years. Sometimes it does. Often it does not, and the removal does more harm than the moss ever would.
Moss itself is not the problem. It is a symptom, and what it tells you is that part of your roof stays damp longer than the rest. That is worth reading properly, because the same conditions that grow moss are the ones that rot battens and freeze into mortar.
This is what moss actually does, how to tell whether yours matters, and what to avoid if you do have it taken off. Some of it costs me work. It is still the right advice.
Read the symptom
Why it grows where it grows
Moss needs damp and shade, so it appears on the north-facing slope first, then under overhanging trees, then along the bottom courses where the roof stays wet longest after rain. If yours is evenly spread across a south-facing slope in full sun, something is holding water up there that should not be.
It grows on concrete tile more readily than on slate, because concrete is porous and its surface roughens with age, which gives spores somewhere to sit. A twenty year old concrete interlocking roof with moss on it is behaving exactly as expected. A ten year old one is telling you something about drainage or shade.
Leeds gives it a good run. Wet autumns, a long damp winter and enough mature trees through the older suburbs mean most roofs here carry some, and a roof with none at all in Headingley or Meanwood is usually one that has been treated recently rather than one that is somehow better built.
When it is worth acting on
Five times moss actually matters
- It is holding water against the tiles
- A thick mat stays wet for days after rain. On concrete tile that accelerates surface erosion, and in a hard frost the water in it expands and takes the face off.
- It is in the gutters and valleys
- This is the real cost of moss, and it is not on the roof at all. Clumps break off, block the gutter, and the overflow runs down the wall instead. Most damp patches blamed on the roof start here.
- It is lifting slates
- Moss growing under the tail of a slate can lift it enough to break the lap. On slate roofs that is worth acting on rather than watching.
- It is bedded into mortar
- Moss in the ridge or verge mortar means the mortar has already gone porous. The moss is the flag, not the fault, and pointing is the actual job.
- You are selling
- A mossy roof reads as neglect to a buyer and to a surveyor, whether or not it is doing harm. That is a presentation reason, and it is a legitimate one.
How it comes off
Roof moss removal methods, and what each does to the roof
On a dry day, worked from the ridge down, with a stiff brush or a scraper shaped to the tile profile. It is slow and it is the only method that takes the moss off without taking anything else with it. Gutters get cleared afterwards, because most of what comes off ends up in them.
A fungicidal wash kills the growth and the roof sheds it over the following months in the rain. It looks like nothing has happened for weeks, which is why people distrust it. It is the gentlest option and the one that lasts longest, usually applied after a scrape rather than instead of one.
Rain washes traces of metal down the slope and discourages regrowth. It works slowly, it works best on the courses nearest the strip, and it does nothing to moss that is already there.
It is fast, it looks dramatic, and it is the reason a lot of roofs need work afterwards. On concrete tile it strips the surface and shortens the life of the tile. On slate it drives water under the laps and into the roof. The NFRC warns about it and so does Google's own summary of this question. If somebody quotes to jet wash your roof, that is worth knowing about them.
Diluted bleach does kill moss. It also runs off the roof onto whatever is below it, which is usually a garden, and it can mark the tiles unevenly. A proper roof biocide costs little more and is made for the surface.
Afterwards
Keeping it from coming back
Nothing keeps a roof moss free permanently in this climate, and anyone promising that is selling a treatment cycle. What you can do is slow it down and stop it mattering.
Cut back whatever overhangs the roof, because shade is the single biggest factor. Keep the gutters clear so the bottom courses drain and dry. Have the ridge and verge mortar looked at, since porous mortar is where moss gets a foothold that is hard to remove. If the roof is being worked on anyway, that is the moment for zinc strips, not a separate visit.
Then leave it. A light growth on a north slope is not a job. Treating a roof every two years because a leaflet came through the door costs more over a decade than the moss ever would.
Related: what moss removal costs, theroof cleaning and moss removal I do around Leeds, andguttering, which is where most of the damage blamed on moss actually happens.
Not sure whether yours needs doing?
Send me a photo of the roof. I will tell you whether it is worth touching, and I will say so if it is not.
Call 07770 902292Roof moss: common questions
Is moss on a roof bad?
Usually not, in itself. Most moss on a UK roof causes no structural harm and can be left. It matters when it is thick enough to hold water against the tiles, when it breaks off and blocks gutters or valleys, when it lifts slates, or when it has bedded into ridge mortar that has already gone porous. In that last case the mortar is the problem and the moss is just how you spotted it.
What is the fastest way to remove moss from a roof?
Scraping it off by hand on a dry day, working from the ridge down with a brush or a scraper shaped to the tile. Faster methods exist and they are the ones that cause damage. A pressure washer clears a roof in an afternoon and can take years off the tiles doing it.
Can I pressure wash moss off my roof?
You can, and it is the main way roofs get damaged by cleaning. High pressure strips the surface off concrete tiles, which shortens their life, and on slate it forces water under the laps and into the roof. The National Federation of Roofing Contractors warns against it. If a quote proposes jet washing, ask what they are doing about the water that goes under the tiles.
What kills moss on a roof?
A fungicidal roof biocide, applied and left to work. The roof sheds the dead growth over the following months in the rain rather than coming clean on the day, which is why people assume it has not worked. Diluted bleach also kills it, but it runs off onto whatever is below and can mark tiles unevenly.
What time of year should moss be removed?
Late spring through summer, on a run of dry days. The moss needs to be dry to come away cleanly, and biocide needs time on the roof without heavy rain straight after. Doing it in autumn means working wet and putting a scraper across a roof that is already slippery.
Should I remove moss from my roof before selling?
It is worth doing for presentation. A mossy roof reads as a neglected house to a buyer and gets flagged in a survey even when it is harmless, and that costs more in negotiation than the clean does. Have it scraped and treated rather than jet washed, because a survey will also pick up damaged tiles.
Sources
- National Federation of Roofing Contractors, guidance on moss and power washing
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