Look up at almost any street in Bridgend, Pencoed or Porthcawl and you will see roofs with a green tinge along the lower courses. Homeowners are often told this is an emergency that needs fixing today, and just as often told to ignore it completely. The truth sits in the middle, and it depends on what your roof is made of and where the moss has taken hold.
Moss has no roots that bore into tiles. It sits on the surface and holds moisture, so on its own it does not eat through a concrete tile or a slate. The damage it causes is indirect, and that is where the real risk lies.
The two problems worth taking seriously are water and blockages. A saturated moss cushion sitting against the head lap of a tile can wick rainwater sideways and upwards, past the point the tile was designed to shed it, particularly on shallow pitches under about 25 degrees. And when clumps break away in a gale off the Bristol Channel, they end up in your gutters, hoppers and valleys. A blocked valley on a semi is a far more expensive problem than the moss ever was.
Bridgend typically sees somewhere in the region of 1,100 to 1,400mm of rain a year, spread across a lot of damp, mild days rather than a few downpours. Moss does not need much beyond moisture and shade, and our winters rarely get cold or dry enough to kill it off.
North and north-east facing slopes are almost always the worst, because they get the least sun to dry out. Add overhanging sycamore or leylandii, common on the older estates and along the valley sides towards Maesteg, and you have ideal conditions. Concrete interlocking tiles from the 1960s to 1980s hold moss far more readily than Welsh slate, because the surface is porous and the sand facing weathers off over a few decades.
If the moss is a light green film on the tile faces and your gutters run clear, there is no urgency. Cleaning it purely for appearance is a fair reason, but it is not maintenance.
Act when you see thick cushions sitting proud of the tiles, moss packed into the interlocks or valleys, gutters overflowing in heavy rain, or clumps regularly appearing on the drive. On a slate roof, moss growing along the nail line matters more, because it keeps the fixings permanently wet and accelerates nail sickness. If the roof is already nearing the end of its life, with slipped or delaminating slates, cleaning is money better spent towards recovering it.
Never let anyone pressure wash a tiled or slate roof. A jet washer strips the protective surface from concrete tiles, forces water under the laps and can shift slates off their nails. The damage is often not visible until the following winter, and it is the single most common way a cheap roof clean turns into a repair bill.
The safe method is scraping and brushing by hand from a scaffold tower or ladder with a stand-off, working downwards, then clearing the gutters and valleys properly afterwards. A biocide treatment applied after cleaning will kill the remaining spores and slow regrowth, usually buying two to four years before it starts to return. Anyone working on your roof should be insured and working from a proper access platform, not standing on the tiles.
For a standard three bedroom semi in the Bridgend area, hand cleaning with gutter clearance and a biocide treatment tends to land somewhere between roughly £350 and £900, depending on access, pitch, roof size and how much scaffolding is needed. Detached properties and anything needing a full scaffold sit higher. Anyone quoting a flat price without seeing the roof is guessing.
Be cautious of doorstep callers who claim they have spotted urgent problems from the pavement, and of anyone recommending a sealant or roof coating on the back of a moss clean. Coatings applied over damp or ageing concrete tiles frequently trap moisture and are rarely a genuine alternative to repair. If you are unsure whether the moss on your roof is cosmetic or causing harm, ask for photographs from roof level before you agree to any work.
Rarely on its own, but it can. Moss packed into valleys, interlocks or gutters holds water where the roof was designed to shed it, and that is what eventually gets inside.
No, that is the part that genuinely needs doing. Blocked gutters and hoppers cause damp fascias, soffits and wall heads, and clearing them once or twice a year costs a fraction of repairing that damage.
In this climate, a properly cleaned and treated roof usually stays reasonably clear for two to four years, less if it is north facing or under trees. Annual gutter clearance matters far more than repeated full cleans.
Got a roof job in mind after reading this? Give Paul a call or send a photo, and you will get a straight answer and a free quote.