Between the sea air off Swansea Bay and the steady rain rolling in over the Gower, moss grows very well on local roofs. Some of it is harmless, but thick growth can block gutters and shorten the life of your tiles. Here is how to tell whether yours needs attention and how to remove it without doing more harm than good.
A light green haze on a sound roof is mostly cosmetic. It becomes a problem once it forms thick cushions. These hold water against the tiles like a sponge, and in a cold snap that water freezes and expands, which can crack concrete tiles and lift slates. The cushions also break off in clumps that wash into gutters and downpipes, so you get overflows and damp patches on the walls below.
In Swansea we see the worst growth on north-facing slopes, on roofs shaded by trees in areas like Sketty and the Uplands, and on exposed coastal homes around Mumbles and Langland where the roof rarely dries out properly.
The method we recommend is slow and unglamorous. First, scrape the moss off by hand with a plastic or wooden scraper, working down the roof so debris doesn't get forced up under the tile laps. Next, clear the gutters and downpipes. Then apply a biocide treatment that kills the remaining spores and roots. The biocide carries on working for months afterwards, and the tiles keep getting cleaner as the rain washes the dead growth away.
Biocide needs a dry spell to soak in, usually at least 24 hours without rain. Swansea's weather makes that the hardest part, so late spring and early autumn are usually the best windows. Cover ponds and delicate plants, and don't wash scraped debris into surface water drains.
Jet washing looks impressive in before and after photos, but it is one of the most common causes of roof damage we're called out to. High pressure strips the protective surface off concrete tiles, so they become more porous and moss comes back faster. It can also crack slates, dislodge ridge mortar and drive water under the tiles into your loft insulation.
Be cautious of doorstep traders offering same-day jet washing and a roof coating. Many coatings are unnecessary, and some trap moisture or make future repairs harder. If anyone knocks unannounced, get a second opinion before agreeing to anything.
Falls from roofs and ladders cause serious injuries every year, and wet moss is extremely slippery. If you're thinking about DIY, stick to what you can reach safely: gutters from a properly footed ladder, or a single-storey extension roof with a low pitch. Steep two-storey roofs need proper roof ladders or a tower, and on many homes scaffolding is the safe option.
Some homes built between the 1960s and 1980s have fibre cement slates that may contain asbestos. Never walk on them, scrape them hard or jet wash them. Get them looked at first.
Prices depend on roof size, pitch, how thick the moss is and what access is needed. As a rough guide, hand cleaning and a biocide treatment on a typical Swansea semi often falls somewhere between £400 and £900. Larger detached homes or roofs that need full scaffolding can cost more. A good roofer will also point out any slipped tiles or tired ridge mortar while they're up there, so you can deal with small faults before they turn into leaks.
Most roofs here benefit from a biocide treatment every 3 to 5 years, or more often on shaded or coastal properties. Keeping the gutters clear between treatments makes a big difference.
Yes, eventually, because spores are always in the air. A biocide treatment after hand removal slows regrowth considerably, often for several years.
On a low single-storey roof you can sometimes reach it safely, but you still need to clear the thick cushions and gutters afterwards. On taller roofs the spray won't cover evenly from the ground, and the dead moss can block your drainage.
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.