In short
A light covering of moss is not damaging your roof, but a thick growth is: it holds water against the tiles, slows the run-off into the gutters and, when it slides off, blocks them. The NFRC's guidance says heavy moss that affects drainage should be carefully removed, by scraping rather than pressure washing. We clear it by hand from boards, clear the gutters, and can fit copper strip to slow regrowth.
This is a job for our Roof Cleaning & Moss Removal service.
What moss does to a roof
The NFRC's guidance note on moss and lichens is clear that the main effect of moss is that it holds water. Run-off into the gutters is slowed and the tiles stay wet for far longer than they should. In a Lincolnshire winter that wet tile then freezes. The same note says moss takes hold on rough or sanded tile surfaces, on low-pitched and north-facing slopes, and where trees overhang, which describes a lot of the bungalows we look at around Spalding, Bourne and Market Deeping.
The second problem is what happens when it comes off. Clumps of moss slide down into the gutter, block the outlet, and the gutter overflows down the wall behind the fascia. A good share of the "leaks" we are called to in autumn are a blocked gutter with a roof full of moss above it.
Leave it or clear it
- Thin patches on a steep, dry roof: leave it. It is cosmetic.
- Thick mats, or moss lying in the tile laps and the gutters: clear it. The NFRC says severe growth that affects drainage should be carefully removed.
- Moss on an asbestos-cement roof: the NFRC says never scrape or brush it. We will tell you if that is what you have.
Why we do not pressure wash
The NFRC does not recommend removing moss by pressure washing because it can force water into the roof space, particularly when sprayed upward from the ground. Marley, whose tiles are on a good share of the roofs round here, lists pressure washing among the things that shorten a tile's life, alongside walking on the roof and poor installation. A jet wash also strips the surface off a concrete tile, which is precisely the rough surface moss likes, so the roof greens up faster than before.
What we do instead
- We work from crawling boards or a properly supported ladder so our weight is spread and no tile takes a boot.
- The moss is scraped and brushed off by hand and bagged, not left in the gutters.
- We clear the gutters and downpipes, because that is where the moss ends up.
- If you want it to stay clean, we can fix copper strip along the ridge. The NFRC note explains that rain running over copper carries ions down the roof that inhibit regrowth.
- We look at the tiles while we are up there. A cleaned roof shows its cracked and slipped tiles, and it is cheaper to swap them on the same visit.
We do not paint or coat a roof after cleaning. The NFRC warns that surface treatments can invalidate the tile manufacturer's guarantee, and the photograph on our roof cleaning page, half moss and half bare tile on the same slope, is what a clean roof looks like without one.