Moss on concrete tiles versus slate roofs

Why concrete tiles grow so much

A new concrete tile has a smooth coloured face. Over years of weather that face wears away and exposes the rough, porous body of the tile underneath. Rough and porous is exactly what moss spores want: somewhere to lodge and somewhere that stays damp. The deep profile of many interlocking tiles also creates troughs and laps where grit, leaf litter and water gather. So a concrete tile roof of a certain age tends to grow thick cushion moss across the whole slope, not just in the shade.

This guide sits under our Roof Cleaning work, covering 20-mile radius of Plymouth.

Why slate grows less, and where it still does

Natural slate is dense and sheds water quickly, so it gives moss very little to hold on to. On a slate roof you typically see growth where debris collects rather than across the open face: along the ridge, in valleys, around chimneys, at the tail of each slate where they overlap, and on the shaded slope under trees. Lichen, rather than moss, is often the bigger issue on older slate, forming crusty grey patches that bond tightly to the surface.

How each is cleaned

On both coverings we remove heavy growth by hand first and bag it, then apply a biocide treatment across the roof to kill what is left and slow it coming back. The difference is in how gently we go. Concrete tiles lose their factory face under high pressure, which makes the next moss colony arrive faster, so they are scraped and treated rather than jetted. Slate is treated even more gently, because slates can crack under careless weight and the fixings on an old slate roof are not something to test. We use roof ladders that spread the load and only step where it is safe.

Stopping it coming back

Biocide treatment kills the spores so regrowth takes years rather than months. What decides how many years is mostly outside anyone's control: shade, tree cover, pitch and exposure. A shallow-pitched concrete roof under an oak will always grow faster than a steep slate roof facing south. Keeping the gutters clear helps both, because a gutter full of moss is a seed bed right at the eaves. On a concrete roof, avoiding pressure washing matters most of all; stripping what remains of the tile face is the quickest way to guarantee a greener roof in future.

Clay and other coverings

Clay tiles sit somewhere between the two: less porous than worn concrete, more textured than slate, and prone to orange and grey lichen on exposed slopes. They are also more brittle underfoot. The same principle applies: remove by hand, treat, and let the weather finish the job rather than forcing it.

Knowing which you have

If you are not sure what is on your roof, a photograph of the slope and one of the edge where the tiles meet the gutter usually tells us. Interlocking concrete tiles are thick and chunky at the eaves; slate is thin and flat. It changes how we plan access, how long the job takes and what result is realistic on the day.