Why Do Concrete Roof Tiles Get So Mossy?

A concrete tile is made from sand and cement, pressed and cured, then usually finished with a coloured coat. Look closely at one and you will see it is not smooth like glazed pottery. The surface has tiny pits and a gritty texture, and the material underneath can soak up a little moisture. After rain, a thin damp film lingers in those pits long after the sky has cleared.

Moss spores travel on the wind and land on every roof in the street. Most of them fail. On a surface that dries quickly they have nowhere to take hold, but in a damp, gritty pit they find water, a rough grip and a little dust or leaf litter to feed on. Within a season or two, a green fuzz appears, and the roots of the moss then hold even more water against the tile.

That is why two neighbours with the same roof can have quite different results. The difference is usually how long each tile stays wet, not how well the house is kept. A south-facing, windy slope dries quickly, while a sheltered one beside a tall hedge stays damp and greens up first.

New concrete tiles carry a smooth, tinted finish that sheds water fairly well. Over the years sun, frost and rain wear that finish thin, and the textured body of the tile shows through. Marley gives roughly 60 years as a typical span for concrete tiles, though it adds that the figure carries no warranty. By the time a roof is well into that span, the weather has had a long time to roughen it, and the tint has often faded unevenly across the slope.

On an older roof the surface is often well worn, so moss and algae find plenty of purchase. The way to find out how yours stands is to look: a roofer on a ladder or tower can see whether the colour coat is intact, patchy or gone.

A worn tile is not a failed tile. Plenty of weathered concrete tiles still do their job well. They simply need to be kept clear of growth, because a thick mat of moss keeps the tile soaked and can pile up in the overlaps and the gutter.

On almost any roof, one slope turns green first, and the cause is nearly always how long it stays wet. A slope that faces away from the midday sun, sits under a tree or runs next to a taller building dries slowly, so a film of water sits there for hours more than on a sunny slope. Leaves, twigs and seeds that settle in the overlaps add a nutrient layer that moss loves.

Low-pitch roofs hold water too, since rain runs off them more slowly. The joints where one tile locks into the next are another favourite spot, because the interlocking profile creates small channels where moisture and debris collect. If you see moss creeping up from the gutter line, check whether the gutters are overflowing or sitting full of silt, because a constantly damp eaves course will go green first.

Our answer on green algae on a roof covers the thinner green film that often turns up before the moss itself.

The usual fix is two steps: take off the growth, then treat the surface so it does not return quickly. Moss is lifted away by hand, with brushes or low-pressure washing, and the tile is then treated with a biocide, a product authorised to kill living growth. Our guide on how biocide roof treatment works explains what happens after the spray goes on.

Be wary of anyone who offers to blast your tiles clean. Marley advises against powerful jet washes, because they can scar the face of a tile and cut short its working life. On concrete the blast can strip the very coat that keeps the surface smooth, which leaves it rougher and greener sooner. Our crews work at gentle pressure and rely on hand work and treatment.

It also pays to look for cracked or slipped tiles while the roof is clean, since moss can hide them. Once we have been up and looked, you receive a single written total, backed by pictures of the roof's present state. Cracked or slipped pieces are far easier to spot on a clean surface, and swapping them is a small job compared with water reaching the loft. The main page for this work is roof cleaning and moss removal.