Roof cleaning and moss removal in Basingstoke

What we find

Moss does not eat roof tiles, but it does change how a roof behaves. A mat of moss acts like a sponge. It soaks up rain and holds it against the tile surface long after the sky has cleared, so the covering stays wet for days instead of drying in a few hours. On concrete tiles, the most common covering on the post-war and 1980s housing around Basingstoke, that constant dampness slowly roughens and softens the surface skin of the tile. Moss roots then grip into the rough surface, and the cycle feeds itself.

The worst growth gathers where water lingers. We find it thickest along the lower edge of each course of tiles, in the shaded angle of a valley (the trough where two slopes meet), behind a chimney stack, and in the lap where one tile slides under the next. In those laps a thick cushion can block the path water is meant to take, and in a hard frost the water held inside the moss freezes and expands against the tile edge. Left for years, tiles on the north slope of a house can show a flaking, pitted face that scrapes off in your hand.

What we find

Gutters are where a mossy roof announces itself. Moss that dries out in summer breaks into crumbs, and rain washes those crumbs, along with the loose grit shed from weathered concrete tiles, down into the gutter. There they mix with leaves and dust into a dense, dark paste. The paste sits in the bottom of the run, grows its own moss, and sprouts the seedlings we often find standing in gutters above a front door.

A gutter choked like this stops doing its job. Water that cannot get to the downpipe spills over the front edge, runs down the wall below and can soak the fascia board behind the gutter, which is the timber board along the eaves that the gutter fixes to. The overflow can also get behind the bottom course of tiles and into the roof timbers at the eaves. The weight matters as well. A gutter full of wet sludge is much heavier than an empty one, and old plastic brackets can bend or snap under it.

What we find

People ring asking for a pressure wash because that is what they have seen on a driveway. For a roof, it is rarely the first thing we reach for. Each method does a different job. Hand scraping removes thick growth and is gentle on the tile. Biocide kills what is left and slows regrowth. Washing shifts the surface staining and the loose film of dead growth.

The right choice depends on the covering and on how bad the growth is. A modern concrete tile in good order will tolerate a careful low-pressure wash. An old clay tile, a hand-made tile or a thin slate may crack, shed its surface or have its nibs and nail holes strained by water forced under the laps. A roof with a thick moss blanket needs scraping first whatever else follows, because washing thick moss just mashes it into the laps. The cheapest-looking quote often leaves out the step that does the real work, which is why two quotes for the same roof can differ widely.

What we find

A tile is made to shed water running down it, not water fired up at it. A jet aimed from below the lap can drive water up and under the tile, into the batten space and onto the underlay, the waterproof sheet that lies beneath the tiles. On a sound roof the underlay copes with a small amount of that. On an older roof, where the underlay has split or was never fitted, the water goes straight through to the loft insulation and ceiling.

Pressure also damages the tile face. Concrete tiles are coloured by a thin surface layer, and a hard jet cuts through it, leaving a pale, rough, thirsty surface that holds moss better than before. We have seen roofs that were cleaned hard and looked bright for one summer, then greened faster than the neighbour's roof that was never touched. On clay and slate, a hard jet can chip edges, loosen pegs and lift weathered tiles that were held in place mainly by their own dirt. The runoff matters too. A jet sends moss, grit and chemicals everywhere at once, into flower beds, patios, neighbours' windows and car roofs.

What we find

Moss is not the only thing that grows on a roof. Black or grey streaks running down from the ridge are usually algae, a thin film that forms on damp, shaded surfaces and darkens with age. Grey-green and orange crusts, flat against the tile like spilled paint, are lichens: a partnership of an alga and a fungus that grows slowly and clings hard.

Streaks are mostly a matter of looks. They tend to appear first on the slope that stays wet longest and spread as the film thickens. Lichen is different. It grips the surface with fine root-like threads that get into pores in the tile, and it is much harder to remove than moss. If it is scraped off dry, it often takes a layer of tile with it, leaving a pale, scarred patch. Orange and yellow lichens love the nutrients in bird droppings and in dust from nearby roads, so we often see them on ridges where gulls and pigeons perch, and on roofs facing a main road or a field.

Neither is a sign that the roof is failing. They do suggest a surface that stays damp, so we look at why.

What we find

Walk around almost any house in Basingstoke and you will find the same pattern. One slope is green, the other is clean. The green side faces away from the midday sun, so it dries slowly after rain, and moss and algae need constant dampness more than they need anything else. The effect is stronger where a neighbouring house, a tall hedge or a tree throws shade for much of the day.

Trees add a second problem. Overhanging branches drop leaves, seeds and twigs onto the roof, and the debris collects at the foot of each slope and in valleys, where it rots into a feeding bed for moss. Branches that touch the tiles also rub the surface and keep the area beneath them damp. On estates where gardens were planted decades ago, trees that were once small now brush the roofline, and the shade has deepened with them.

The older Victorian and Edwardian streets of the surrounding towns show the same thing on clay and slate: green on the shaded roof pitch, and clean tile on the sunny side. The covering itself is rarely the cause. The shade, the trees and the damp around the roof are.

What we find

Cleaning is the right job for a roof that is sound but dirty. It is the wrong job for a roof that is worn out, and the trouble is that moss hides the difference. Under a thick mat you cannot see a slipped tile, a cracked one, or a tile whose surface has crumbled to nothing. Once the moss is off, the roof is bare and you may find what it was concealing.

The roofs we are most cautious about are the old ones: clay tiles with softened faces, slates that have delaminated into thin layers, and 1960s concrete tiles that have thinned with weathering. Another warning sign is the ridge. If the mortar bedding under the ridge tiles has cracked or fallen away, those ridge tiles may be loose and will not tolerate scraping. Inside the loft, daylight through the roof, dark water stains on the rafters or damp insulation tell you the covering is already letting water in, whatever it looks like on top. Cleaning it would not stop the leak, and it would cost money that belongs in the repair.