Roof Moss Removal and Roof Cleaning in South London

The moss itself is not the problem. What it does with water is. A saturated mat behaves like a sponge held permanently against the head of the tile and up inside the lap, so that part of the roof never dries out. Water sitting in a porous tile expands by about nine per cent when it freezes, and the face of the tile comes off a flake at a time.

The bigger damage happens lower down. Clumps break off in wind and heavy rain, wash down the slope and pack the gutters, the hopper heads and the valleys. That is where most moss-related water damage comes from — not through the tiles, but over the back of a full gutter and down the wall, or backed up in a valley until it passes the lead. On a Victorian butterfly roof, with its central valley gutter hidden behind the front parapet, a packed valley puts water straight into the roof space.

A pressure washer takes the moss off in minutes. It takes the surface of the tile with it. Concrete tiles are made with a fine cement-rich face that carries the colour; a lance strips that back to the aggregate and leaves a rougher, more porous tile than the one you started with. The moss then returns faster and thicker, because it has been given a better surface to grip.

The second problem is where the water goes. A roof sheds downwards and relies on the head lap between courses to stop water travelling back up. A jet aimed uphill defeats that lap and puts water onto the underlay. On an older roof with perished felt underneath, that is how a clean becomes a ceiling stain a fortnight later. On slate it breaks the fragile edges and works the fixings loose.

Several tile manufacturers exclude pressure-washing damage from their warranty, which tells you what the trade thinks of it. Jet washing has its place on a patio, a path or a driveway. A roof is not one of them. We scrape by hand and brush down instead, which takes longer and leaves the roof in the condition it was in before the moss arrived rather than worse.

The order matters. There is no point treating a roof before the mat is off it, or clearing the gutters before the debris has come down. The work runs the same way every time.

Once the mat is off, the roof still carries what is left in the surface: spores, root and a film of algae that no brush reaches. A biocide deals with that. Most roof biocides sold in the UK are built on a quaternary ammonium compound, applied dilute and left to work. Ask which product we are using and for its safety data sheet before anything goes on; it belongs in the quote.

It is not instant. The treatment kills what is left over days and weeks, and the dead residue weathers off with the rain over the following months. A roof still looking patchy a fortnight after a clean is behaving normally. It is also not a coating and not waterproofing. It changes nothing about how the roof sheds water.

Expect three to five years on a shaded slope before it needs looking at again, and longer on an open, sunny roof. Two things extend that more than any product: cutting back whatever overhangs the roof, and keeping the gutters clear so the bottom courses can dry between showers. A strip of copper or zinc along the ridge helps as well — rain carries trace metal down the slope and suppresses regrowth — though it works on the top third of a slope and does little by the time it reaches the eaves.

Worth doing: a concrete or clay tile roof with a real mat on it, gutters that fill every autumn, a valley you cannot see the bottom of. Every three to five years on a shaded South London rear slope is a sensible rhythm, and it is cheap set against the gutter, fascia and damp work that follows if it is left.

Not worth doing: a slate roof with a green tinge, unless you simply want it gone for the look of it. Nor a roof you already know is being replaced within a couple of years — hold the money for the re-roof. And if the reason you are asking is water coming in, a clean is not the answer. That is a repair, and the leak wants finding first.

We do not sell a roof coating after a clean, and we would advise against buying one. A sprayed acrylic seals a tile that needs to dry out, bridges the laps so a single tile can no longer be lifted for a repair, and does nothing about the nails and the underlay, which are what actually end a tiled roof. Surveyors tend to treat a coated roof as something with a problem hidden under it, which can cost you at a sale. If there is money left after a clean, the gutters or a tired flashing is the better place for it.