Can you use moss killer on a roof yourself?

Moss killer is a biocide: a chemical made to kill living growth. Some products are sold for paths and patios, some for roofs, and the label says which. A patio product on a roof can stain a tile or leave a residue you cannot see until the next rainstorm turns it white, so begin by reading what the manufacturer says about roofing surfaces, dilution and how long to leave it.

The label also tells you what to keep it away from. Plants, ponds, fish, pets and a neighbour's washing are all in the line of fire when a spray is lifted by a breeze. A product that is fine for a shed roof at the bottom of the garden may be a poor choice over a conservatory or beside a greenhouse. Treat the label as the instruction, not the advice of a shop assistant, and stick to it.

A garden sprayer on a pole reaches a few rows up the slope. Moss does not stop at the first few rows, and the growth that is out of reach carries on dropping spores onto the treated area. The result is a roof with a clean-looking band along the bottom and a thick green mat above it, which looks worse than doing nothing and gives a false sense that the problem has been dealt with.

Getting higher means a ladder or a roof access system, and this is where the real risk lies. A ladder leant against a plastic gutter is unstable, damp moss is slippery, and a roof that looks sound can break under a foot placed on an old tile. If a treatment cannot be done from the ground or from a properly secured platform, it belongs with someone equipped for work at height. Our page on roof cleaning and moss removal explains how that access is arranged.

Dead moss does not vanish. It browns, dries and loosens, and then the weather slowly carries it down the slope and into the gutters. That can take a while, and in the meantime the roof can look worse than before you started. Our answer on how long a biocide takes to work explains the waiting period, and why a treatment is judged on the weeks that follow rather than the day it goes on.

The part people overlook is the debris. Sooner or later it lands in the gutters and the downpipe, where it can block the outlet and push rainwater over the lip and onto the wall below. Someone has to clear it, ideally in stages. A treatment plan that ignores the gutters has only moved the problem from the tiles to the guttering, and the gutter is the part most likely to cause damp inside the house.

Some coverings need more than a bottle and a pole. Natural slate is hard-faced but brittle at the corners and easily broken by anyone stepping on it, and old clay can be porous and quick to mark. A cement-based sheet roof, usually on a garage or outbuilding, may be asbestos cement. If you think yours could be, do not brush it, scrub it or spray it. Leave it alone and have it identified first.

Roofs with solar panels, skylights or a steep pitch add to the difficulty. So does a roof with loose tiles, cracked pieces or failing mortar, because treating it hides the symptoms of a fault that needs repairing. Our survey photographs show the covering up close, so the type of tile or slate is known before anything is sprayed on it, and that choice shapes which product goes on and how strongly it is applied.

If the roof is high, steep, slate or visibly in poor shape, book a survey. We examine the covering, check for loose or cracked pieces, choose a product that suits the surface and apply it from a safe working position. When we finish, the garden is left tidy and the dead moss that has come down off the slopes goes with us, rather than sitting in a heap by the shed.

Not every roof needs a visit, and where a small, sound roof is within reach of a careful homeowner, we are happy to say so and let you get on with it. There is more on the whole process in our guide to removing moss from a roof, and our page on whether roof cleaning is safe covers the risks to the roof and to the people working on it. A conversation costs nothing, and photographs from the ground are often enough to begin. Tell us the roof covering if you know it, the rough height of the eaves and whether any trees overhang, and we can say what we would look at first.