Can You Use Bleach on Roof Tiles?

What bleach does to moss and algae

Household bleach is a solution of sodium hypochlorite. It attacks the green and black growth on a roof and gives a quick visual result, because algae streaks often fade within hours. It does not dissolve moss or lift it, though. The dead growth stays on the tile until rain or brushing removes it, and the roots and spores can survive in the joints, so regrowth is common. It also evaporates and breaks down quickly, so it gives little lasting protection against the next season's growth. Our guide to roof biocide treatments and how long they last explains the difference.

Damage to the tiles

Bleach can lighten the pigment in coloured concrete tiles, leaving pale patches that look worse than the stain. It can etch the surface of some clay and concrete tiles and make them rougher, which gives moss a better grip afterwards. On a roof that is already weathered, the damage may not show for a season. If your tiles are under a warranty, using household bleach may also cause problems, as our answer on whether roof cleaning can void a tile warranty explains.

Damage to metal, plants and paint

Hypochlorite is corrosive to lead, zinc, aluminium and some painted surfaces. Run-off down a roof passes over lead flashings, valley liners, zinc strips and metal gutters, and can leave them pitted or stained. It also harms garden plants and lawns it falls on, and it can discolour render, brick and paving. Fish and other aquatic life are very sensitive to chlorine compounds, so run-off must not reach ponds, streams or surface-water drains. Our guide to roof cleaning, run-off and wildlife covers the wider points.

Safety risks to you

Bleach splashes burn skin and eyes, and the fumes irritate the lungs, which is worse in a confined spot under a roof edge. Mixing it with an acid cleaner, an ammonia-based product or some moss killers releases toxic gas. Working on a roof while applying a liquid from a sprayer or a pole adds the risk of a fall. We would rather you leave both the roof and the chemicals to someone who has the equipment and access. See also our answer on whether moss treatments are safe for pets.

Why homeowners reach for it

The appeal is easy to understand. Bleach is cheap, it is already under the sink, and the first result looks dramatic. A black-streaked roof can seem cleaner within an afternoon. The problem is that what you see is the pigment fading, not the cause being removed. Moss that has died is still moss, and a bleached roof can look blotchy where it was applied unevenly, because tiles in sun, shade and different ages react differently. Many people find they have spent a weekend on a ladder and still need a proper clean a year later.

The legal and environmental side

Chemicals that wash off a roof end up in the surface-water drain, which usually runs to a watercourse, not to a treatment works. Disposing of contaminated rinse water onto the ground or into a drain is something the council and environmental regulators care about. Check your council's website, for example basingstoke.gov.uk, before pouring any treatment liquid away. Neighbours with ponds, vegetable beds or pets are affected by run-off over a boundary too, so tell them if you plan to treat a roof near the fence.

If you have already used it

If bleach has already gone on the roof, rinse the gutters and any plants below with plenty of clean water as soon as you can, and check lead and zinc for white or pitted patches. Do not apply a second product over the first. Tell any roofer who comes about the roof exactly what was used and when, because the residue can react with the treatment they plan to apply. A survey will show whether the tiles have faded or etched, and whether a gentler treatment can still be used.

What works better

Hand removal of the bulk of the moss, followed by a roof biocide made for tiles and applied at low pressure, is gentler and lasts longer. The product is designed to leave the tile surface unharmed and to stay active for a time. Washing is sometimes used on algae and lichen, though it carries its own risks on soft tiles. Our guide to moss removal methods compared sets out the options, and soft washing is another choice.

What we do

We survey the roof, photograph the growth and the tile condition, name the product we propose and explain where the run-off will go. The fixed written quote lists the method. For the full range, see our roof cleaning and moss removal in Basingstoke page.