The short answer
Bleach is a poor roof cleaner. It bleaches the colour out of the growth so the roof looks brighter for a while, but it does not reliably kill the roots, it can discolour and weaken some coverings, it corrodes metal fixings and flashings, and the runoff harms plants and drains into the gutters. A purpose-made biocide used at low pressure does the job better.
Why bleach looks as if it works
The appeal of bleach is easy to understand. It is cheap, it is already under the sink and the effect can be seen within minutes. Dark streaks fade, green patches turn pale, and from the ground the roof seems cleaner. The difficulty is that bleach is very good at removing colour and much less good at removing the thing that has the colour.
Moss and the dark algae-type growths that cause streaking have a structure that holds on to the surface. Bleaching the top layer changes how it looks without necessarily killing it at the base. The growth can then recover, and it often does within a season, leaving you with a roof that is a little worse than before and a bottle of bleach less to spare.
A clean roof is one where the growth has been killed and removed, not simply made pale. The difference matters most for the length of time before it comes back.
What it does to the roof itself
A roof is made of more than one material, and bleach is rough on several of them. Concrete tiles carry a coloured face that can fade or turn blotchy when strong chemicals sit on it. Clay can be stained by salts left behind as the bleach dries. Natural slate and stone slate can lose their surface character, and a patch cleaned with bleach may stay lighter than the rest of the slope for a long time. We look at how the colour settles in our answer on whether cleaning changes a roof's colour.
The metals matter even more. Bleach is corrosive to lead, zinc, aluminium and the steel in fixings and clips. The flashings around chimneys, the lead in valleys and the nails holding slates are all in the path of runoff, and a bleach rinse can speed up their decline. Failed metalwork is a far bigger problem than a mossy roof, and it is expensive to put right.
Bleach does nothing for the moss that has grown into joints, and it may leave a white bloom on some surfaces as it dries. That residue can take several rains to wash off and looks worse than the green it replaced.
Mortar bedding at the ridge and verge, and the pointing around a chimney, can also be softened by strong chemical treatment, which leaves them open to frost and rain.
What happens to everything below
Strong chemicals also react to weather in unpredictable ways. In rain, a half-dried bleach film can run in streaks that look worse than the original growth. In hot sun it can dry on the surface before it has had any effect. An unpredictable result at height, where you cannot easily go back and correct it, is another reason to leave bleach in the cupboard.
The roof is at the top of a drainage system. Whatever goes onto it goes into the gutters, down the downpipes and onto the ground. Bleach runoff will scorch lawn edges and borders, harm plants and shrubs at the foot of the wall, and mark paving and render. It can stain painted fascias and soffits as it runs over them, and it will leave streaks on stonework.
Around Bath, runoff down a stone wall is a particular worry. Soluble salts can be drawn into porous stone and show later as marks or surface decay. For houses where the stone is part of the character, that risk is not worth taking. For stone slates in particular, see cleaning stone roofs.
There is also the plain safety side. Bleach fumes and splashes are hard on eyes and skin, and mixing it with other cleaners can release harmful gases. Using it at height, with a sprayer, makes careless mistakes more likely.
What works better
Roofers use a biocide made for the purpose. It is applied at low pressure, left to soak in and allowed to work over several weeks. It reaches the roots, so the growth dies and releases its hold instead of being merely lightened. The covering, the metals and the garden are better served by a product chosen for the roof than by a household one.
Moss is also lifted off by hand first, which no spray can replace. Heavy moss makes a damp mat that keeps the roof wet; killing it in place and leaving it there simply feeds the next cycle. If you are tempted to try something yourself first, read removing moss yourself for the sensible limits.
Slow, gentle and thorough is the pattern. It takes a few weeks for the result to show, but it tends to last longer than a quick bleach job, and it leaves the roof in better condition.
If bleach has already been used
If you have already put bleach on the roof, there is no need to panic. Rinse the gutters and any plants that took runoff with plenty of clean water, and keep an eye on the lead and the fixings over the next season. If the roof looks blotchy, a proper clean can often even it out, though a coloured surface that has faded will not come back.
Tell us what was used and where when we survey. Knowing it helps us choose the right treatment and check the flashings. The survey costs nothing, the photographs are of your own slopes, and the price that follows is fixed and written down. The full service is described on our roof cleaning page, and you can call 01225 285181 to arrange a visit.
