Cleaning a roof puts a chemical on a surface that drains to the outside of your house. The treatment kills moss, lichen and algae, which is its job, and it can also affect plants, a pond, a family pet or the wildlife that shares your roof. The risk is manageable, and it is manageable by following the product label, planning where the water goes and timing the work. This page explains how roof cleaning chemicals are controlled, what happens to run-off, and how we protect the garden and the animals around a house.
Roof cleaning chemicals are regulated products
Products that kill moss and algae are classed as biocides, and they are regulated in Great Britain. In practice this means the maker has to show the product is suitable for the job it is sold for, and the label has to state how it may be used. The label is not advice, it is part of the product's authorisation. It tells you what the product is for, how to dilute it, how much to use, how long to leave it, what protective equipment to wear and what to keep it away from.
Two things follow. First, a product sold for patios or decks is not necessarily suitable for a roof, and a roof product should be used only on the surfaces it names. Second, mixing your own treatment from household chemicals puts you outside any label. Bleach, for example, is not a roof treatment, and our answer on whether you can use bleach on roof tiles explains why it is a poor choice for the tiles, the fittings and the garden. We will not name particular products here, since the range changes, but we use roof products and follow the label every time.
Where the wash water goes
A roof sheds water to the gutter, and from there it goes down the downpipe. Where the downpipe ends is the thing to check, because it decides who or what receives the run-off.
Soakaway or garden
Many downpipes discharge to a gully, a soakaway or onto the ground. Treated run-off here can reach lawns, borders and vegetable plots. Plants differ in tolerance, and some are sensitive. The label may call for protection of plants, and we cover borders and beds with sheeting or wet them down before application to dilute any drift.
Surface water drains
On modern houses, rainwater from the roof often goes to a surface water drain, which in many areas leads to a stream, ditch or river without treatment. Wash water carrying a biocide should be kept out of such drains as far as the label requires. That can mean not rinsing heavily, blocking the gully during the work, or choosing a method that produces little run-off. If you are unsure where your downpipe goes, tell us, and we will check.
Foul drains
On some older properties, rainwater joins the foul sewer. Discharging there has different implications again. Rules and practice vary, and your water company or the local authority can advise on connections. We do not need to settle it for you, but we do need to know.
Water butts and ponds
If a water butt is fed by the roof, disconnect it or cover it before treatment, and leave it disconnected for as long as the label directs. The first rain after a treatment carries product with it, and you do not want that on tomatoes. Ponds, streams and ditches close to the house need particular care, since aquatic life is sensitive. We keep treatment away from the water and tell you what to do about the gutter outlets.
Low-impact methods
The less water and chemical that leave the roof, the lower the risk. Methods that rely on hand removal of thick moss, a measured low-pressure treatment and no heavy rinse produce much less run-off than a long pressure wash. They also keep the product on the roof, where it works. Our description of what soft washing is and the guide to roof biocide treatments set out how that is done. Pressure washing creates a large volume of dirty water and moss debris, and it is the method we least recommend for that reason, as well as for the damage it can do to tiles.
Moss and debris that come off the roof need clearing from paths, gutters and flower beds, and not rinsing into drains. We collect the material and take it away, in line with the aim of leaving the site clean. Our short answer on whether roof cleaning makes a mess of the garden explains what to expect.
Pets, children and people
Keep animals and children out of the treated area while the product is wet and while it dries, as the label states. Pets are drawn to wet areas, and some products are harmful if licked or if paws are cleaned afterwards. Bring in toys, bowls and washing. Close windows and vents on the day, and keep ground-floor doors shut if spray could drift. Our answer on whether roof moss treatments are safe for pets deals with this in detail.
Neighbours matter too. Spray can drift in wind onto cars, washing and gardens next door, and adjoining roofs may be affected by runoff. A word beforehand, and a calm day, avoid most trouble.
Wildlife on and under the roof
Roofs are homes. Nesting birds use gaps under tiles and in eaves, and bats may roost in roof spaces and behind fascias, and both are protected, so work may need timing or a survey first. Beyond that, other creatures such as insects, spiders and small mammals share the space, and a clean should be done without destroying nests, so we check before we start.
In practice we look along the eaves, the ridge ends and the gaps by the verges for nests and droppings before we begin. If there is an active nest, the work on that part waits. If we suspect bats, we stop and advise you what to do next. This is another reason for a survey: a thorough look beforehand costs little and avoids harming wildlife or breaking the rules.
Rainwater harvesting and the first flush
Some households collect rainwater from the roof for the garden, the toilet or the washing machine. A treated roof changes what that tank receives. The first rain after a treatment carries product with it, and the label sets out how long the system should be isolated. We ask at the survey whether any tank, butt or soakaway is fed from the roof, and note the point at which it can be reconnected. On a tank, the sensible step is to divert the first rainfall to waste, then reconnect once the label's period has passed.
Moss, dead growth and the compost heap
The moss removed from a roof is not always harmless. It may carry residue from a previous treatment, and it can contain grit from the tiles. Putting treated moss on the compost heap or the veg bed is a bad idea. We bag it and take it away. If you clear any yourself, put it in the general waste or the garden waste bin as your council allows, and do not wash it down the drain. The council's waste pages will give the local rule.
Asbestos cement and other special cases
Some roofs are not cleaned at all. Older garages and outbuildings may have asbestos cement sheets, which must not be pressure-washed or scrubbed, since damaging them can release fibres. The HSE sets out the position on its asbestos pages, and our answer on whether an asbestos cement roof can be cleaned explains the options. Water running off such a roof is a concern in its own right.
Roofs that feed rainwater harvesting tanks, roofs near a sensitive watercourse and roofs on commercial sites with their own drainage rules also call for more care. Tell us at the survey, and we will plan around it.
Weather on the day
The weather decides much of the environmental risk. Wind carries spray onto neighbours, cars and borders. Rain soon after application washes product off the roof and into the gutter in larger amounts than planned. Heat dries the product too fast and can leave marks. We prefer mild, calm weather with no rain forecast for a day or two, and we move the job if it does not suit. A cleaner who works regardless of conditions is passing the risk to your garden.
Why we say 'it depends' about run-off
You may find this page unsatisfying if you wanted a single rule. The honest position is that a roof cleaning job touches a drain, a garden, a tank and a neighbour in a different way on every property, so the safe method is worked out per house. That is the reason for the survey. A bungalow with a gully on the lawn, a terraced house whose downpipes join a shared drain and a farmhouse with a pond all call for different plans, and none of them is hard once the facts are known.
What to ask any cleaner
| Question | What a careful answer sounds like |
|---|---|
| What product will you use? | A roof product, named, with the label to hand |
| Where will the run-off go? | They have checked the downpipes and protected drains and ponds |
| How will you protect plants and pets? | Sheeting, wetting down, keeping animals indoors |
| Have you checked for nests and bats? | A look along eaves and roof edges before starting |
| What happens to the moss? | Collected and removed, not rinsed into drains |
How we handle it on your job
The free survey records the covering, the growth, the downpipes and where they discharge, any ponds, water butts and sensitive plants, and signs of nesting. It tells us what method will keep run-off low and where we need protection. You see photographs of the roof and a note of the plan, and a fixed written quote follows. If wildlife or drainage call for a change in timing, we say so in the report.
Our work is covered by a written ten-year guarantee on workmanship, and nothing is left behind in the garden or the gutters when the crew leaves. Find out more at roof cleaning in Basingstoke, and if you have a pond, a pet or a protected nest you want us to know about, say so when you ring 01256 213956 or message us.
