The short answer
Roof biocide is made to kill living growth, so it should be kept away from pets, fish, ponds and food plants while it is applied and until it has dried. Used as the product label directs, with animals indoors, ponds covered and plants wetted and protected, the risk to pets and gardens can be managed. Ask for the product name and its safety data sheet.
What a roof biocide is
A biocide is a product made to kill living things, and on a roof the targets are moss, lichen and algae. It is sprayed or brushed over the covering once the bulk of the growth has been removed, and it keeps working for some weeks afterwards, which is how it slows regrowth.
Because it is made to kill, it is not harmless, and no sensible roofer will describe it as completely safe. The question is what the exposure is and how it is controlled. The product label and the safety data sheet, a document the maker publishes with hazards and handling advice, are the sources to go by. Ask for the product name before the day, and read both.
Products differ. Some are more hazardous to water life than others, and some leave less of a mark on stonework, paving or paint. Our general page on roof cleaning and moss removal sets out the full process, and the guide to roof biocide explains how the treatment works.
Pets and other animals
Dogs and cats should be kept indoors while spraying is under way and until the surface has dried, and they should stay away from any puddles or runoff in the garden. Animals lick paws, drink from standing water and sniff around damp ground, which is how exposure usually happens.
Small animals need extra thought. Rabbits, guinea pigs, hens and tortoises that live outdoors should be moved well out of reach of drips and spray, along with their bedding, feeders and water bowls. Bird baths and feeders on the side of the house that is being treated should be emptied and covered or moved.
If an animal does come into contact with the product, wash it off with clean water and call your vet for advice, taking the product name with you. The safety data sheet is what the vet will want. Keep the product name and the date of treatment somewhere you can find them, since it is easy to forget what was used.
Plants, lawns and food crops
Runoff from a treated roof reaches the beds along the walls first. Plants that sit under the eaves or beneath a downpipe are the most exposed, so the standard precaution is to wet them thoroughly with clean water beforehand and cover those that are delicate. Water on the leaves dilutes anything that lands there. Sheeting over a bed, held down with bricks, keeps drips off.
Lawns usually cope. Shrubs, young plants and anything in a pot close to the wall are more at risk. Move pots out of the zone, and keep an eye on climbers growing up the wall or across the gutter line.
Edible plants need their own care. Vegetable beds, fruit bushes and herbs below a treated roof should be covered during the application, and the label should be checked for guidance on when produce may be eaten. If you are growing food within reach of the roof, tell the roofer at the survey. Our page on black streaks on roofs shows another case where a gentler treatment may be enough.
Ponds, water butts and drains
Water life is the most sensitive of all. Fish and pond plants can be harmed by even small amounts of some biocides, so ponds near the house should be covered with sheeting for the duration of the work and until the roof has dried. Water butts connected to the downpipes should be disconnected, or the pipe diverted, so that nothing from the roof reaches water you use on the garden.
It is also worth thinking about where the gutters discharge. A downpipe running into a soakaway, a drain or a ditch is a different matter from one running over a garden. Rinsing the roof edge and gutters in a controlled way, and keeping runoff on to hard standing or into the foul drain where permitted, is the sort of detail a careful roofer plans for at the survey. Ask what will happen to the water.
Neighbours' gardens count as well. A word with them beforehand is courteous, and it lets them move pets and cover plants.
Questions to ask, and what we do
A short list is enough for most jobs.
- Which product will be used, and can I see the label and safety data sheet?
- How long should pets stay in, and when is it safe to use the garden again?
- How will ponds, water butts, drains and plants be protected?
- What happens if it rains soon after application?
We give you the product details at the survey, ask about pets, ponds and planting then, and agree the precautions before a date is fixed. If the weather turns, we hold the treatment back. Where a roof does not call for chemical treatment, we leave it out of the quote.
For roofs where growth is slight, hand removal and a soft wash may be enough. The page on cleaning listed roofs shows how we treat sensitive properties, and the page on roof coatings and mortgage valuations covers a separate question. The survey costs nothing and we photograph the roof, then the written quote gives one fixed price. Tell us at that visit about anything outdoors that matters to you, from a pond to a vegetable patch to a rescue hen, because it changes how we plan the day and which product we reach for.
