The short answer
Whatever goes on a roof eventually comes off it, rain carrying it along the gutters and down the pipes. Runoff from a treated roof can reach plants, ponds, lawns and drains, so the product label's conditions on rain, drains and watercourses matter. Protect plants and ponds, divert or contain water where the label asks, and plan the work for a dry spell.
Where the water goes
Follow a raindrop from the ridge. It runs down the tile, drops into the gutter, travels along it to an outlet and falls down a downpipe. From there it goes to one of a few places: a drain connected to a surface water sewer or soakaway, a gully, a water butt, or simply onto a path, border or lawn.
Anything on the roof travels the same route. Dead moss, dirt and any product residue are washed along with it. For a mechanical clean with plain water, that is mostly debris. For a roof that has been treated with a biocide, the water can carry some of the product.
That is why the end of the downpipe matters. If it discharges over a flower bed, the plants take the water. If it empties onto a lawn where a dog plays or into a pond, the product goes there. If it connects to a drain, it enters the drainage system, and some systems lead to a watercourse.
The route is easy to check. Walk round the house on a wet day and note where each downpipe ends. Our page on moss in gutters explains why gutters get clogged and what the clearing sends downstream.
Tell the roofer where your downpipes go, and about any water butt, pond or soakaway. They shape the plan.
What the label asks
Each approved roof treatment is a biocide with authorised uses, and the conditions are printed on its label. Those conditions often deal with runoff directly. A label may say how long the roof must stay dry after application, that the product must not enter drains or watercourses, that water butts must be disconnected and that the treated area must not drain onto soil or planted areas.
The Biocidal Products Regulation holds users to those conditions, so the label is not optional reading. HSE also keeps professional-only biocides from householders, partly because the controls they ask for are beyond most people.
A roofer working to the label will check three things before starting. They look at the forecast, to avoid rain during the period the label sets. They look at where each downpipe discharges. And they decide how the runoff will be handled, whether by diverting it, containing it or protecting what is below.
If the label says the product must not reach a drain, that rules out letting it run into a gully, and a plan is needed. If it cannot be met, the product is not the right one for that roof.
Ask the person doing the work to show you the label and explain what it requires for your property.
Protecting ponds, plants and drains
There are practical ways to keep runoff where it belongs.
- Cover or move plants and borders below the roof edge, with sheeting where the label asks for it.
- Disconnect downpipes from water butts, and divert them into a container or onto hard surfaces that drain safely, as the label allows.
- Cover or block gullies and drain gratings where the label says the product must not enter them.
- Keep ponds covered and out of the runoff path, and tell the roofer if you keep fish.
- Do the work in a dry spell, so rain does not carry the product before it has had time to work.
- Rinse paving afterwards with clean water, where the label permits.
The treatment itself is chosen with this in mind. A product with strict runoff conditions on a roof that drains straight to a pond would be a poor match, and a roofer should say so.
Not every roof needs a biocide. Where the growth is light, a gentle mechanical clean with plain water has little runoff concern beyond debris, and gutters can be cleared by hand.
Our page on roof cleaning chemicals and pets deals with animals, which are the other main reason runoff matters.
Planning a clean with runoff in mind
The survey takes in the garden under the eaves along with the roof itself. We note the downpipes and where they discharge, any pond, butt, soakaway or planted border, and neighbouring boundaries where water might run.
Then we say whether a treatment is needed and which product suits the site. If the label sets conditions you cannot meet, we explain what the alternatives are. Marley advises against high-powered jet washing, since tile surfaces suffer, and a jet would also scatter water and debris, so avoid it.
You see the label of the product, hear what it asks of the site and agree the arrangements beforehand: where sheets go, which downpipes are diverted, when pets are kept in and what the weather allows.
The price follows the survey as one written total, with any protection and access included. The roof cleaning and moss removal hub lists the other services connected with this one.
If you have a pond, a stream at the bottom of the garden or a neighbour with a vegetable plot, tell us. Those are exactly the details that change how a clean is done.
