What actually comes off the roof
A clean sends three kinds of material down the slope: the moss and lichen itself, the grit and dust that has built up beneath it, and whatever liquid was used to loosen or treat the surface. Most of it ends in the gutter, and from there in the downpipe and whatever the downpipe feeds.
Where that is a drain, the main trouble is blockage. Moss in lumps, mixed with granules shed from tiles, can pack into an outlet or the first bend of a downpipe. Where the downpipe empties onto a bed, a path or a water butt, the same material arrives in the garden instead.
The liquid is the part that needs thought. Some treatments are made to stay on the roof and kill what grows there, which is the point of them, but a product that is hard on moss may not be kind to a plant below the eaves. That is a matter for the label, not for guesswork, and it differs between products.
So the practical question is where each downpipe goes and what is under the eaves. A quick walk round the garden with the roofer, before work, answers most of it.
Protecting plants, ponds and water butts
Protection is mostly simple preparation done before the first brush goes on the slope:
- Cover low planting and borders beneath the eaves with sheeting.
- Move pots, furniture and anything delicate out of the drip line.
- Disconnect or cap a downpipe feeding a water butt, so the first flush does not collect there.
- Cover or screen a pond near the house, and keep the sheeting out of the water.
Tell the roofer about koi, fish or wildlife ponds specifically, as they will want to know before a hose is run. A pond that sits under a long run of gutter is more exposed than one further down the garden.
Sheeting is lifted away at the end and the ground looked over for debris. A rake round the beds and a sweep of the path is part of leaving the garden as it was found.
If you grow vegetables or fruit near the house, say so. They are worth a conversation about the product and whether a covered bed is enough or whether the harvest should be rethought for a short period.
Rainwater harvested into butts is the one place where it is easy to forget the roof is connected.
Reading the product label
Any treatment applied after cleaning comes with a label, and the label is the working document. It says how the product is diluted, how it is applied, what surfaces it suits and what to do about run-off, plants and water. A roofer who follows it is doing the job within what the maker sets out.
Ask to see the label or the product data sheet for what is used on your roof. You do not need to be a chemist. You want to know three things: what the product should not touch, whether it needs to be kept out of drains or ponds, and how long to wait before the area is used or planted again.
Those three points are the ones to settle, because the label, not a roofer's opinion, is the authority on a product. If the label and the garden clash, the answer is a different method, extra protection or a different day, not an argument with the label.
The page on roof biocide treatment explains what these products are for and where they fit in the clean.
Gutters and downpipes after the clean
The last job on any clean is the gutter line. Whatever has come off the roof has landed there, and leaving it means the first heavy rain washes it into a downpipe or over the lip onto the wall. Hand clearing the gutters and flushing the outlets finishes the work.
Which? Trusted Traders (November 2025) put clearing the gutters, front and back, at a typical UK range of £75 to £350 ex VAT, with prices higher in the South East. If gutter clearing is part of your clean, it should be in the one written price, not a surprise afterwards.
It is worth looking at where each downpipe discharges. A shoe that sends water onto a flower bed or across a path can be turned, and a blocked gully at the foot of the pipe can back water up until it spills, which is easy to mistake for a fault on the roof.
If moss in the gutters is a repeating problem, moss blocking gutters goes into the causes. The service as a whole is on the roof cleaning and moss removal page.
