The short answer
Moss and dirt are brushed or scraped into a collection point, caught on sheeting or in gutters, bagged and taken away with the other waste. Rinse water is kept low in volume and directed away from drains, flower beds, ponds and neighbours' ground. Gutters and downpipes are protected during the work and cleared afterwards, and the site is left clear when the job finishes.
How moss is removed before any water is used
The best way to control debris is to remove as much as possible dry. Thick moss is lifted from the tiles or slates by hand, or with a soft brush, before any wash is carried out. It is gathered into heaps on the roof or passed down in bags, not pushed off the edge.
This also protects the roof. Moss is often thick and holds a lot of water, so a heavy mat swept off in one go can drop into the gutters or break a slipped tile on its way. Working from the top of the slope down, a section at a time, keeps the debris moving in a controlled direction and lets the roofer see the surface as it is uncovered.
Each slope is different, so the order of work depends on the roof. A small gabled house is a simple job. A roof with valleys, dormers and a parapet needs more care, because valleys and parapet gutters collect the debris you do not want to wash into a downpipe.
Protecting gutters and downpipes
Gutters are where cleaning debris most often causes trouble. Moss crumbs and grit washed from the roof run into the gutter and settle at the outlet, and a blocked gutter is the start of a leak and of damp at the eaves.
To prevent that, gutters can be protected with temporary sheeting or lined while the roof is brushed, and the debris scooped out by hand before it reaches the downpipe. Outlets are covered or plugged during the work and opened again afterwards. Once the roof is clean, the gutters are cleared and flushed so that the first heavy rain does not carry a load of silt into the drain.
It is the same reason that moss and gutter blockages are linked. Our page on moss blocking gutters explains how a mossy roof sheds material into gutters long after the cleaning has been finished, and why clearing them is part of a proper clean.
Where the water goes
Roof cleaning does not need a large volume of water. Soft washing relies on a gentle spray and a cleaning solution rather than force, which keeps the run-off modest. MyBuilder's roof cleaning price guide warns that hard jets can harm fragile tiles, a further reason to keep the pressure down. Heavy rain during the job matters too. If the forecast is poor, the work is paused, because a downpour turns loose moss into a slurry that reaches the drains before it can be collected.
The run-off from a soft wash still carries treatment, moss residue and fine grit. It is directed to the gutters where possible, and then kept off planting, ponds and shared ground. Where a roof drains into a soakaway or onto a neighbour's side, that changes the plan, and it is something to raise with your roofer before the work starts.
The product label sets the rules for any treatment, including how to keep it off plants and how long it needs. If pets, children or a pond are involved, tell the roofer, and read about whether biocide is safe for pets.
Waste, bags and clearing the site
All the moss and debris is collected and removed with the other waste. It is bagged, carried down and taken off site, not left on the lawn or dropped over a fence. Moss is light when dry but heavy and messy when wet, and a wet pile left in a garden bed can smother planting underneath.
At the end of the job the ground is cleared: sheeting folded away, bags removed, paths and patios swept, and any debris that landed on the ground cleared up. Leaves, spores and fine grit are a nuisance on a path, so a final sweep matters. Treated moss is not composted on site. Once dry it is light, and it goes out with the rest of the waste, so nothing is left to blow back onto the roof or into a neighbour's garden. Fine grit on windowsills, cars and paving below the eaves is brushed or rinsed away before the team leaves.
On a house with access constraints, such as a terrace with no rear lane, the debris route is worked out before the job starts. In Bath that is often a question of carrying it through the house or down a narrow side passage, and it affects the plan and the time.
Flat roofs, special roofs and selling
The same principles apply on other roof types with a few changes. On a flat roof, debris is swept to the outlets and lifted out by hand, as described under cleaning a flat roof. On a roof with lichen, the crust is softened and lifted gently, as explained on our lichen on roofs page.
If you are tidying a roof ahead of a sale, the same debris plan applies, with the added aim of leaving the roof looking right for a viewing or a survey. We cover that in cleaning a roof before selling.
Our survey is free and includes photographs of your roof. The written fixed price sets out how the work, debris and waste will be handled. The full service sits on the roof cleaning hub.
