The short answer
Yes. Flat roofs can be cleaned with soft brushes, low-pressure water and a suitable cleaner, and the right approach depends on whether the covering is felt, EPDM or GRP. Avoid scrapers, hard jets and strong solvents, which can tear or thin the surface. Clearing the outlets and standing water matters as much as removing green growth, because ponding is usually what feeds it.
Why flat roofs need a different approach
Pitched roofs throw water off quickly and rely on hundreds of small overlapping units. A flat roof relies on one continuous waterproof skin, with joints, laps and upstands that all have to stay sealed. The covering is thinner, softer and less forgiving, and a gouge that would not matter on a tile can be a leak on a membrane.
Flat roofs also gather what a pitched roof lets go. Leaves, silt and moss settle where the fall is gentle, and standing water helps them stay. The growth you see is often a symptom of drainage that is not working properly. BS 6229 sets out falls for flat roofs: they should be designed at 1:40 so that the finished roof achieves 1:80, and where it does not, puddles form and green follows.
Our guide to roof moss removal methods for Stroud homes compared deals with pitched roofs. This answer is for the flat ones, which need gentler handling.
Felt, EPDM and GRP: what each tolerates
Built-up and torch-on felt has a mineral-chip or bare bituminous surface. It can be swept and washed gently, but scraping can lift the top layer, and a hot or strong solvent can soften bitumen. Felt that is cracked or blistered needs a repair assessment before any wash.
EPDM is a synthetic rubber sheet. It tolerates a soft brush and clean water well, and many cleaners suit it, but sharp tools cut it and some oil-based products can attack it. Joints and edges, where the sheet is glued down or cut to fit, need extra care, and so do the corners around rooflights and pipes, where a brush can catch and peel the seal.
GRP, glass-reinforced plastic, has a hard glossy topcoat that responds well to a gentle scrub with a mild cleaner and plenty of clean rinse water, after which it often looks close to new. Abrasive pads can dull the gelcoat, and cracks in the surface should be sealed rather than washed over. MyBuilder's 2026 guide gives felt at £70 to £100 per m², EPDM at £90 to £130 and GRP at £95 to £150 for replacement, which shows why protecting the existing covering is worth the care.
Safe cleaning on a flat roof
The method is gentle and methodical. Sweep off loose debris and leaves first, by hand, working towards the outlet so that nothing is pushed into the drain. Ease any moss off in sheets using a broom-type brush or a blunt plastic blade, never a metal one, and bag it as you go. Apply a cleaner suitable for the covering and leave it for the time given on the label.
Rinse with low-pressure water, again towards the outlet, and keep an eye on the roof edges and upstands, where water may be forced behind a flashing. Check the outlets and gutters at the end and clear them. Cleaning that sends silt into a blocked drain simply moves the problem.
Walking on a flat roof also needs thought. Boards spread the load and keep boots off the membrane. In MyBuilder's 2026 guide, cleaning a flat roof is typically £300 to £600, a national range and not a Supreme price. A survey of your own roof comes first, and our written quote follows.
Ponding, outlets and what the green is telling you
Look at where the growth is thickest. If it follows a hollow, the roof is probably holding water, and cleaning will only buy time until the deck or insulation is dealt with. If it clusters around an outlet, the drain is likely slow or blocked. If it runs in lines from a wall or a tree, the source is overhead.
Standing water after a day or two of dry weather is worth reporting. The covering may be near the end of its life, as suggested by the typical ranges for flat roofing: MyBuilder's 2026 flat roof guide quotes 15 to 30 years in general, and felt is the shortest-lived. Where the fall is poor, relaying is a different job from cleaning, and we will say so on a survey with photographs.
Gutters on an adjoining pitched roof also affect the flat one, because they discharge onto it. That is another place where black streaks on a roof may begin before they reach the flat section.
How long it takes and when to ask for help
A flat roof is often quicker to clean than a pitched one because of the smaller area and easier access, but the preparation and drying times matter. See how long a roof clean takes for the factors involved.
Ask for help if the roof is blistered, cracked or ponding, if there are patches of soft deck underfoot, or if it is above a living space and you cannot reach it safely. Small garage and porch roofs may be fine to sweep from a stable position; anything higher needs proper access. Before any wash, we look at laps, upstands and flashings, because a flat roof that is already letting water in should be repaired first, not cleaned over. If you can, send us a few pictures of the surface after rain.
The whole service is described on our roof cleaning and moss removal page. Our work carries a 10-year workmanship guarantee in writing.