The short answer
Clean a flat roof with a soft brush, clean water and a mild treatment for algae or moss, working gently and keeping the roof drained. Avoid pressure washers, wire brushes, scrapers and solvents, which can tear EPDM, scratch GRP and strip the granules from felt. Always check the membrane's condition first, because cleaning a failing roof can open the leak you were trying to avoid.
Start by looking at the roof, not cleaning it
Before any water touches a flat roof, identify the covering and judge its condition. A flat roof is waterproofed by a single covering, and cleaning can only be as safe as that covering allows. EPDM is a synthetic rubber sheet. GRP is fibreglass laid in resin and finished with a topcoat. Felt is bitumen-based sheeting, either torch-on or built up in layers.
Walk the edges from a ladder or look at photographs if you can, and note blisters, splits, lifted laps, cracked topcoat, open seams and any place where water stands. A roof with those faults should be repaired before it is cleaned, because brushing across a split membrane will make it worse and washing it will drive water through the gap.
Also think about access. A flat roof is easy to step onto, and easy to slip on once it is wet with algae. Fall protection, edge protection and a safe route up matter more than the cleaning itself, which is why most owners leave this job to a roofer.
Cleaning EPDM
EPDM is tough but it is not indestructible. It tolerates water and soft brushing well. What damages it is sharp tools and some chemicals. Use a soft broom or a soft brush with clean water and, if there is staining or algae, a mild cleaner of the kind the membrane maker recommends. Rinse well and let the roof drain.
Keep away from petrol-based products, strong solvents and anything abrasive, because they can mark or soften the surface and weaken seams. If you are unsure, check the manufacturer's cleaning guidance for your membrane.
Pay particular attention to the seams, since a flat roof has no second line of defence beneath its covering if one opens up. Check the upstands, where the sheet is lapped up a wall, and the corners. These are glued areas and the first to open if scrubbed hard. A well laid EPDM roof is typically expected to last 20 to 30 years or more, depending on exposure and workmanship, and gentle cleaning should not shorten that.
Cleaning GRP
GRP has a smooth, glossy topcoat, which sheds dirt well and is easy to wash. A soft brush, clean water and a mild detergent will deal with most dirt and green algae. A soft sponge or cloth works on stubborn patches.
The surface is a coating on top of the glass fibre, and it shields the laminate beneath. Abrasive pads, stiff brushes and pressure washing can scratch or wear the topcoat, which leaves the laminate under it exposed to sun and water. A dull, chalky or crazed topcoat is a sign that the roof may need re-coating, not just washing.
Look closely at the joints where the roof meets the edge trim and at any fibreglass upstands. Hairline cracks here tend to show up once the dirt is gone. It is much better to find them while cleaning than to find them through a ceiling stain. Where the topcoat has aged, a coating may be the right next step, as covered in our guide to roof coatings.
Cleaning felt
Felt is the most delicate of the three to clean. Modern felt roofs are finished with a mineral or granular cap sheet, and those granules shield the bitumen underneath from sun. Brushing hard or scraping lifts the granules, and a bare patch of bitumen ages quickly. Mineral chippings gathering in the gutter are a warning that the cap sheet is shedding its protection.
Remove loose leaves and silt with a soft brush, then wash lightly with water. For moss, soften it with a treatment and lift it gently rather than raking it off. Do not use a pressure washer on felt, and do not use a scraper or a spade.
Older felt that has gone brittle or blistered will not tolerate cleaning at all. Typically felt lasts around 10 to 20 years, and longer for a good multi-layer system, so a felt roof that is old, cracked and covered in moss may be telling you it needs renewing and not washing. Judging whether it is worth saving needs the surface examined closely, and we say directly whether the roof wants cleaning, repair or renewal.
Debris, drainage and what the dirt is hiding
Most of what lands on a flat roof is leaf litter, silt and seeds, and it collects around the outlets. Clear the drainage first, so that rinse water has somewhere to go. A blocked outlet or gutter turns a cleaning job into a pond, and standing water is the main cause of algae in the first place.
BS 6229:2018 recommends designing a flat roof to a fall of 1:40, so that finished falls of 1:80 are achieved. A roof that holds puddles for days after rain is not falling as it should, and no amount of cleaning will fix that. Our page on where roof cleaning debris goes explains how waste and rinse water are kept out of the drains.
Cleaning a flat roof in the UK typically sits within a UK range of around £300 to £600 according to MyBuilder (2026), with the fixed price confirmed once the roof has been surveyed. If you are also weighing up moss strips, note that they suit pitched roofs, as explained in zinc strips and moss. For the full picture, see our roof cleaning service.
