The short answer
A flat roof needs gentler cleaning than a tiled one, because its waterproofing is a single skin. Clear leaves and debris by hand, keep outlets open, wash with a soft brush and clean water, and avoid blades, scrapers and high-pressure jets that can cut or lift a membrane. A clean roof is also the best moment to check for ponding, blisters and open seams.
Outlets, ponding and falls
The first job on a flat roof is to keep water moving. Leaves, moss and silt collect in outlets and gutters and block them, and water that cannot leave stands on the surface. Standing water, called ponding, loads the roof, ages the covering faster and finds any weak seam.
Clearing by hand is the start: leaves, twigs and dirt lifted out, outlet grilles cleaned and the gutter run checked. Do it from a safe position, as flat roofs are often reached from a ladder at the edge and not by walking on them.
A roof is designed to drain. IKO says the design fall for a single-ply flat roof is 1:40, whereas the finished minimum under BS 6229:2025 is 1:80 across general roof areas and internal gutters. That standard is for flexible waterproof coverings and does not cover lead.
A roof that still holds water for a day or more after rain may have a fall problem, not a cleaning one. Sagging boards, a worn deck or settled supports can all produce hollows. Cleaning will not cure those, but it will show you where the water sits.
Our page on flat roofing covers the repairs that follow, and the one on moss in gutters explains the neighbouring problem.
Membrane-safe methods
Flat roofs are covered in felt, single-ply membrane such as EPDM, or GRP, a glass-reinforced plastic. Each has a skin that can be cut, lifted or scoured, and cleaning has to respect that.
Safe habits include the following.
- Use a soft brush or broom, clean water and a mild cleaner.
- Keep metal scrapers, wire brushes and blades away from the surface.
- Do not aim a high-pressure jet at seams, edges or upstands, where it can lift the membrane.
- Rinse thoroughly, so no cleaner film remains.
- Stay on foot-friendly parts of the roof, and avoid dragging heavy objects across it.
Moss and algae can be treated, but the product must suit the covering and be approved. HSE guidance bars householders from biocides made for professionals alone, so treatment is a job for someone using the right product to the label.
Lifespans vary by covering and are typical ranges, not promises. HomeBuilding gives EPDM 25 to 30 years and Less gives 30 to 50, so the sources disagree. HomeBuilding gives GRP 25 to 30 years and felt 10 to 15, and Less gives 25 to 40 for GRP and 10 to 20 for felt.
An old, brittle covering may not stand cleaning. If it is tired, a gentle sweep and an inspection may be the most that is wise.
Inspection while it is clean
A clean surface shows its faults. Once the debris is gone, look for these signs.
- Blisters or bubbles, where trapped moisture has lifted the covering.
- Open or lifting seams and edges, especially at upstands and corners.
- Cracks, splits or crazing in felt, or pinholes in a membrane.
- Worn or cracked flashings at walls, chimneys and rooflights.
- Ponding marks, tide lines and stained patches showing where water has stood.
- Damaged or loose edge trims and drip details.
Photograph each and note where. Small faults caught early are small repairs: a patch, a resealed seam or a new section of flashing. Left, they let water into the deck, and a wet deck rots.
Inside the building, check the ceiling beneath for stains, especially after rain. Water from a flat roof can run along the deck before it shows, so a stain can be well away from the fault.
Our page on roof cleaning access explains how a flat roof is reached safely for this kind of inspection.
Do not walk on a flat roof you do not know. The deck may be soft where water has got in.
What we do and when
We look at the roof before any cleaning is agreed. The survey covers the covering, the seams, the upstands, the outlets and the ceiling underneath, and you see the photographs with a plain explanation.
If the roof is sound, cleaning is simple: clear the debris, wash gently and treat any growth with an approved product. If it is failing, we say that, and we explain whether a repair or a new covering is the better route. Cleaning a roof that needs replacing is poor value.
How often a flat roof needs attention depends on trees, shade and the covering. A look in spring and autumn, with outlets cleared after leaf fall, is a sensible habit. Between visits, a quick check from a window after heavy rain tells you whether water is draining.
The price follows the survey and covers the whole job as one written total. Access, area and condition decide it. The roof cleaning and moss removal hub explains the wider service for roofs around Farnborough, Aldershot and Fleet.
