How Do You Clean a Flat Roof?

Know what the roof is made of

Flat roofs are covered in a membrane, and each type needs a slightly different touch. EPDM is a synthetic rubber sheet that tolerates water and gentle cleaning. GRP is a hard fibreglass laminate finished with a topcoat, which can be damaged by abrasive tools. Torch-on felt is layers of bitumen bonded to the deck, often with mineral chippings, and it is the most easily scuffed. If you are not sure, look at the surface from a window, or send us a photo. Our guide to the best flat roof material explains how they differ.

Step one: clear the debris

Leaves, twigs, grit and moss hold water and block outlets, so remove them by hand or with a soft broom, working towards the edge, then bag them. Lift debris, do not drag it, since grit under a broom scratches. Take particular care at the outlets, gutters and any scupper, where blockages cause ponding. Our answer on standing water on a flat roof explains why drainage matters so much.

Step two: wash gently

Use a soft-bristled brush or a soft broom with clean water and a mild, non-abrasive cleaner suitable for the membrane. Work in small areas, scrubbing lightly in one direction and rinsing as you go. For EPDM, a dilute washing-up-type solution is generally fine. For GRP, take care not to scour the gel coat. For felt, brush gently, as chippings may loosen. Rinse with a low-pressure flow from a hose, not a jet, and let the water run to the outlet.

Algae, moss and stains

Green and black algae form on flat roofs that stay damp or shaded. A mild cleaner and a soft brush removes most of it. Moss that has taken hold in cracks or laps should be lifted carefully by hand, not scraped with a blade. A biocide made for the membrane can slow regrowth, but check that it is safe for the roof type, as some products attack rubber or bitumen. Solvents, acids, bleach and petrol-based cleaners should stay away from the roof.

What not to do

Do not use a pressure washer. It can force water under laps and seams, lift felt edges and cut thin membranes. Do not use a scraper, wire brush or sharp tool. Do not walk on the roof in shoes with hard soles or grit stuck to them, and do not drag ladders or equipment across the surface. Do not seal a damaged area without finding out why it failed. Our guide to whether pressure washing damages roof tiles explains the risks, and they are greater on a thin membrane.

Edges, trims and flashings

The weak points of a flat roof are the edges and the junctions with walls. Clear the edge trim and the gutter lip, because debris left there dams water and stains the fascia. At the wall upstand, the vertical strip where the membrane turns up the wall, brush gently along the flashing and do not pull at it. If the sealant or lead looks cracked, do not wash over it. Note it for a repair instead. Moss and leaves lodged in a corner are the first things to cause a leak, so a patient hand clean here is worth more than a quick pass over the open field.

Drying and finishing

Let the surface drain and dry after washing. Check that no pools remain after a day or two, since a pool that stays is ponding and has a cause other than dirt. Put back any loose chippings on a felt roof. Do not leave brushes, buckets or leaves on the roof, because they will be blown back into the outlets by the next gust. Wash tools and shoes before you go up again next time.

Safety

A flat roof looks easy, but edges are unprotected, surfaces can be slick and the deck beneath may be soft if there has been ponding. Falls from low extension roofs are a real hazard. Use a proper ladder, tied and footed, never lean over the edge and do not go up alone. If the roof is high, fragile or has rooflights, leave it to a roofer with edge protection. Rooflights can break under a person's weight.

Check as you clean

Cleaning is a good chance to inspect the roof. Look for blisters, splits, lifted laps, cracked edges, damaged flashings and soft patches underfoot. If you find any, photograph them. We can check the roof, tell you whether it needs repair and show you the photos. See the main roof cleaning page for what we offer.