The short answer
Yes, more than on a pitched roof. A flat roof has little slope to shed water, so a mat of moss that holds moisture, blocks an outlet or hides standing water can shorten the life of the membrane and lead to a leak. Moss needs removing gently, by hand and with a suitable treatment, never with a hard scrape or a pressure lance.
Why a flat roof holds water
Flat roofs are not truly flat. A slight slope carries rain to a gutter or outlet. Under BS 6229:2025 the finished fall must be at least 1:80 across general areas and internal gutters, and IKO advises designing for 1:40. Those gradients are tiny, so any hollow, sag or blocked outlet leaves water standing.
Standing water is where moss thrives. Spores settle in a damp film, then the growth spreads along the line where water sits, and a roof that is meant to dry quickly stays wet for days. The moss then slows drainage further by acting as a sponge and a dam. The problem feeds itself.
Ponding of this kind is not always visible from the ground. You may see green patches and dark staining around the edge of a puddle, or see a roof that never seems to dry after rain. A moss-covered flat roof is therefore worth inspecting for the cause of the wetness, not only for its surface.
Outlets, upstands and the membrane
The most damaging place for moss is around the outlet or gutter. Growth and debris that collect there block the exit for water, so the roof fills at the low point and the depth of the pond increases. A blocked outlet can overload the roof and push water over the upstand, the short vertical edge at the wall, or into the building.
Moss can also lodge along the edges and the junction with a wall, where flashings and trims are fixed. It holds moisture against the seams and the sealed laps of the membrane, and it can lift the edge of a trim as it thickens. Ageing felt, which HomeBuilding rates at 10 to 15 years and Less at 10 to 20 years, is the most vulnerable, though EPDM and GRP can also suffer where the water sits.
Roots are less of a worry on a smooth membrane than on a tile, but seeds blown into a crack or a dirt-filled hollow can sprout. Weeds that take root in the grit at a corner will lift a membrane over time.
If you see water standing for long after rain, or green growth at the outlet, the roof needs a look. Our answer on moss blocking gutters covers the drainage side.
Cleaning a flat roof gently
A flat roof covering is a thin skin. Felt, EPDM and GRP can all be cut, scuffed or lifted by rough treatment, so the usual cleaning methods for tiles do not suit them. The pressure lance is the worst choice: a hard jet can drive water under the laps and lift the edges. Scrapers and wire brushes score the surface and open the way for leaks.
The kinder approach is to lift loose moss away by hand, clear the outlets, and apply a suitable treatment to whatever remains. The dead growth then breaks down and rinses away. Between visits, the roof needs light attention only: clearing leaves and checking that water flows to the outlet.
Foot traffic is a risk. Walking on a flat roof can crush insulation or scuff the membrane, particularly when it is old, and some surfaces become very slippery when wet. A roofer will use boards to spread the weight, and take care at the edges.
Biocides are regulated and the professional-only kinds are not for householders, as the Health and Safety Executive advises. Run-off from a flat roof often goes to a single outlet, so the treatment is planned around where that water ends up.
What to check besides the moss
When we survey a mossy flat roof, the growth is the starting point and the drainage is the real question. We look for hollows where water stands, outlets and gutters that are blocked or cracked, edges and upstands that have lifted, and blisters or splits in the membrane. Inside, we look at the ceiling beneath for stains.
If the roof has lost its fall, cleaning will improve it only temporarily, because the water will return to the same low spots. In that case the realistic options are a repair to the affected area, adding a tapered layer, or a new covering. Which one fits depends on how old the membrane is and how it looks, and photographs show you why.
Do not climb onto a flat roof to scrape it yourself, even if it looks safe from the window. A photograph taken from an upstairs window or a ladder at the edge is enough to start.
The roof cleaning and moss removal page lists the treatments we use, and the survey ends with a single written total.
