A flat roof does not shed water the way a tiled slope does. It relies on a continuous waterproof skin, a gentle fall towards an outlet or gutter, and a set of edges and upstands that are sealed shut. When one of those slips, water has nowhere to go but through. The slip is usually small and slow: a drain half full of silt, a trim that has started to lift, a patch of moss spreading in the one damp corner.
That is why maintenance on a flat roof is cheap compared with repair. Twice a year, in spring and again in autumn after the leaves have come down, you look, clear and note what has changed. The routine below takes under an hour on most house roofs and garage roofs. It works from the ground, a window or a safe set of steps, because the aim is to spot trouble, not to put you on a surface you cannot test. Where the covering is felt, EPDM or GRP, the notes show what to watch for on each, and where a repair is needed, we explain how to tell a patch from a replacement.
Set up a twice-yearly check
Choose two fixed dates and put them in the diary: a mild week in spring, once the frosts have gone, and another in late autumn, after most of the leaves are down. Spring shows what winter did. Autumn gets the roof ready for the wet months. A third look after any severe storm is worth the effort too.
How do you inspect a flat roof safely?
Start from the ground with binoculars, then from an upstairs window if the roof sits below one. If you must go up a ladder, have someone foot it, never lean it on the membrane or the edge trim, and do not step off onto the surface unless the maker's guidance says it will take your weight. Cold, wet and frosty mornings are the wrong time. Many flat roofs are brittle in frost and slippery with algae at any time. Where you cannot reach, a roofer can do the check for you and show you photos.
What should you write down?
Note the date, the weather and what you see. A stain on a ceiling, a spot where water stands, a split in a seam, a lump in the surface. Take pictures from the same place each visit, so that changes are plain. A simple log builds up a record that helps a roofer later, and it shows whether a small blister is growing or has stayed the same since last year. Keep the notes with your other house papers, and read them over before each check.
Outlets, gutters, debris and a light clean
Drainage is the first thing to check and the most common thing to fix. A flat roof drains to a gutter, a hopper and downpipe, or an outlet in the surface that feeds a pipe. If any of these clog, water backs up across the roof and finds the weakest joint.
What blocks a flat roof outlet?
Leaves, moss, silt, and the loose grit that sheds from some felt finishes. Seed heads, a tennis ball or a bird's nest will do it as well. Clear the surface debris by hand or with a soft brush and a scoop, wearing gloves, working towards the outlet and never pushing grit across the membrane. Lift out what has collected in the outlet itself, then pour water in and watch it run away. If it gurgles, rises or drains slowly, the pipe below is partly blocked and needs rodding from a ladder at the gutter or by a drainage firm.
Do the gutters count?
Yes. Many flat roofs end at a plain gutter, and when it overflows it soaks the edge trim and the fascia as well. Clear it out, check the brackets and watch for drips at joints. Our guide to gutter maintenance covers the routine for the gutters themselves. Overhanging branches drop leaves and rub on the surface in wind, so cut them back or ask the tree's owner to. Our answer on a blocked flat roof outlet goes through the signs and the fixes in more detail.
How should moss and algae be cleaned off?
Moss and algae grow in damp, shaded corners and in the silt along an edge. They hold water against the covering and make the surface slippery. A soft brush, plain water and a mild cleaner are the starting point. Skip pressure washers and stiff wire brushes, which can scour the surface or push water under a lap. For roof coverings generally, Marley advises against high-powered jet washes, and the caution fits membranes too. Anything biocidal needs more care: the HSE guidance is that the public should avoid products meant only for professional users, so read the label and follow it. Moss removal is also a job you can hand over, and Checkatrade's April 2026 guide puts it at £8.50 to £14 per m².
Ponding, falls and what standing water tells you
Puddles on a flat roof are common. Some are harmless. Some are the first sign of a deck that has dipped or a fall that was never right, and they are worth watching.
How much ponding is too much?
Look an hour or two after the rain has stopped, and again the next morning. A film of damp is normal. Pools that stay for long after the sun has come out, or that grow wider from one check to the next, mean the water has nowhere to go. BS 6229:2025 keeps a minimum finished fall of 1 in 80 for general roof areas, so a roof that holds water in several places is below what the standard expects. Standing water also carries dirt that settles into a ring when it dries, which is a handy way to see where a pool has been.
What can you do about it?
Clear the outlet first, because a blocked drain is the easiest cause to remove. If the water still stays, mark the edge of the puddle with chalk or tape and photograph it, then ask a roofer to check the deck beneath. Sagging boards, a lost fall or a settled structure need correcting; no amount of cleaning will help. Persistent ponding speeds up wear on any covering, especially where the water meets a seam or a trim. Our answer on flat roof ponding water sets out how a roofer approaches it.
Upstands, flashings and edge trims
The middle of a flat roof is simple. The edges are where the work is. An upstand is the short vertical turn of the covering up a wall, a parapet or a roof light kerb. The trim, or drip edge, is the metal or plastic profile along the front that guides water into the gutter. A flashing, in lead or another metal, covers the junction between the roof and a wall.
What do you look for at the edges?
Look for trim that has lifted, sagged or come away from the fascia, sealant that has cracked and shrunk, covering that has pulled away from a wall, and mortar that has fallen out above a flashing. Check every pipe, vent and roof light where it passes through. Water that gets behind a loose trim usually runs down the fascia and into the soffit, so look at those for dark streaks and peeling paint as well. Our answer on a leaking flat roof edge trim explains what the repair involves.
Is resealing a fix?
For a hairline gap in sealant, a clean, dry surface and a compatible sealant can buy time. Do not smear mastic over a wide gap or over a wet surface; it hides the problem and traps water. Check the maker's guidance first, as sealants and cleaners that suit one covering can harm another. If the gap keeps opening, the fixing behind it is probably loose or the wall has moved, and that needs a proper repair.
Care notes for felt, EPDM and GRP
The routine above applies to every flat roof. These notes show what each covering tends to show first. If you are unsure which you have, look at the surface: a gritty or mineral-flecked finish is usually felt, a smooth black rubber sheet is EPDM, and a hard, glossy or painted shell is GRP.
What to watch on felt
Blisters, splits along the laps, and bare patches where the mineral finish has worn through to the bitumen. Small blisters can sit for a long time. Splits and open laps let water in and need attention. Loose grit collecting in the gutter is a sign the surface is ageing.
What to watch on EPDM
Lifting seams, loose corners and cuts or punctures from a ladder or a branch. EPDM is a single sheet, so a problem tends to show at a join, a corner or a fixing rather than across the field. Keep solvents, fuel and strong cleaners away from it unless the maker approves them.
What to watch on GRP
Hairline cracks in the top coat, especially along a board joint, around an outlet or at a corner, and crazing in the colour. A crack can be a sign that the deck underneath is moving. Where the surface is only dulled, a clean may be enough. Where it is cracked, a roofer will want to see how far it has gone. Our answer on GRP roof cracking goes into the causes.
When maintenance stops being enough
Maintenance prolongs a covering; it does not renew one. At some point a repair stops making sense, and it helps to know the signs so that you can plan the spend rather than meet it as an emergency.
What signs point to a repair?
A single split, a lifted corner, one failed seam or a leaking joint around a pipe. These are local faults. Typical UK repair rates from Checkatrade's June 2026 guide are £100 to £140 per m² for bitumen, £120 to £170 per m² for EPDM and £130 to £180 per m² for GRP, excluding VAT and for small areas of up to 3 m². Small patches cost more per square metre than a full roof, because the set-up is the same however small the job.
What signs point to a replacement?
Damp insulation or a wet, spongy deck, repeated leaks in different places, a covering that has cracked or blistered across wide areas, and ponding that comes back after the outlets are clear. When most of the roof is in that state, patching only delays the day the whole covering has to come off. Our answer on whether to repair or replace a flat roof helps you weigh it up. Our flat roofing work begins with a look at the roof, pictures for you to keep, and a single written total. To get a view from us, send photos of the outlet, the edges and any stained ceiling.
Part of our Flat Roofing in Crawley
Flat roof upkeep, answered
How often should a flat roof be checked?
Twice a year is a sound habit: once in spring to see what winter did, and once in late autumn after the leaves have fallen. Add a look after any severe storm. Checks from the ground or a window are enough to catch most problems, and a roofer can inspect the surface itself.
Is it safe to walk on a flat roof to clear it?
Only if the maker says the covering will take it, the deck is sound and you can get on and off safely. Many roofs are fragile in frost or slippery with algae. If you are unsure, clear what you can reach from a ladder at the edge, and ask a roofer to do the rest.
Can I paint or coat a flat roof to make it last longer?
A coating can help an ageing surface, but it needs a clean, dry, sound base and a product that suits the existing covering. Painting over blisters, splits or wet felt traps the fault. Ask the product maker whether it is suitable, and have a roofer look at the deck first.
