Flat roof care
Flat roof maintenance: what to check and when
A flat roof forgives very little neglect. A few short checks through the year keep drains running and edges sealed, and they show you early when a repair is turning into a replacement.
Flat roofs sit quietly over extensions, garages and porches, so they only get attention once a stain appears on a ceiling. By then the membrane may have been letting water in for a while. The sensible habit is to look at the roof on a schedule, from the ground or a window, and to write down what you see.
Most of what follows needs no ladder. You are looking for four things: blocked outlets, water that stays put, edges and upstands (the short upward turns where the membrane meets a wall) that have lifted, and a surface that has changed since your last look.
We cover a year of checks, outlets and gutters, ponding, edges and details, what is safe to do yourself, and how to judge a roof that has had its time. Where a repair is the right answer, that is the answer you should get before anyone mentions a new roof.
01Side by side
A year of checks in four visits
Tie the checks to moments you already notice: leaves coming down, the clocks changing, the first dry weekend of spring. Four short visits cover the year, and each one has a different job.
On each visit, stand back and look at the whole surface, then at the edges, then at the room below. Look up as well as out. A new stain on the ceiling, a damp smell in the cupboard under the roof, or paint lifting near the wall can all point to a leak that the roof surface does not yet show.
Autumn is an easy visit to skip, and it is one that pays back well. Gutters and outlets fill with leaves and seed pods well before the heavy rain, and a roof that goes into winter with its drains clear starts from a far better position than one that has to be rescued in January with the drains already overflowing and the light going by four o'clock.
Photograph the same view each time. A phone picture taken from the same window every season tells you far more than memory does, and it gives a roofer something solid to compare when you ring.
| When | Look at | Why it matters |
|---|---|---|
| Early autumn | Outlets, gutters, debris | Leaves arrive before the wet months |
| After a strong wind | Edges, trims, loose items | Gusts lift what was already tired |
| Late winter | Puddles, splits, ceilings | Shows what the cold months did |
| Summer | Upstands, seams, blisters | Dry weather is the time to book repairs |
02Gutters
Outlets, gutters and where the water goes
A flat roof is not flat to the water. It is a gently sloping surface that has to carry rain to an outlet, a gutter or a downpipe. Anything that stops that journey turns into a puddle, and a puddle becomes a leak.
Start with the outlets and gutters:
- Lift out leaves, seeds and silt by hand, wearing gloves, and bin them rather than washing them down the drain.
- Check that the water is running to the outlet, not stopping short of it.
- Check each gutter run for a sag between its brackets, because a sagging run holds water.
- Check the joints and the downpipe for drips and green staining on the wall below.
For a plastic gutter, Brett Martin, a manufacturer, advises a fall of 1 mm for every 600 mm towards the outlet. That is a very gentle slope, so even a small sag is enough to leave water standing. A membrane roof with internal gutters falls under a single standard: NFRC reports that BS 6229:2025 asks for a minimum 1:80 finished fall across the main roof areas and in those gutters too.
A downpipe that looks clear from the top can still be choked lower down. Pour a bucket of water into the outlet and watch how quickly it clears. If it backs up, the blockage is in the pipe, and that needs rodding rather than another look at the roof.
Flat roof upkeep questions
How often should a flat roof be looked at?
Four short looks a year is a workable rhythm: early autumn, after a strong wind, late winter and in summer. Each takes a few minutes from a window or the ground. Look at outlets, edges, the surface and the ceiling below, and photograph the same view each time so changes show.
03Key points
Ponding: when a puddle matters and when it does not
Water that lies on a flat roof for a day or two after rain is called ponding. A shallow film that dries off by the next day is not the same worry as a pool that is still there on the third morning, or a pool that has left a tide mark of grime.
The numbers explain why puddles appear at all. NFRC says BS 6229:2025 sets a minimum 1:80 finished fall for general roof areas. IKO, a membrane maker, reports that to achieve 1:80 the code of practice calls for a roof designed for 1:40. A roof laid to a poor fall, or one whose deck has sagged since, can leave hollows that hold water.
Standing water is not harmless. It loads the deck, keeps the surface wet, and sits on every seam and detail. Note where the pool forms, how deep it looks, and how long it stays. Then use these rough pointers:
- A pool beside an outlet can mean the outlet or its pipe is partly blocked.
- A pool in the middle of the roof, away from any outlet, points to a dip in the deck.
- A pool that appears only after a split or a lifted seam needs a repair first, then a look at the fall.
Our page on ponding water on a flat roof goes through the causes and the options for each.
04In brief
Upstands, edges and trims
The field of a flat roof, the big open surface, is the easy part. The trouble starts at the details, where the membrane has to turn up a wall, wrap a rooflight kerb, run past a pipe or finish at an edge.
An upstand is the short upward turn the membrane makes at each of those points. It is held and sealed at the top, usually under a flashing, a trim or a coping. Walk the perimeter with your eyes and check each one.
Is a puddle on my flat roof a problem?
A shallow film that dries by the next day is less of a concern than water that is still there after two or three days. Persistent pools load the deck and sit on every seam. Note where the pool forms and whether it sits beside an outlet, as that points to a blockage.
- At wall junctions, look for a gap where the sealed top edge has pulled away from the brick or render.
- At the edge, a drip trim that has bent, lifted or come loose lets wind under the membrane.
- Around pipes and vents, a hairline crack or a collar that has slipped is a direct route for water.
- On rooflight kerbs, check the corners, because that is where the membrane is folded hardest.
Upstand failures can start small: a bead of sealant that has hardened and cracked, or a fixing that has backed out. Caught then, they are a short, inexpensive job. Left through a winter, they let water behind the membrane and into the deck, which turns a patch into a larger repair.
Do not push a tube of general-purpose sealant into a gap and call it done. Filler hides the movement without stopping it. A roofer will work out why the edge moved before sealing it again.
05Choosing a roofer
What you can do yourself and what needs a roofer
Cleaning leaves from an outlet within reach of a window or a low roof is a sensible job for an owner. Almost everything else should stay with someone who has proper access and knows the membrane.
A membrane is not a floor. Boots with grit in the sole scuff and cut it, and a roof that has been used as a storage platform carries a pattern of damage. If you do have to cross a flat roof, put a board down and keep the route short.
These are jobs to leave:
- Any work from a ladder against a roof edge in wind.
- Cutting out or patching a blister or split, because the method depends on the material underneath.
- Resealing an upstand or flashing.
- Anything involving a torch or hot-air equipment.
Think about the person doing the job, too. Roof edges are low, often without any guard rail, and a flat roof looks safe precisely because it is level. The fall that matters is the one off the edge. If reaching the outlet means leaning out of a window or stepping onto the roof in the wet, leave it for a day with better conditions or for a roofer.
A visit also needs checking against the rules if the plan grows. In Approved Document L (2021 edition), paragraph 11.2(d) lists replacing a flat roof's waterproof membrane as renovation. When more than 50% of the roof's surface is renovated, para 11.3 expects the entire element to be upgraded to the Table 4.3 improved figure of 0.16 (the threshold is 0.35), where that is technically and functionally feasible. So a re-cover is not only a waterproofing job: it can bring insulation with it. Ask your building control team or your roofer how that applies to your roof.
06In figures
Keeping a log and knowing when upkeep is not enough
A one-page log earns its place. Write the date, what you looked at, what you saw, and a photo reference. Over a few years it separates a stable roof from one that is getting worse, and it helps a surveyor answer the question you actually care about: repair or replace.
Lifespans for the common coverings are published as ranges, and HomeBuilding and Less disagree, so here are both.
Can I walk on my flat roof to clear it?
Only if you must, and with care. A membrane scuffs and cuts under gritty boots, and edges are unsafe in wet or wind. Clear leaves from an outlet you can reach safely, lay a board down to spread weight, and leave patching, resealing and anything involving a torch to a roofer.
| Covering | HomeBuilding | Less |
|---|---|---|
| EPDM rubber | 25-30 years | 30 to 50 years |
| GRP fibreglass | 25-30 years | 25 to 40 years |
| Torch-on felt | 10-15 years | 10 to 20 years |
These are typical ranges, not promises, and neither says how much upkeep sat behind the figure. Read them as a rough guide to when a roof deserves a closer look.
Keep the log somewhere you will find it again, such as a note on your phone with the photos filed alongside. If you sell, a dated record of checks and repairs is a useful thing to hand to a buyer's surveyor.
Signs that upkeep is no longer enough include repeat leaks in different places, a surface that is crazed or brittle across its whole area, and a deck that feels soft underfoot at a repair. For money, MyJobQuote gives an average flat roof replacement at between £1,500 and £6,000 in 2026, while Which? puts felt on a typical single garage (6m x 3m) at £1,400 to £3,500, excluding VAT. Your written quote is one total price after a survey. Our flat roofing page explains how we weigh repair against replacement, and there is more on repairs by material in our other post.
Tell us what's wrong with the roof.
You'll get one written total price.