How long does a flat roof last? Anywhere from 10 to more than 30 years, and the material decides most of that. Traditional built-up felt usually gives 10 to 15 years. A modern torch-on felt system, an EPDM rubber roof or a GRP fibreglass roof can last 20 to 30 years or more when it is laid properly.
The rest comes down to workmanship and upkeep. A well-built felt roof can outlast a badly laid rubber one, and a roof with blocked outlets will fail early whatever it is made of.
That matters in South London, where a lot of flat roofs were laid on back additions, dormers and kitchen extensions in the 1980s and 1990s. Many are now at the end of their working life, and the first heavy storms of winter are usually when an old roof gives up. This guide covers the flat roof lifespan you can expect from each material and the warning signs to check before the weather turns. It also explains how to decide between a repair and a new roof.
How long does each type of flat roof last?
| Roof covering | Typical lifespan | Where you usually see it |
|---|---|---|
| Traditional built-up felt | 10 to 15 years | Older extensions, garages and outbuildings |
| Modern torch-on or self-adhered felt | 20 to 30+ years | Multi-layer systems with a mineral cap sheet |
| EPDM rubber | 25 to 30+ years | Single-sheet roofs on extensions and dormers |
| GRP fibreglass | 20 to 30 years | Dormers, balconies and roofs with awkward shapes |
The upper figures come from independent testing. The British Board of Agrément (BBA) certificate for one leading EPDM system gives a life of at least 30 years. The certificate for a modern multi-layer bitumen system puts it at more than 35. One established GRP manufacturer guarantees its standard system for 25 years and its heavy-duty system for 30.
Those numbers assume the roof was designed and installed to the manufacturer’s specification. In practice the lifespan you get is often shorter, because most early failures come from the details rather than the membrane itself.
Traditional felt
The felt roofs on most older extensions were built up in layers of bitumen felt, often finished with stone chippings. They were cheap and quick to lay, and they are the reason flat roofs earned a reputation for leaking. After 10 to 15 years the bitumen dries out, cracks and starts to blister, especially on roofs that face south.
Modern felt systems
Today’s torch-on and self-adhered systems are a different product. They use polymer-modified bitumen in two or three bonded layers, topped with a mineral-surfaced cap sheet that protects against sunlight. They cope well with foot traffic, and a damaged area can be patched with compatible felt.
EPDM rubber
EPDM is a synthetic rubber membrane laid in one sheet, so a small or medium roof has few joins or none at all. It stays flexible in frost and heat and resists sunlight well. The weak points are the edges, corners and the details around rooflights and outlets, so the quality of the detailing matters more than the membrane.
GRP fibreglass
GRP is a resin and glass-fibre laminate laid wet and left to cure into a hard surface with no joins. It suits roofs with awkward shapes and roofs that people walk on. It needs dry weather and temperatures above the manufacturer’s minimum to cure properly, which can limit when it is laid in winter.
What shortens a flat roof’s life?
Most flat roofs that fail early do so for one of six reasons.
Water that sits on the roof. Flat roofs are not truly flat. British Standard BS 6229, updated in 2025, sets a minimum finished fall of 1 in 80 so that water runs off to the outlet. Where the roof has sagged or was never laid to a proper fall, puddles form and slowly break down the surface. In winter that water freezes and expands in every small crack.
Blocked outlets and gutters. Leaves, moss and debris block the outlet. The water backs up and finds its way under the edges. A roof with only one outlet and no overflow is particularly exposed.
Failed upstands and flashings. Most leaks start where the roof meets a wall, not in the middle of the roof. The upstand runs up the wall and the flashing covers the top of it. If the flashing has pulled out of the brickwork or the upstand has cracked, rain gets in behind.
Sun and temperature swings. Ultraviolet light breaks down unprotected bitumen. A dark roof can swing through a wide range of temperatures in a single day, and that constant movement opens up joins and splits.
Foot traffic. Roofs used as an unofficial balcony, or as a platform for cleaning windows, wear out faster than they were designed to. So do roofs where satellite dishes, air conditioning units or solar panels were fixed without proper protection.
Condensation from below. Many older roofs are cold roofs, with insulation between the joists and no ventilation or vapour control. Warm, moist air from the room below condenses under the deck. The timber rots from the inside while the surface still looks sound.
Signs your flat roof needs attention before winter
October and November are the best time to look, before the storms and before the first hard frost. You can check most things from an upstairs window or the top of a properly footed ladder. Do not walk on an old felt roof, and do not climb a ladder when there is no one else at home.
From outside, look for:
- Blisters or bubbles in the felt surface
- Splits and cracks, especially along joins and edges
- Standing water that is still there two days after rain
- Bare patches where the stone chippings or mineral finish have worn away
- Moss, plants or debris around outlets, gutters and trims
- Edge trims that have lifted or come loose
- Flashings that have pulled away from the wall
Then check inside. Damp patches or brown staining on the ceiling below, a musty smell, peeling paint or a ceiling that has started to sag all point to water getting in. A leak often shows some distance from where the water enters, because it runs along the deck before it drips.
If you find something, clear the outlets, take photos and get the roof looked at before the weather turns. A small repair in November costs far less than a new ceiling in February.
Repair or replace? A simple test
Three questions settle most cases.
1. How old is the roof? A traditional felt roof over 15 years old, or any roof well past its expected life with repeated problems, is nearing the end. Repairs will buy time, but not much of it.
2. How much of it is affected? A single split, a failed flashing or a blocked outlet is a repair. Problems spread across the roof, or leaks that turn up in a new place every winter, are signs a flat roof needs replacing.
3. What condition is the deck in? The covering sits on timber boards. If the deck is soft, swollen or rotten, patching the surface will not last, because there is nothing sound for the new material to bond to. A rotten deck means a new roof.
There is one exception worth knowing. If you are planning a loft conversion or a rear extension in the next year or two, the roof may be rebuilt as part of that work anyway. In that case a careful repair to see you through is often the sensible choice, so mention your plans when the roof is surveyed.
What a flat roof replacement involves
A full replacement usually follows the same six steps.
- Survey. Measure the roof and check the falls, the deck, the outlets and the junctions with the house.
- Strip. Remove the old covering and inspect the deck. Replace any rotten boards.
- Insulate. On most renovations this means a warm roof, with the insulation laid on top of the deck. It keeps the timber at room temperature and avoids the condensation problems of older cold roofs.
- Correct the falls. Tapered insulation or timber firrings give the roof a proper fall to the outlet.
- Lay the new covering. Form the upstands and fit the flashings, edge trims and outlets.
- Sign off. Arrange the building control inspection where one is needed.
A small roof over a back addition or dormer is often finished in two or three working days, although the weather sets the pace. Felt systems can go down in colder conditions than GRP resin or some EPDM adhesives, which need dry weather and minimum temperatures.
Do you need building regulations approval?
Often, yes. Re-roofing is usually building work under the regulations, and insulation is where most people get caught out. The trigger is renovating more than half of a roof’s surface, or more than a quarter of the whole building’s outside envelope. Past that point, the roof has to be brought up to current insulation standards. In practice that covers most full flat roof replacements. The Planning Portal’s re-roofing guidance explains the rules.
Adding insulation on top of the deck can raise the roof level by 100mm or more. That affects the upstands under any doors or windows that open onto the roof, so it needs planning at survey stage rather than on the day.
Flat roofs on dormers and rear extensions
Dormers. A dormer roof is small, high and exposed to the weather. Leaks show up in the ceiling of the loft bedroom, and repairs usually need scaffolding or a tower for safe access. If your dormer was added as part of a loft conversion, find out what system was used and when. If you are planning a loft conversion, the dormer roof should be specified for the long term from the start.
Rear extensions and back additions. On the Victorian terraces of Brixton and Streatham, many back additions and the kitchen extensions built behind them have flat roofs. The weak point is usually the junction with the main house wall, followed by the upstands around rooflights and lanterns. If you are thinking of extending, decide early whether a flat or pitched roof suits the house. Our extension builders page covers the options.
Quick answers
My flat roof is leaking. Can it be repaired in winter?
Yes. Patch repairs, flashing repairs and outlet clearing can be done in most weather. A full replacement depends on the system and a few dry days.
Does one leak mean I need a new roof?
Not usually. A single leak often comes from a flashing, an outlet or one split. Repeated leaks in different places are the sign to replace.
Felt vs EPDM vs GRP: which lasts longest?
On certified figures, modern felt systems and EPDM come out slightly ahead, with GRP close behind. On a real roof, the quality of the installation matters more than the choice of material.
Get your flat roof checked before the weather turns
Prime Build Works has been building in South London since 2012, from our base in Balham. We look at flat roofs on extensions, dormers and garages, tell you plainly whether a repair will do the job, and give you a written quote broken down by stage.
See our roofing services, call 07472 137237 or ask for a free quote.
