Flat roofs are everywhere in London and the South East. They sit on rear extensions, dormers, garages and porches, and they cover many offices, shops, commercial units and blocks of flats. They do a good job when they’re built and looked after well. But they don’t shed water the way a pitched roof does, so small faults turn into leaks more quickly.
This guide covers the main flat roof materials, the faults we see most often, what causes them and how to judge which ones need attention now. It ends with what a flat roof survey looks at.
The main flat roof materials
Each type of flat roof ages in its own way. Published lifespans vary, and the ranges below come from Checkatrade’s flat roof guide. Treat them as rough guides, not promises.
| Material | Often lasts | Faults to watch for |
|---|---|---|
| Felt (built-up bitumen) | around 15–20 years | Blisters, cracking, split laps, worn mineral surface |
| EPDM rubber | around 30–50 years | Failed seams, damage at edges and outlets, punctures |
| GRP fibreglass | up to about 30 years | Cracks at joints and edges, crazing of the top coat |
| Mastic asphalt | varies | Cracking, blistering, crazing, failed upstands |
| Single-ply membranes | varies | Failed welds or seams, punctures, shrinkage at edges |
How long any roof lasts depends on how it was laid, how well it drains and how it’s been looked after. Our guide to how long roofs last compares flat and pitched roofs.
Ponding
Ponding is water that sits on the roof for a long time after rain. A flat roof should have a slight fall so water runs to an outlet or gutter. Where the deck has sagged, the fall was never right, or an outlet is blocked, water collects instead.
Standing water adds weight, speeds up wear on the surface and finds any weak spot. A shallow puddle that dries within a day or so is common on older roofs. Large or deep areas that stay wet, or ponding that gets worse over time, need looking at. Sagging can also point to deflection in the deck or the structure below. A photo of the roof taken from a window a few hours after heavy rain shows where water collects.
Blisters and bubbles
Blisters form when moisture or air trapped under a membrane expands in the sun. They’re most common on felt and asphalt. A small, firm blister that isn’t growing may be left and watched. Blisters that have split, are spreading or sit where people walk are more of a worry, because once the top layer cracks, water can get under it.
Blisters can also be a sign of trapped moisture in the roof build-up. That’s one place where a thermal roof survey can help, since wet insulation holds heat longer than dry insulation.
Want a roof surveyed? We cover London and the South East, with written reports and photos from £150 including VAT. Book a survey or call 07833 053749.
Splits, cracks and failed joints
Flat roofs move as they heat up and cool down. Over time, that movement opens up splits in felt, cracks in asphalt and GRP, and gaps at seams and laps. Splits often run along joints in the deck below, or around corners and edges where movement is greatest.
On EPDM and single-ply roofs, the weak points are usually seams, edges and details rather than the main sheet. Punctures from dropped tools, foot traffic or fallen branches are common too.
Upstands, edges and outlets
Many flat roof leaks start not on the flat surface but at the details:
- Upstands where the roof meets a wall, with a lead or membrane flashing above;
- Edges and drips where water leaves the roof into a gutter;
- Outlets that get blocked with leaves and silt, or whose seal to the membrane has failed;
- Rooflights and vents passing through the roof;
- Dormer cheeks and abutments on loft conversions.
These are worth checking on any flat roof, however good the main surface looks. Upstands are often too short on older extensions, and flashings above them are sometimes missing altogether. Outlets are the other big one. A single blocked outlet can turn a well-laid roof into a shallow pond after one heavy downpour.
Which problems need attention now
As a rough checklist, these are signs to get a flat roof looked at soon:
- water coming through the ceiling below, or new stains;
- splits or cracks you can see from a window;
- large areas of ponding that don’t dry, or a visible dip;
- blisters that have burst or keep spreading;
- a soft or springy feel reported by someone who has been up there;
- outlets or gutters that overflow in heavy rain.
Worn surfaces, a little moss or small, stable blisters are often things to watch rather than act on straight away. A survey helps you tell the two apart. Safe checks you can do yourself include looking from an upstairs window after rain, checking the ceiling below for stains and clearing leaves from outlets you can reach without climbing.
What a flat roof survey looks at
A flat roof survey covers felt, asphalt, EPDM rubber, GRP fibreglass and single-ply roofs on houses, extensions, garages, dormers, blocks of flats, offices and units. We look at the surface, laps and seams, upstands, edges, outlets and any rooflights. We look for ponding, blistering, splits and deflection, and at the ceiling below where we can.
Where the source of a leak is hard to find, electronic leak detection can pinpoint holes in suitable membranes. Our flat roof survey starts from £150, with larger commercial roofs quoted. In one real example from our case studies, a flat roof survey on a five-storey office in Camden found membrane failure, drainage problems and deflection.
Older flat-roofed garages and outbuildings sometimes have cement sheet roofs. On buildings from before 2000 these may contain asbestos, according to the HSE. We may flag them, and a specialist asbestos surveyor should check them before they’re disturbed. To book, contact us.
Quick answers
Is ponding on a flat roof normal?
Small puddles that dry within a day or so are common. Large or deep areas that stay wet, or get worse, need looking at.
Should I worry about blisters on a felt roof?
Small, firm blisters can often be watched. Split or spreading blisters are more of a concern, because water can get underneath.
How long does a felt flat roof last?
Checkatrade puts felt at around 15–20 years. It depends on how it was laid, drainage and upkeep.
Do you survey commercial flat roofs?
Yes. Offices, shops, units and blocks of flats are included. Larger commercial roofs are quoted.
Related services
More advice
- How Long Does a Roof Last? Slate, Tile, Flat
- Why Roof Leaks Are So Hard to Find
- How Electronic Leak Detection Works
- Green Roofs and Blue Roofs: Inspection Needs
- Commercial Roof Surveys: Rooflights and Sheeting
- All advice
This guide is general information about roofs and roof surveys. It isn’t legal, insurance or structural engineering advice; for those, speak to a suitably qualified professional.