Flat roofing
Three systems, and the roof decides which one
Torch-on felt, EPDM rubber and GRP fibreglass all make a sound flat roof. They fail differently, cost differently and suit different shapes — the choice should follow the roof, not the fitter’s preference.
Comparing the three systems
| System | What it is | Suits | Watch for |
|---|---|---|---|
| Torch-on felt | Two or three layers of bituminous membrane, heat-bonded on site | Larger areas, complex shapes, roofs with several penetrations | Needs an experienced hand with the torch near timber and adjoining walls |
| EPDM rubber | A single sheet of synthetic rubber, usually with no joints in the field | Simple rectangles, garage and extension roofs, quick installations | Joints and upstand details are the weak points; the field almost never fails |
| GRP fibreglass | Glass-reinforced polyester laminated and cured in place | Balconies and walk-on areas, roofs where a hard surface is wanted | Needs dry, mild weather to cure, and a stable deck — it does not like movement |
Named systems we use include IKO for torch-on, ClassicBond for EPDM and Cure It for GRP.
What actually makes a flat roof last
Flat roofs are not flat. They are laid to a fall so water leaves; where they fail early it is almost never the membrane in the middle.
Falls and drainage
Water standing on a roof for days after rain — ponding — is the clearest sign the falls are wrong or the outlet is in the wrong place. It ages every system faster and it is a deck problem, not a covering problem. Re-covering over it buys a few years and no more.
Upstands and terminations
Where the covering turns up a wall, meets a door threshold, or stops at a drip edge is where nearly every flat roof leak starts. An upstand that is too short, or dressed into a chase that has not been sealed, will let water in whatever the membrane is made of.
The deck
A soft or springing deck fails whatever goes on top. Where boards have been getting wet, they are replaced before the new covering goes down — quoting a re-cover without checking underneath is how a two-year-old roof ends up needing doing again.
Insulation and ventilation
A cold-deck flat roof needs a ventilation path; a warm-deck construction puts the insulation above the deck and does not. Mixing the two, which happens whenever a roof is upgraded piecemeal, causes condensation that looks exactly like a leak.
Flat roofs we have laid
Case studies are published here as jobs finish, with photographs taken on site before, during and after the work.
Flat roofing questions
How long does a flat roof take to install?
A straightforward extension or garage roof is typically one to three days on site once access is set up, depending on the size and on whether the deck needs work. GRP adds a curing window and cannot be laid in cold or wet weather.
Can a flat roof be repaired, or does it all have to come off?
Localised damage can be patched, and a sound roof with one failed detail is worth repairing. Where the covering has reached the end of its life across the whole area, or where water has been getting into the deck, patching is money spent twice.
Why does water sit on my flat roof after rain?
Because the falls are insufficient, the deck has deflected, or the outlet is blocked or badly positioned. Ponding is worth correcting when the roof is re-covered — it is a deck-level fix, and it is far cheaper to do then than to do later.
Do I need planning permission for a new flat roof?
Replacing an existing flat roof like for like is normally maintenance rather than development. Changing its height, adding a balcony or a roof terrace, or working on a listed building or in a conservation area can all change that. Check with the local authority before committing — we can tell you when we think it is a question worth asking.
Can I insulate at the same time?
Yes, and re-covering is the sensible moment to do it. Bringing a cold-deck roof up to a warm-deck construction while the covering is off costs a fraction of doing it as a separate job later, and it removes the ventilation problem that comes with cold decks.
Not sure what your roof needs?
An inspection ends with a written report and photographs, whether or not you go ahead with the work.
Inspection fee: £99.00.