EPDM is a synthetic rubber sheet, usually laid in one piece on a small roof and bonded to the deck. Its appeal is that a domestic garage or extension roof can often be covered with no joints at all, and a sheet with no joints has nothing obvious to fail. What varies between quotes is almost never the rubber. It is the deck underneath, the falls, the trims and whether anyone is correcting the problems that made you replace the roof in the first place.
EPDM flat roof cost: you are buying a deck, not a sheet
The membrane is a small part of the cost and the least variable part. Stripping the old covering, replacing soft or delaminated boards, correcting the falls so water actually reaches the outlet, and fitting new edge trims is where the money goes. A quote priced by the square metre of rubber with no allowance for boarding is a quote that will change once the roof is open, so ask what happens if the deck is wet.
Falls are the thing people skip
A flat roof is not flat. It needs a designed fall to an outlet, and a great many older domestic flat roofs never had one or lost it as the joists sagged. Laying a perfect membrane over a roof that ponds gives you a perfect membrane that ponds. Correcting the fall means firring pieces or a tapered insulation layer, which costs more and is the single best thing you can spend money on.
Warm deck or cold deck
If the roof is being stripped anyway, that is the moment to decide whether insulation goes above the deck as a warm roof or stays between the joists. Approved Document L sets the thermal expectation when you renovate a thermal element, and building over old insulation without thinking about where the dew point lands is how a new roof rots an old deck quietly. Ask which build up is being quoted.
Where rubberised roofing and liquid systems differ
A single ply rubber sheet is a manufactured membrane bonded down. A liquid applied system is painted or rolled on and cures in place, which suits roofs with many penetrations, awkward upstands or shapes a sheet cannot follow cleanly. Liquid systems generally cost more per square metre and depend heavily on preparation and on the weather during application. Neither is better in the abstract, they suit different roofs.
Guarantees are two things
There is the manufacturer's material warranty and there is the installer's workmanship guarantee, and almost every real world failure is workmanship at a detail rather than the sheet giving up in the field. Ask how long the installer has been trading, whether the guarantee is insurance backed, and what happens at an upstand or an outlet, because that is where a flat roof leaks.
Questions people ask about rubber roofing
How long does an EPDM roof last?
Manufacturers quote long service lives for the membrane, and well detailed domestic installations routinely run for decades. The realistic answer for any given roof is that it will last as long as its worst detail, which is usually an upstand, an outlet or a trim rather than the open field of the sheet.
Can rubber go straight over an old felt roof?
Sometimes, but it is rarely the right call. Overlaying hides the condition of the deck, adds nothing to the falls, and makes the eventual strip more expensive. If the old covering is sound and the falls are good, an overlay can be defensible, and the installer should say why in writing.
Is rubber garage roof cost lower than an extension roof?
Per square metre a detached garage is often simpler, because there are no abutments to the house, fewer penetrations and easier access. An extension roof usually has a wall upstand, a door threshold or a window above it, and those details are where the labour is.
Do I need building regulations approval to replace a flat roof?
Replacing a significant proportion of a roof covering is normally notifiable, and the renovation triggers a thermal requirement for the element. Some contractors can self certify through a competent person scheme, otherwise it goes to building control.