EPDM flat roof costs area times the installed allowance plus confirmed extras and tax; the default example is about £2,100.
Separate the membrane from what is underneath
A flat roof has more to it than the visible waterproofing. Measure length and width to find its plan area, then select felt, EPDM rubber or GRP fibreglass. If there is a meaningful slope, enter the pitch so the existing roof geometry helper adjusts the surface. The membrane allowance covers installation, but deck boards, insulation, access and edge detailing can have different inclusions between contractors.
The deck percentage prices only the portion being replaced. Set it to the surveyed share of the roof and enter a before-VAT deck rate for that area. A warm-roof insulation quote applies across the whole measured roof when the toggle is on. Additional upstands and trims use your measured length times the quoted rate per metre. The calculator requires a price when one of these measured extras is selected; an unpriced upgrade is not silently treated as free.
Worked example: 18 m² of EPDM
With a 6 m by 3 m roof at zero pitch, the roof area is 18 m². The net EPDM triple is £77.50 / £96.25 / £115 per m². The middle installation allowance is £1,732.50. Standard 20% VAT is £346.50, making £2,079, displayed as approximately £2,100. This example replaces no deck, adds no insulation and includes no extra trim or access quotes. Confirm the removal scope with the installer before using it as the full project budget.
New flat roof cost per m2
The published ranges below use pounds per square metre of installed membrane. Select the material before comparing the rate; board replacement and edge details need their own scope.
Rubber roof cost calculator: EPDM
EPDM is a rubber membrane system. Ask about the substrate, perimeter fastening or adhesion, penetrations and the particular product warranty. A single sheet does not eliminate the need for good outlet, edge and wall details. A deck that is damp or soft needs inspection and an appropriate repair specification before the membrane is fitted.
Fibreglass roof cost calculator: GRP
GRP is a fibreglass system whose preparation and detailing need a suitable deck and compatible components. Compare the whole specified assembly rather than purchasing a resin allowance alone. The felt choice is a bituminous installation allowance, not a promise that a thin shed felt is suitable for an occupied extension.
| Membrane | Checkatrade before VAT, GBP/m² | MyBuilder installed, GBP/m², tax unstated |
|---|---|---|
| Felt / bitumen | £50–80 | £70–100 |
| EPDM | £80–100 | £90–130 |
| GRP | £95–120 | £95–150 |
Read 1 October 2026: Checkatrade flat roof replacement and MyBuilder flat roofs. The first guide explicitly excludes VAT and several extras. The second guide has different preparation scope and does not state its tax basis. The calculator accommodates that uncertainty in the net range; a confirmed before-tax installer rate narrows it.
Insulation and Part L
Planning Portal's thermal-element guidance explains that significant roof renovation can require an insulation upgrade. This is England guidance, with project-specific conditions. Do not treat a percentage of the roof alone as a universal compliance test. Building control can advise on the applicable renovation trigger, insulation standard and ventilation arrangement. A quoted insulation cost does not certify that the assembly complies.
For an unheated outbuilding, try the garage roof replacement cost calculator UK. Use the UK roof replacement cost calculator for the pitched house, the conservatory roof replacement cost calculator for glazed-room systems, and the gutter slope calculator for drainage measurements.