The cheapest flat roof today is often the most expensive one over thirty years. Here is what each system costs to install and what it costs to own.
Installed cost per square metre
High performance torch on felt typically costs 85 to 110 pounds per square metre installed. EPDM rubber sits at 100 to 140 pounds. GRP fibreglass is the most expensive at 130 to 180 pounds per square metre.
On a typical 20 square metre garage roof that means roughly 1,700 to 2,200 pounds in felt, 2,000 to 2,800 in EPDM and 2,600 to 3,600 in GRP. An extension roof of 25 to 30 square metres with upstands, an outlet and a skylight will sit at the upper end of each band.
Cost over thirty years, not thirty days
Felt gives you around 15 to 20 years. EPDM is commonly quoted at 30 years or more and a single sheet installation has very few joints to fail. GRP lands at 25 to 30 years with a hard wearing surface you can walk on.
Divide installed cost by expected life and the picture inverts. Felt on a garage is sensible value. Felt on a habitable extension you plan to keep for decades usually is not.
The hidden costs under the covering
Deck condition is the biggest variable. Rotten OSB or chipboard is common on roofs that have leaked for a while and replacing it adds materials and a day of labour. We always quote for the deck we can see and state a rate for boards found rotten once stripped.
Insulation is the other one. Where a flat roof over a habitable room is fully replaced, building regulations expect the thermal performance to be brought up to standard, which adds board cost and can raise the roof height enough to need new edge trims and flashings.
Choosing by use, not by price
For a garage or shed, felt does the job at the lowest outlay. For a rear extension over a kitchen or living space, EPDM is usually the best balance of price and lifespan. For a balcony or a roof that will be walked on or used as a terrace, GRP earns its premium.
Dormers and awkward shapes suit EPDM because the sheet can be dressed around detail without joints. Very small complicated roofs sometimes suit GRP because it is laid wet and moulds to the shape.
Which flat roof system is cheapest overall?
Felt is cheapest to install. EPDM is usually cheapest per year of service once you account for lifespan and the fact it rarely needs interim repair.
Can EPDM be laid over an existing felt roof?
It can in some cases if the deck is dry and sound, but we generally advise stripping. Laying over trapped moisture is the most common reason a new flat roof fails early.
How much does a garage flat roof cost to replace?
For a typical single garage of around 18 to 22 square metres, budget 1,700 to 2,200 pounds in felt or 2,000 to 2,800 pounds in EPDM including strip, deck checks and new trims.
Do flat roofs need to be perfectly flat?
No. Every flat roof needs a fall, usually around 1 in 80, so water reaches the outlet. Ponding water is the enemy of every system regardless of price.
Membranes carry manufacturer guarantees and we give a separate written workmanship guarantee. Ask for both in writing rather than accepting a verbal figure.
Evans Roofing & Maintenance Ltd, Witney, Oxfordshire. Call 07960 482233 for a free written quote.