Why online prices rarely match
Search for flat roof cost and you'll find figures per square metre that look tidy and precise. The trouble is they assume a perfect roof: easy access, a sound deck, square edges and nothing coming through it. Real extension and garage roofs rarely look like that. Two roofs of the same size can differ widely in price once the strip-off reveals what's underneath. That's why we visit, measure and look before putting a price in writing, and why we don't publish figures here.
Roof size and shape
Area matters, but shape matters nearly as much. A plain rectangle over a garage is quick to cover. An L-shaped extension with a rooflight, a soil vent and two walls to turn up against involves far more cutting, forming and detailing. Every corner and penetration adds labour, and those are the parts that must be done slowly.
Which system you choose
EPDM roof cost, GRP roof cost and felt roof replacement cost each depend on different things:
- EPDM rubber: one sheet goes on quickly on simple roofs, so labour is often modest; complex corners take longer. More on our EPDM rubber roofs page.
- GRP fibreglass: laid and cured in place, it needs good weather and careful laminating, with topcoat finishes adding to the job. See GRP fibreglass roofs.
- Torch-on felt: a multi-layer build-up; the number of layers and the cap sheet chosen affect the cost. See felt flat roofs.
We recommend the system that suits the roof, then price it. If you'd like a second system priced for comparison, just ask.
The deck underneath
This is the biggest unknown. Once the old covering is off, the boards are checked. Damp, swollen chipboard or rotten planks have to be replaced, because no covering lasts on a failing deck. Sometimes the joists need attention too, or firring strips are needed to give the roof enough fall to drain. A good quote explains how unexpected deck work will be handled, so there are no surprises.
Upstands, trims, outlets and flashing
Where a flat roof meets a house wall, the covering is turned up and protected by flashing. If the old flashing is tired, renewing it adds to the price but closes off one of the most common leak points. New edge trims, drip details and outlets are part of doing the job properly, and leaving them out is how cheap roofs stay cheap for only a short time.
Access and height
A single-storey garage at the end of a drive is easy. A dormer on the second floor or an extension reached only through a narrow side passage needs more planning, sometimes scaffolding or a tower, and more time carrying materials and waste. That all shows in the quote.
Waste and strip-off
Heavier old roofs with several layers of felt and chippings take longer to strip and produce more waste than a single old membrane. Older corrugated sheets on garages may need checking for asbestos before removal, which changes how they are handled.
Repair costs versus replacement
Flat roof repair cost and felt roof repair cost are driven by the size of the damage and whether the boards beneath are wet. Repairs make sense when the rest of the roof is sound. When the surface is failing all over, our flat roof repair page explains why we'd suggest replacing instead.
Compare the inclusions before comparing the totals
Put two written proposals side by side and check whether both include removing the existing covering, deck preparation, perimeter details and disposal. A price for a membrane laid over retained boards is not directly comparable with a complete strip and rebuild. Ask how additional timber work will be assessed and agreed if it cannot be seen before opening the roof. Clear inclusions help you understand the figure without needing an unreliable online rate per square metre.
Intended use also matters when weighing repair against replacement. A dry storage garage and an extension over a heated room may require different consideration of insulation and moisture control. Our flat roof lifespan guide explains how installation and upkeep affect service life. Use that context alongside the condition of your roof rather than assuming a material name guarantees a particular lifetime or makes a cheaper proposal better value.
Need it sorted?
Book a flat roof replacement visit and we'll measure up, check the deck where possible and send a free written quote. Or use the quote form below. For an isolated defect, book a flat roof repair assessment instead; you can send the details through the quote form without deciding on a whole replacement first.