Flat roofs are priced by the square metre and paid for by the detail. Two roofs of identical area can differ by half again because one is a plain rectangle over a garage and the other has an upstand against a house wall, a rooflight, a parapet and an outlet that has to be reformed. The system on top is the part everyone asks about and the smallest part of the decision.
What is under the membrane sets the price
Almost every flat roof replacement turns into a deck job. Chipboard that has been wet swells and never recovers, softwood boarding rots at the ends where it meets the wall, and a deck that has sagged holds water in the middle of the roof for days after rain. A new membrane over any of that is money spent on a surface that will move. Ask whether the quote assumes the deck is sound, and what the price becomes if it is not.
The four systems, and how each one fails
Reinforced bitumen membranes are laid in layers and fail at laps and upstands, usually after decades and usually repairably. Fibreglass is laid as a single continuous surface and fails by cracking where the roof moves, which makes it excellent on a small stiff deck and poor on a large or timber-framed one. Rubber sheet is a single piece that tolerates movement well and depends on the quality of its bonding and its edge trims. Single ply is a commercial system, welded rather than bonded, and wants a trained installer.
Falls, outlets and the standing water problem
Standing water is what turns a small defect into a leak, and it is a design fault rather than a material one. A flat roof needs a fall to a designed outlet, and where the original firrings have sagged the new roof has to correct them rather than follow them. When you are given a price, ask what the finished falls will be and where the water is going, because a roof that ponds will find whichever weakness the membrane has, whatever it is made of.
Questions people ask about flat roof replacement cost
How long does a flat roof last?
Published lifespans for the same product vary enormously because the deck, the falls and the detailing do more work than the covering. A well laid roof on a sound deck with real falls outlives a premium membrane laid over a ponding one, which is why the survey matters more than the brand.
Can a new membrane go over the old one?
Sometimes, on a bitumen roof whose deck is dry and sound. It saves the strip cost and it hides whatever is underneath, so it is only sensible where somebody has established that what is underneath is dry.
Do I need building regulations approval to replace a flat roof?
Replacing more than a quarter of the roof covering is controlled work under the Building Regulations, and most flat roof replacements clear that threshold easily. Either building control signs it off or a contractor registered on a competent person scheme does.