Nobody plans a flat roof. It arrives with the house, usually over a kitchen extension, a garage or a back bedroom that somebody added in the eighties, and it gets ignored until a brown ring appears on the ceiling below. Then the questions start. Can it be patched? Does the whole thing need to come off? Why has it only lasted fifteen years when the pitched roof next to it has been fine since the war?
The honest answer is that most flat roofs fail for dull, predictable reasons, and almost all of them were built in at the start.
Water that has nowhere to go
A flat roof is never actually flat. It should fall at least 1 in 80 towards an outlet, and the good ones fall 1 in 40. When the deck was laid level, or the joists have sagged, rainwater sits in shallow pools. Ponding does not leak straight away. It slowly cooks the surface in summer, freezes in winter, and loads the structure with weight it was never designed to carry. By the time the ceiling stains, the deck underneath is often soft.
Walk on a flat roof the morning after rain. If there are puddles still sitting there, that roof has a drainage problem no amount of sealant will solve.
The wrong material for the job
Three systems dominate domestic flat roofing in Britain, and each fails in its own way.
Built-up felt, torched on in layers, is the traditional choice and still perfectly good when it is done well. It fails at the laps and at the upstands, where the felt turns up a wall or around a pipe. A felt roof with a single poorly bonded lap will let water in for years before anyone notices.
GRP, the fibreglass system, is laid wet over a timber deck and cures into a single sheet. No laps, no seams, which is why people like it. It is also rigid. If the deck moves, and timber decks move with every change in humidity, a GRP roof laid on a poor substrate will crack along the board joints. It also needs dry weather and a competent installer; a GRP roof laid on a damp day will blister within a year.
EPDM, the rubber membrane, is the most forgiving. It stretches with the building, tolerates standing water better than the other two, and comes in sheets big enough to cover most domestic roofs without a seam. Its weakness is adhesion at the edges and the quality of the trims. Cheap trims and a rushed bond at the perimeter are how a thirty-year membrane ends up lifting in five.
Repair or replace
A repair makes sense when the failure is local and the deck is sound: a split lap, a cracked upstand, a flashing that has pulled away from the wall. Those are a morning’s work.
A repair does not make sense when the roof is ponding, when the deck feels spongy underfoot, when there are multiple patches already, or when the insulation has been wet for a season. Wet insulation does not dry out. It sits there losing most of its thermal value and feeding damp into the timber. At that point you are paying for repairs to a roof that is already finished.
There is also a regulatory point that catches people out. If more than half of a flat roof is being renewed, current building regulations in England treat it as a thermal element and require the insulation to be brought up to standard. That often means a warm roof build-up, with the insulation on top of the deck, which raises the finished height and changes the detailing at doors and thresholds. A roofer who quotes to simply strip and re-cover without mentioning this is either unaware of the rule or hoping you are.
What a good replacement looks like
A proper job starts with the deck. Rotten boards come out, falls are corrected with firring strips, and the outlets are repositioned if the original ones were in the wrong place. Then insulation, then the membrane, then the details: upstands of at least 150mm against walls, drip trims that throw water clear of the fascia, and a flashing chased into the brickwork rather than stuck to the face of it.
It is the details that separate a roof that lasts twenty-five years from one that lasts eight. Anyone looking at flat roof replacement in East London should ask to see how the installer handles the upstands and the edge trims before they ask about price, because that is where the next leak will come from.
Five questions worth asking
Ask which system they are proposing and why that one for this roof. Ask what fall the finished roof will have and where the water will go. Ask whether the insulation will be upgraded and whether the quote includes the extra height at the thresholds. Ask what the guarantee covers and who backs it, because a manufacturer-backed guarantee on the membrane is worth more than a roofer’s promise on the labour. And ask whether they will photograph the deck once it is exposed, so you can see what you are paying to fix.
A flat roof is not a lesser roof. It is simply one with fewer places to hide a mistake, and the mistakes tend to be made in the first week of its life rather than the last.
Image- https://www.pexels.com/photo
