Can a Leaking Flat Roof Be Repaired?

Where Flat Roofs Actually Leak

Almost never in the middle. The field of a flat roof is a continuous sheet with nothing happening to it. Everything interesting is round the edge.

The upstand, where the roof meets a wall. The covering has to be carried up the wall to an adequate height and either tucked into a chase or covered by a flashing. Carried up too low, or with the flashing over it lifted, and driving rain simply runs down behind it. A flat roof that only leaks when rain comes from one direction is nearly always an upstand.

The edge trim or drip. The perimeter detail that holds the covering down and throws water clear into the gutter. Wind lifts hardest at edges and corners, and once it has an air path under one corner it works at it every time it blows.

The outlet. Blocked outlets are the single most preventable flat roof problem there is. A blocked outlet holds water at exactly the point where the covering is most likely to have a joint, and holds it permanently.

Penetrations. Anything coming through — a soil vent, a flue, a rooflight kerb — is a joint in the covering and joints are where water gets in.

What the Covering Is Made Of Changes the Answer

Built-up felt. Layers of bitumen felt bonded together, usually three. Repairable while the top layer has life in it; once it has gone brittle, crazed or blistered across the whole area, patching is buying months.

EPDM rubber. A single sheet, so there are very few joints in the field and the failures are at the perimeter and the details almost by definition. Splits and seam failures are patchable with the right materials.

GRP fibreglass. Rigid, so it fails at movement points: cracks along a line where the deck flexes, and crazing across the surface with age. Repairable, though a crack that has appeared because the deck moves will reappear unless the deck is dealt with.

Single-ply membranes. Generally welded seams and generally repairable by somebody with the right kit for that system.

The one thing all of them have in common: the deck underneath matters more than the covering on top. If the boards have got wet and gone soft, no covering laid over them will last.

The Ponding Question

A flat roof is not flat. It should be laid to a fall — typically a minimum of around 1 in 80 finished, which usually means designing to something steeper to allow for deflection — so that water drains rather than sits.

Pools of standing water that are still there a day or two after rain mean the fall is inadequate or the deck has deflected. Ponding itself is not what causes a leak; what it does is keep the covering permanently wet, magnify the effect of any tiny defect, accelerate ageing and, in freezing weather, put ice loads where you do not want them.

So it is a symptom worth taking seriously, and it is also a reason a repair may not last: patching a covering that sits under water for weeks is a different proposition from patching one that drains.

How to Tell Repairable From Finished

Repairable: a sound-looking covering with a defect you can point at — a lifted trim, a failed upstand, a split at a corner, a blocked outlet. The rest of the surface is flexible, uncracked and doing its job.

Finished: crazing across the whole surface, brittleness so the covering cracks when it is pressed, widespread blistering, several existing patches from previous repairs, or a deck that gives underfoot. Any one of those means the next patch buys a season.

The multiple-patches one is the most reliable signal in practice. A covering that has needed three repairs in five years is not a covering with three faults; it is a covering at the end of its life producing a new one each year.

The Repair That Is Not a Repair

Liquid coatings applied over a failed flat roof are sold hard and are a genuine product with genuine uses. Applied over a sound covering as a refurbishment, they work. Applied over a covering that is already saturated, or over a deck that is wet, they seal the moisture in and the roof deteriorates underneath while looking fine on top.

The same goes for a new layer of felt torched over an old one that has failed. It traps whatever is underneath.

So the question to ask before any overlay is not "will this stop the leak" but "what is the condition of what is under it". If nobody has checked, nobody knows.