When a flat roof leaks, people look at the middle of the roof. That is usually the least interesting part. The main area is one continuous surface that stays in good condition for twenty or thirty years.
It goes wrong where the covering has to do something: go up a wall, over an edge, around a pipe, against a rooflight. Every time a roofer has to bend, cut or join the covering, a spot is created that will one day come under strain.
The six places where it goes wrong
The upstand. Where the covering goes up against a wall or a roof edge. If the covering shrinks or sags, it pulls at the bend. This is the most common cause on flat roofs on an extension.
The roof edge. Roof edge trim and roof edge profiles expand and shrink with the temperature. Loose screws or a seam that has opened let water get behind the covering. See
roof edge finishing.
The roof penetrations. Vents, cables, mechanical ventilation. Every pipe through the roof has a collar or a flashing piece that ages. See
roof penetrations.
The rooflight. The edge around a rooflight is a joint and also a place where water flows past. Old rooflights also develop hairline cracks in the plastic.
Standing water. Where water cannot drain away, it is always sitting on the covering. That speeds up ageing and puts strain on seams. See
fall on a flat roof.
The rainwater outlet. A blocked or undersized outlet lets the water rise above the lowest seam. Then a roof that is otherwise completely fine starts to leak.
“On a flat roof it almost never goes wrong in the middle. It goes wrong where the covering had to do something.”
Fouad, owner of Astrodak Service
Blisters, cracks and a spongy feel
There are three signs you can see yourself without knowing anything about roofing. Blisters in the covering mean trapped air or moisture and are a sign that the layer is no longer stuck down everywhere. See
blistering.
Small cracks in the top layer, especially at the bend of an upstand, mean the material has lost its stretch. And if the roof feels spongy when you walk on it, there is water in the insulation. That last one is the least good news, because then there is more going on than sealing a seam. See
wet insulation.
A leak? Do not let it spread.
A leak you leave for a season usually costs ten times as much in follow-on damage. Call or send a message: you will get an answer today, from me personally, not from a scheduler.
Why a flat roof sometimes only leaks days later
On a pitched roof, water gets out again quickly. On a flat roof it does not. Water that comes through a seam first spreads through the insulation and only finds its way inside much later. That is why the stain on your ceiling can appear a day after the rain and be metres away from the real cause.
That makes tracing a leak on a flat roof harder than on a tiled roof, and it is why on a flat roof we first check whether the insulation is dry before we repair a seam. A repair over wet insulation keeps the water in rather than out.
Repair or renew the covering
On a roof less than fifteen years old with dry insulation, a targeted repair is usually enough and you are set for years again. If there is water in the insulation, if there are already several layers of bitumen on it, or if small cracks are appearing in different places, you are repairing a roof that is reaching the end in more places.
Why a test hole is sometimes needed
With a flat roof the question is not only where the leak is but also how far the moisture has spread. You cannot see that from outside, because the covering is vapour-tight and the insulation underneath can be saturated without it showing on top.
A test hole, a small square that is cut open and then neatly sealed again, gives you that answer in ten minutes. Making a hole in a roof that leaks may not sound appealing, but it is considerably better than laying a new covering on wet insulation.
Frequently asked questions about flat roof leaks
