I know from experience that locating a leak in a flat roof can be very difficult and expensive. In our home it took three different roofers, making multiple visits, to identify the place in our flat roof that a leak was coming from. The reason why pin-pointing leaks in flat roofs is difficult is that very often the place where the leak shows in the ceiling below can be quite far away from the place at which the leak is occurring in the flat roof itself.

The best places to look for leaks on a flat roof are flashing or joints or seals of any kind. Now, flashings are the point at which the roof joins adjacent non-roofing material such as brickwork. For example above our kitchen extension we have a parapet wall that is at one edge of the roof and flashing closes the gap between the roof and the wall.

As described at flat roofing, a capable roofer should first examine such flashings to i.d. gaps through which water could perhaps pass. Flashings are typically made of a metal material such as lead .

It is also usually difficult to test for leaks on a flat roof in the way you would on other types of roofs. The reason is because it could take hours for water to trickle down to the ceiling below from the moment you pour it on a suspect area on the roof. This means that in practical reality it's rather hard to test and then track multiple suspect places on the roof. This is explained a bit more at the epdm roofing webpage.

The key to effective flat roof repair is to know in advance the areas that are likely to be vulnerable. The best test unfortunately is to carry out the work to close suspect gaps and then wait to see if the leak persists. Hence flat roof repair depends greatly on the intelligence, honesty and experience of the roofer you hire.