After every heavy rainfall, flat roofers are sure to get many phone calls from building owners reporting “roof leaks.” It is very common to diagnose the leak as originating from a place other than the roof. However, the leak will appear to be a roof leak from the perspective of the building owner.
There are many different places that water can come from other than your roof. A common leak is from a clogged roof downspout distal from the conductor head just outside the through-the-wall scupper. When the downspout is blocked and water fills the conductor head, the water will find its way into the building through the masonry wall behind the conductor head. Because masonry is not waterproof, the water in the conductor head will make its way through the masonry into the building. Because the scupper is above the ceiling, the water will present at the wall/ceiling interface just below the scupper.
Another common leak is a blocked roof drain. If the roof drain is not equipped with a backflow preventer when the drain becomes clogged, the water will back up onto the roof. The head pressure of the water will force the water under the roof membrane. The leak will present near the drain; however, depending on roof design, the leak could present quite a distance away from the drain.
Water intrusion through a masonry wall is also very common. Masonry walls are drainage systems, and they are not waterproof. If water gets behind the masonry wall, the water will drip down the wall to the base flashing (if the wall was flashed correctly). If the masonry wall stops at a flat roof, the leak will appear to be coming from the roof. These leaks can happen through any type of siding, not just masonry. When these leaks are diagnosed, it is often hard to explain to the building owner that the leak is not from the roof.
Other leaks originate from open windows, guttering that is full of debris, and condensation from HVAC equipment in the ceiling.
It is always important to thoroughly study the leak. Get another person to give their opinion and think, think, think. I’ve been wrong about leaks many times, and each time I take note of what I have learned so that I don’t make the same mistake again.