Since flat roofs cannot shed water like a sloped roof, there are three different types of drain systems that are used to drain the water off the rooftop. They are gutters, drains, and scuppers. For each type of draining system there are pros and cons, how well it works is dependent upon your flat roof; what roofing system you have, any parapet walls, lots of trees and so on. Regardless of what type of draining system you have, it still needs to be maintained by keeping it clear of debris because any small amount of debris can cause a blockage for the water to exit the roof. Here are the three common drain systems for flat roofs.
Many flat roofs still use the gutters. Because the roof is flat there needs to be some kind of lean to direct the water to the gutters. Most of the flat roofs with gutters are not surrounded in all directions by parapet walls so the gutters are be installed where there is no wall and normally are bigger than the traditional 5” and 6” gutters on sloped buildings. If there are gutters on a flat roof they still need maintenance and care. Depending on the material that the gutters are, major neglect can cause rust, erosion and gutters can begin to rot. In order for them to work properly they need to be clear of debris. If they are not maintained they can get to the condition you see in the photos where the water just ends up sitting in the gutters and not draining down the downspouts.
Scuppers are similar to gutters except you don’t have the gutters but you have downspouts. Scuppers are common for flat roofs that have parapet walls. Many older buildings will have scuppers instead of the drain pipes. The way scuppers are is there will be an opening through the wall on the roof in a square or rectangle normally. These are on the very bottom of the wall so that water will flow into them; the water flows into the opening and into the downspouts going down the side of the building. These also need to be kept free of debris or anything blocking the opening in the wall. Bigger downspouts can be used to help keep from blockage if the building is surrounded by a lot of trees.
The last type of system used is the drain pipes that are actually under the roof and in walls. Instead of water traveling into a downspout on the side of the building the pipes are under the roof and run down walls to the ground. There are strains that are installed onto of the hole to help keep debris out but any little bit of debris can cause a blockage for the water and the roof will have ponding water issues. For large size of commercial flat roofs, the drains are common to help drain water away from the building. If a blockage does happen a professional has to be called to fix the problem and this can be a more costly repair than scuppers or gutters.