Flat Roof Drain Strainers are Important
If appropriately designed, a strainer is supposed to keep larger objects from going down a drain, while allowing smaller debris through with the water flow. Most strainers, or domes, that cover a drain on flat roofs, are poorly designed because they block smaller debris and the strainer becomes clogged, preventing any water flow. Twigs, sea shells, and rocks should not be able to get through strainer holes or slots. These types of objects will cause blockage in drain pipes.
Drain Installation on a Flat Roof Video Transcription
Drain and Strainer Position Can Cause Ponding- Video
The video above shows how to correct the position of a drain and its strainer to prevent ponding.
Old Style Drain Strainers
There are many old styles cast iron drain strainers found on roofs. These old cast iron strainers were not well-designed for smaller debris to flow through with water because the slots are too narrow.
Metal vs. Plastic Strainers
There are many companies that manufacture drains and strainers. Most plastic strainers end up cracking and breaking due to harmful UV exposure over a long period of time.
Metal strainers last longer because they endure the outside elements and are heavier, therefore, will tend to stay on a drain without having to be bolted down, which makes for easier cleaning.
Cast Iron Drain Strainers
These images are from roofs that still exist. The drain and strainer were not replaced when a new roof was installed because they are costly. Ponding or pooling water is likely to occur due to the poorly designed drain which will cause stress and possible leaks on the new roof.
Plastic Drain Strainers
It is our experience that plastic strainers are the worst to install. They degrade quickly after only a few years due to constant harmful UV light exposure. With the force of water during rainstorms, we see how the brittle strainers shatter leaving the drain to become clogged with large debris.
Drain and Strainer Combination
The drain and strainer combination in the picture below is an example of a good drainage system. Slots on the strainer are big and wide enough for small debris to flow through. If debris collects in the catchment area, around the strainer, it is low enough, so water could flow over the top of the debris and down the drain.
Custom Made Strainer
In the picture below, we have found that the best strainer for any flat roof is one shown; it is a 2 x 2 ft custom made metal strainer with big slots or openings to let fine debris through. This is ideal when there is a catchment area around the strainer. The strainer has a lower profile than a dome-shaped strainer, which will prevent a high debris build-up around the strainer.
This part of the roof was lowered so if debris collects around the strainer, water could still flow over its top and down the drain.
As you have read, a strainer, is an important part of a drain system. If well-designed, it will require only minimal regular maintenance.
The conclusion to Drain Strainers
Often we are called to address large pools or ponded water that has collected on a flat roof because of improper drainage. These pools of water cause the roofing material to deteriorate prematurely and cause leaks.
During the spring season, we have seen some flat roofs in such a state, that not only has moss grown in the damp areas, but other types of plant life have also thrived.
In the winter months, these ponded areas freeze and cause stress to the roof structure, which may affect the interior parts of the dwelling.
We have found a tried and true way to alleviate the ponding of water where there is no slope for proper drainage. We cut in channels that are angled toward the draining area, which can solve a flat roof drainage problem and preserve the life of the roof.
It is a combination of sloping a roof and the installation of a drain basin that will give the best flat roof drainage performance to last for decades.