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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, seashells, 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

Video Transcript of a Drain Strainer

This is an illustration of a roof here. The substrate of the roof, and when most roofers install a drain, they would install it on top of the substrate and the plywood here and then they will install the membrane right over the substrate like this. What happens is, this particular part of the drain area becomes higher than the rest of the roof.

So, when it rains water will fill up in this area, behind this flashing and cause some pooling, unnecessary pooling on the roof, especially when there’s skylights or stuff like this around. Debris will collect around the strainer, and make the water pool even more. This can go as much as 3 or 4 inches. I’ve seen a roof fill up with water like this.

The best solution is:

The best way is to take the substrate and cut through this plywood, where the beams are, and lower this section of the particular roof and make a basin where the drain is supposed to be.

Once the drain is in a basin like this, it’s around 4 or 5 inches lower than the roof itself. Then when it rain, only the basin gets filled with water. And even if debris collects around the strainer, it will allow the water to fill up in the basin, and not on the roof itself, and then flow over the debris into the strainer.

This is the correct way to have a drain on a roof. So, you collect debris in this basin, and it gives you enough time to come and clean up the basin of this drain area instead of having the whole roof flooded.

Summary

The best thing is to get your drains either lower, or when they install a new roof have them lowered to a proper area like this.

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 pooling.

Old Style Drain Strainers

There are many old style 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.

This is a cast iron flat roof drain strainer. This was a badly designed drain strainer due to the small openings
Cast Iron flat roof drain strainers were not a good design

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.

Plastic flat roof drain strainers don't last and often are the cause for roof problems

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.

Cast Iron strainer that's well designed

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.

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, other types of plant life have 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.