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Flat Roof Drain Strainers or leaf Filters Important functions

This video explains the principle of a drain basin

If appropriately designed, a strainer or leaf filter 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.

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/ Leaf Filters

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
Drain Strainer or Leaf Filter. This is an old Leaf Filter on a cast iron drain with very small slots and holes. This is one of the most ineffective leaf filters on a drain
Cast Iron flat roof drain strainers were not a good design
Another image of old cast iron Leaf Filters or Drain Stainers. The holes and slots are very small therefore blocking smaller debris flowing through.

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
Plastic Leaf Filters are the worst to have. They brake and allow debris to clog the drain

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
The Leaf Filter in this image has slightly larger slots which will allow smaller debris to get through

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.

Custom made flat roof drain strainer is by far the best
This image shows a custom made Leaf Filter or Drain Strainer. The idea is to let water flow over the leaf filter after debris collects around the side.

The conclusion to Drain Strainers or Leaf Filters

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 drainage basin that will give the best flat roof drainage performance to last for decades.

https://youtu.be/6J6LBOA5qVM
This video explains the benefits of a drain in a basin with a large leaf filter or strainer keeping debris out
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