Best Flat Roof Drain System

Drain Basins are the most effective drain systems

large strainer for a shallow flat roof drains
Leaf Filters for drains are a very important factor that most people don’t understand

A well-designed drain will prevent leaks on flat roofs

A pleasant surprise about a flat roof drain on a commercial roof we installed more than 15 decades ago. After so many years I am still amazed how well it performs. The owner never bothered to keep the roof clear of leaves and small twigs, but the drain performed as it was intended to.

lowered flat roof drains are the most effective drainage systems
This image shows a drain that was installed in a basin. After many years and collecting debris in the basin, this roof was still without problems

All flat roofs with internal drains should have this sunken base drain. We could still improve on it by lowering it more. But the principle we applied help for less maintenance and a roof that would last much longer.

The best drain system is those drains that are installed in a drain-basin. A drain basin is a recessed area around the drain that is 4 - 8 inches lower than the roof surface itself.

It is best practice to create a basin for a drain to be installed in. 

This is a sketch of a drain in a basin:

A sketch of a drain inside a basin
A sketch of a drain inside a basin

Leaf filters or strainers are extremely important to keep large objects from flowing down the drainpipe and cause a blockage.

There are many styles and types of leaf filters for drains:

  1. Plastic leaf filters - these leaf filters become brittle in the sun and eventually break allowing bigger objects to clog the drain
    Plastic leaf filter cracks in the sun
    Plastic leaf filter cracks in the sun
  2. Metal leaf filters - there are many styles to choose from
    • Aluminum leaf Filters - they are lighter and have larger holes or slots allowing finer debris to go down the drain
      Always make sure you have a large metal drain strainer for a flat roof repair
      Aluminum Leaf Filter with large slots and good height
    • Cast Iron Leaf filters - they are heavy and have smaller slots filtering smaller debris causing the strainer to clog up
      Leaf filter with small slots
      Cast iron Leaf filter with small slots
    • Wire mesh filters - these filters are usually custom-fitted for each drain and are effective because they can cover a larger area of filtering
Large wire mesh leaf filter covers a larger area
Large wire mesh leaf filter covers a larger area

Leaf filters come in different sizes and heights. The best leaf filter is metal and has larger slots to allow small debris through. The height is also important.

See this sketch of a high leaf filter to understand why it is important - The height is important because when the debris clogs the leaf filter at the bottom, water can still flow over the debris into the drain

A sketch of a drain inside a basin
A sketch of a drain inside a basin

The three types are:

  1. Inner Drains - these are the drains normally found on the inside of a roof where water would flow towards the center or where the drain is located.
  2. Scuppers - an opening in the wall of a building through which water can drain from a floor or flat roof
  3. Gutters - a trough along the eaves to catch and carry off rainwater from a roof

When Inner drains are present, it is best practice to have a scupper drain as a secondary drain incase the inner drain clogs up

Watch the video demonstrating a flat roof drain basin. This roof drain system should be on all flat roofs with internal drains.

This is a video showing a drain inside a basin. This is the ultimate roof drain design

The Most Efficient Drain for Flat Roofs

Scuppers on commercial and residential roofs are high efficient drains. Check this page out about scuppers

Flat Roof Drains on Commercial Flat Roofs

Flat roof drains are the most critical components of any commercial roof. Drains are responsible for removing water quickly and efficiently. The strainers over drains are what causes the biggest problem. Most of the time, strainers that come installed with a drain are not well designed. There are several ways you can improve the effectiveness of a drain system.

We designed a roof drain system that is very effective and practical. The most significant problem for flat roof drains is debris that collects around the strainer. It is inevitable to happen, especially if nearby trees or branches are overhanging the roof. In most cases, it means someone has to clear the roof and the strainers continually.

There are several reasons why drains on flat roofs don’t function correctly:

  1. Placement of drain – the roof is not pitched towards the drain
  2. Overhanging  branches can cause drains to block
  3. Strainers slots or holes are too small
  4. Drain size too small for the roof surface

With our video, we demonstrate how to improve an existing drain. With this type of improvement, most drain problems can be solved and increase the roof’s life expectancy. Standing water on a flat roof can lead to material degradation and cause major leaks.

In this 3D video, we illustrate how effective a drain can be by lowering it. This kind of improvement can cost as much $2,000 per drain but in the long run, is worth it. It will save the roof and prevent leaks.

The video above shows:

To cut a square area of the roof decking away around the drain. Then shorten the drain pipe and reconnect the pipe to the drain. After the roof is reconnected to the drain, water can freely flow into the drain.

The debris can accumulate in this drain basin and the water can still flow freely through the strainer. It is important that the strainer has large enough slots so smaller debris can get through. If the slots are too small, then small debris will end up clogging the strainer and water will be prevented to flow freely.

Flat Roof Drain Strainers

There are conditions on a roof that is not practical to lower a drain as illustrated in the videos above. If that’s the case, then an alternative solution is to fabricate a large strainer that will fit around the drain. The design and configuration of a strainer are critical. A strainer with small holes or slots will collect fine debris that will clog the strainer to quick and cause water to pool or back up.

A strainer should allow small debris through and only keep back larger debris like twigs and objects that could get stuck in a drain and cause a blockage.

Self main Strainer or leaf filter. This is an example of a low profile leaf filter we made.

Watch the video on how strainers on a drain can make the difference.

This video shows a drain we installed many years back in a low lying basin. Notice the high leaf filter. This helps when debris collects around the filter that water will rise and flow over the debris into the drain

Watch the video to see how a larger strainer around a drain can be effective.

In this video, we were able to lower the roof only a few inches but made a large strainer with a low profile. When debris accumulates around the strainer and starts preventing water through, the water will rise slightly to flow over the strainer into the drain.

Roof drain strainers to avoid or that should be replaced

Drain strainers on a roof are very important and careful consideration should be given to the size, materials it’s made of and the slot or hole sizes.

There are basically 3 types of drainage options for flat roofs.

The results when a drain is not maintained regularly

This is very obvious that this drain wasn’t cleared for a long period of time. The sad part is that the roof got compromised. Notice all the ripples in the membrane. The pitch of the roof is perfect towards the drain and the drain is positioned perfectly. The strainer had too small slots or holes so that small debris could not get through and or that no one kept it clear from debris.

Scuppers are very efficient drainage systems. If possible, scuppers should always be considered before a pipe drain system

This leaf filter on this roof was never cleaned. The results is a natural garden.

Metal Drain strainer with small holes

The filter on the drain in the image dates to the early 1900s. The irony is that many of these drains and filters still exist in older buildings. When a re-roof is called for, most of the time drains don’t get replaced. The reason is that it is attached to an existing pipe and to replace the drain would take too much time and become costly. In the image above where the plants are growing on the roof, this type of strainer was in place.

A leaf filter with small holes. This is not efficient because small particles clog the strainer fast blocking water from draining off the roof.

Plastic Dome Drain Strainer

This is the worst type of strainer or leaf filter on a drain. The plastic gets brittle due to the ultraviolet lights and heat on the roof and cracks after a few years. Replacing these strainers are a must to avoid future drain blocking.

Flat Roof Drain - Plastic-strainer-or-dome-for-drains
Plastic leaf filters or strainers don’t last long on a flat roof.

Drain is lowered into the roof

Notice the drain is lower than the rest of the roof surface. When we re-roofed this roof, we used the opportunity to lower the drain. The top of the strainer dome is level with the rest of the roof surface. This is a perfect example of how it should be done. it is very important to use metal strainers. If the roof is covered with snow, the drain will be hidden. When someone removes and shovel snow, they will damage the strainer or break it if it’s not metal.

A flat roof drain lowered into the roof
A drain in a basin. Very effective

Lower Roof Drain

Notice how the twigs have accumulated around the drain. This is an excellent example of how well this drain system works. The rest of the roof is clear and most of the debris is in the drain basin, yet the water could still drain out without pooling up on the rest of the roof.

Large Strainer for a Shallow Drain

When it is not practical to lower the drain considerably, then another option is to make a large strainer. The strainer should have a low profile so when debris collects around the strainer and clogs it, then water can still flow over the strainer top side and flow out.

large strainer for a shallow flat roof drains
Another solution to drains, is to make a lower profile strainer or leaf filter

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