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Flat Roof Drains: Location, Installation & Maintenance

Flat roof drains require the proper location, installation, and maintenance because, as a system, it is fundamental to the preservation of a building’s structure. We have seen many times that the location of a drain on a higher level than the rest of the roof which causes it to not function. Most, if not all, structural leaks begin with some drainage issue, so it is essential that flat roofs design with the drains in the proper location.

When drain locations are determined, properly functioning, high quality drains should be installed to complete the system. Without a well-designed drainage system and regular maintenance, commercial flat roof membranes will be compromised and not last the 30 to 40 years as meant.

Flat Roof Drain Issues

  • Location – Drains SHOULD NOT be next to other rooftop objects like A/C units, vent pipes, or skylights.
  • Level – Drains SHOULD always be installed at the lowest level of the roof surface.
  • Strainers/Domes – Drain strainers SHOULD have holes or openings large enough for small debris to pass through.
  • Piping – Drain Pipes SHOULD  be large enough to handle the volume of water typical for rainstorms.
  • Trees – Branches SHOULD be pruned back to prevent hanging over any part of a flat roof.

Flat Roof Drain Maintenance

Flat roof drain maintenance is essential to the life of the roof and ultimately to the building’s structure. As seen in the video, the flow of rainwater, to the drain, is restricted by a small accumulation of debris around the strainer which clogs it and prevents water from reaching the drain. This pile of wet debris left on the roof, without being removed during regular maintenance, will cause the deterioration of the seams and sealants of the roof.

When Drains are Installed Unnecessarily 

In the image below, the drain was installed to remove ponded water next to a gutter system. We determined that not only was this drain unnecessarily, but it was also too close to a wall. This roof was designed to drain water to a gutter system. The solution would have been to build up the area where water pooling or lower the edge of the roof at the gutter.

Flat Roof Drains - This drain was installed unnecessarily and too close to the wall.

Roofs Without Maintenance

The image below is an example of a strainer’s slots being too narrow to allow small debris through. As a result, water pooled around the drain and compromised the seams and flashing of the roof at the wall, which caused leaks.

Flat Roof Drains - Roofs are Compromised Without Maintenance

Flat Roof Drain Level

The video below shows one of the most common problems we find with both commercial and residential flat roofs. A drain should always be installed lower than the rest of the roof. A good roofing mechanic will remove the existing plywood and lower the drain to include a square catchment area around the drain. This will help with the collecting of debris. We lowered this drain and added a double membrane for durability for snow shovel maintenance and standing precipitation.

Flat Roof Drain Basins

It is essential to the design of a new flat roof; provision is made to install a lower roof drain basin. Because of the basin, around the drain area, is much lower than the roof surface, the basin will likely collect debris that has washed from the roof into the basin, allowing room for water to rise above the debris. The basin area can be 3′ x 3′ and up to 5′ x 5′ wide.

Watch the video below which illustrates how a drain basin functions.

Image of a Functional Drain Basin 

When we installed this roof 20 years ago, we created a basin around the drain. You can see that debris collects around the basin catchment and the strainer; however, water was still able to flow over the debris.

Flat Roof Drains - Drain with a catchment basin used to collect debris.

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