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Roof Drains: Types of Roof Drains

Common used roof drains, and information on how to size your roof drain.


Roof Drain
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Roof drains are now a combination of functionality and modern aesthetics, were water will be carried away from the surface being protected. A roof drain installed with the wrong size of pipe, will not be as useful as expected and sometimes it will generate even more problems than the one it is supposed to solve. When roof drains are designed and install properly, they will maintain your area free of water only if they are maintained on a regular basis. Roof drains come in different shapes, sizes, and will vary on the roof and building where they will be installed. Before deciding which roof drain to use, consider the roof size, pitch, drain location, inflow rate, safety requirements and rainfall volume.

How to Size Your Roof Drain

Your roof drains shall be equally spaced and located in low points of the roof or in any area on which water is accumulated has been identified. To determine your roof drain size first you need to calculate the roof area or surface area that the drains will be covering. The next important thing that you must know is the average hourly rainfall rate at the location where the project is located. There is plenty of information from which you can obtain this information. Pick a leader (drain pipe) size, the larger the pipe the fewer the roof drains needed. Now you must determine the roof area drained by each leader. This information could be obtained from the pipe or drain manufacturer. Divide the roof area by the area per each leader to determine the number of drains required.

How to Choose the Right Roof Drain

To choose the right roof drain; you must consider the following factors:

  • Type of roof
  • Roof pitch
  • Volume of average rainfall and storm design criteria (100 years, 50 years, 25 years and so on), normally set by your local building code.
  • Rate of drainage
  • Overflow requirements from local or state building code
  • Roof load (how much water does the roof can hold up if the drains are obstructed). This is really important as too much water could affect the structural stability of the roof.
  • Drain locations
  • Drain sizing
  • Additional features such as vandal proof and others

Flat Roof Drain

The ideal drain if your structure has a flat roof is a flat roof drain, designed to flow part full. This type of drain could be installed in flat or almost flat roof with less than a 2% slope. The drain will be connected to rain gutters and will help divert the water away from the structure. It is recommended that you install a flat roof drain with covers, to prevent debris and other potential obstruction elements get into the drain or the gutter.

Siphonic Roof Drain

A siphonic roof drain is designed to operate at full capacity, when water is vacuumed from the roof into the drain at higher velocities when compared to flat drains. These roof drains allow the designer to install a minimal number of roof drains and have them connected into a single rain leader. The siphonic roof drain incorporates a mechanism that allows water drain faster than using any other roof drain. It normally cost more than a regular roof drain but it works smoothly and reduces the need for maintenance. This type of drain could be connected using horizontal rain leaders instead of having sloped or downward pipes.

Green Roof Drains

The new trend in roof drains is to replace the traditional roof with a Green roof. Green roofs will reduce noise transmission, reduce heat inside the building, and improve air quality. A green roof drainage system will be designed to accommodate drainage while protecting the building structure with heavy waterproofing membranes. Two types of green roof exist: intensive and extensive.

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