Types of Roof Water Drains and How to Size Them

Commonly Used Roof Water Drains and How to Size Them for Your Roof

Cleaning a rain gutter
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Roof drains are designed to carry water away from the roof to protect it. A roof drain installed with the wrong size of pipe will not perform as expected. In fact, it may generate more problems than it solves.

Roof drains come in different shapes, sizes, and will vary on the roof and building where they will be installed. When roof water drains are designed and installed properly, they will still require regular maintenance to do the job well.

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 Water Drain

Your roof drains should be equally spaced and located in low points of the roof or in any area where water accumulates.

To determine your roof drain size, you must first calculate the surface area of your roof that the drains will be covering as well as the average hourly rainfall rate at the location where the project is located. 

Next, 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. Pick a leader (drain pipe) size large enough to handle the volume of water to be drained—the larger the pipe the fewer the roof drains needed. 

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 (i.e., how much water can the roof hold up if the drains are obstructed). This is really important as too much water can compromise the structural stability of the roof.
  • Drain locations
  • Drain sizing
  • Additional features such as vandal proofing and others

Flat Roof Water Drain

If your structure has a flat roof you will need a flat roof drain. This type of drain could be installed on a 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 obstructions from blocking the drain or the gutter.​

Siphonic Roof Water 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 to a single rain leader. The siphonic roof drain utilizes siphoning as a mechanism to drain water faster than any other roof drain. It normally costs more than a regular roof drain but it works smoothly and reduces the need for maintenance.

This type of drain can be connected using horizontal rain leaders instead of having sloped or downward pipes.

Green Roof Water 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 is designed to accommodate drainage while protecting the building structure with heavy waterproofing membranes. Two types of green roof exist: intensive and extensive.