How Many Drains Do I Need On A Roof?


Not all roofs are the same. While your roof’s main purpose is to keep your house and belongings safe and protected, they are not all built the same. Different climates call for different needs.

There are also many different types of roof drains, and the ones you need will depend on the type of roof you have, the material your roof is made of, the size of your roof, and the slope.

How Many Roof Drains Do I Need?

Water will run off of your roof naturally after rain or melting snow. But, it can get trapped and start to accumulate. If your roof doesn’t have a steep slope or there is damage, then water can start to pool.

The roof drains are installed to keep that water from causing damage to your roof. The drains also act to remove water that does run off and keep the groundwater from accumulating and damaging your foundation.

Determining Roof Drains

The two main ways of determining how many drains your roof will depend on the size of your roof and the amount of rain your area gets. The material your drain is made from will also need to be considered.

  1. First, calculate the area of your roof that will receive the most rain. It might be the entire roof, or your roof may be partially covered by trees or a neighboring building.
  2. Get the square footage of your entire roof, and consider where there are other drainage systems, like gutters or drain pipes.
  3. Divide the roof area to determine how many drains your roof will need.

On average, for roof areas of 10,000 square feet or less, two or more drains will be needed. For a roof that measures over 10,000 square feet in area, you may need at least four, if not more.

The overflow rate also needs to be considered. An area that gets a lot of heavy rain would typically require more drainage than an arid climate. The location of the drains will also matter.

Types of Roof Drains

Select a roof drain that provides the required capacity for removing the rainwater based on the flow rate of your area. You may need more roof drains to counter the lack of gutters or other drainage systems.

Roof drains may also need a cover or grate across them to keep out any leaves or other debris that can get stuck in them and cause them to block. This may require you to install additional drains to protect your roof and home.

Siphonic Roof Drain

A siphonic roof drain acts faster than other types of drains. It works by siphoning off the water. If you have a few of these drains, you can often get away with having fewer drains and they can all connect to a single leader.

These are used for flat roofs, more often for commercial buildings. Siphon drains help move the water by force, so the drainage systems can use horizontal leaders instead of sloped pipes, which rely on gravity for drainage.

Scupper Drains

Scuppers are open holes or channels cut into the edge of your roof. Again, they are commonly used on more flat roofs. The roof is sloped toward the scuppers, and water then drains out through the scupper.

They drain into a downspout on the outside of the building, or they may have an extended chute or pipe that directs a stream of water away from the building. These are used when a roof has some type of barrier that encloses the roof.

Inner Roof Drains

These drains are used on flat roofs and include a low section or channel that collects water and sends it into a concealed drain connected to an underground drainage system.

Inner drains are popular because the system is hidden from view. They are resistant to freezing but often require careful maintenance to avoid certain problems.

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