4 Effective Ways to Drain Water from a Flat Roof

water-on-flat-roofFlat roofs are commonly seen on commercial buildings and modern home designs because they are resistant to elements like hail, strong winds, and animals that pose hazards to sloped or arched roofs. They’re also an attractive feature for the adventurous homeowner.

However, flat roofs to have some downfalls, like their vulnerability to water damage. Because gravity doesn’t cause water to flow off the sides like on a sloped roof, a flat roof requires a specially built drainage system. Despite its name, a flat roof isn’t completely flat; it slants slightly toward the drains. There are a variety of drainage systems to consider. We’ll examine their pros and cons to help you determine which is best for your building.

Inner Drains 

True to their name, inner drains are installed within the roof or in the parapet wall—a small wall or railing around the edge of the roof. This prevents visible pipes from hindering the building’s aesthetics. Similar to the drains found in your sink and shower, an inner drain is usually located in the middle of the roof. Because inner drains are generally best for large roofs, they are commonly found on commercial properties.

Because the pipes are underneath the roof or in the parapet wall, they may not have any contact with your roof, greatly decreasing your risk of water damage. The pipes are also naturally insulated and protected by the building’s walls and roof, minimizing cracking or freezing. On the other hand, inner drains have higher costs associated with installation, maintenance, and repair. If the drains get clogged or damaged, it can result in water damage to the walls around the pipes. As a result, this type of drainage system requires regular inspections.


Scuppers are a simple draining method for flat roofs. These are large openings in the building’s outer wall that allow water to flow from the edge of the roof through downspouts installed directly below them. The downspouts ensure water doesn’t damage the walls. This may be the most efficient drain for flat roofs, as scuppers are easy to maintain and don’t tend to clog (as long as the opening is wide enough).

With scuppers, however, it is important to remember to check the downspouts on a regular basis for debris, since these are susceptible to clogs. Scuppers can also be particularly susceptible to freezing, as their drain lines are exposed on the outside of the building. As a result, they may not be ideal for areas with harsh winters.


Gutters are installed along the edges of the roof and direct water to a pipe or downspout. As long as the roof is on the smaller side and tilts slightly toward the gutter, water should flow easily to the gutter. This makes a gutter drainage system a simple and cost-effective option for smaller buildings. 

Be aware that gutters are completely open to the elements and therefore very susceptible to damage and clogging. Debris like leaves and twigs can stick inside the gutters and cause considerable water backup. With regular maintenance and cleaning, however, gutters can be a great choice.


Siphonic Roof Drain 

A siphonic roof drainage system uses a vacuum pump to remove water from a flat roof. The gravity-induced vacuum is connected to a series of drain openings, drawing water away from the roof’s surface at a high speed. This type of system is perfect for large roofs in areas that sustain heavy and frequent rain.


A major benefit of a siphonic roof drain is how fast it removes water from the roof. It’s not prone to clogging, and the system requires little maintenance. The pipes are small, helping to keep your roof clear and the placement of any equipment, like HVAC, unhindered. Because the pipes are small, however, it’s important to have them regularly cleaned and maintained to prevent blockage.


Regardless of which drainage system you choose for your flat roof, it’s important to have a professional like those at Hedrick install, inspect, and repair it when needed. Subscribe to our blog today to learn more about roof installation, maintenance, and repair.