Commercial Flat Roof Drains

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puddles near a drain on a roof with a city in the background.

Commercial Flat Roof Drains


Flat roofs, while aesthetically pleasing and space-efficient, come with their own set of challenges, primarily concerning water drainage. Unlike their steeply pitched residential roof counterparts, flat roofs lack the natural ability to shed water, which makes the role of drains even more critical.

This article delves deep into the intricacies of flat roof drains, examining their types, functions, and the importance of regular maintenance, ensuring the longevity and structural integrity of the roof and the building beneath it.

Key Takeaways

  • Flat roofs require effective drainage systems as they do not naturally shed water like sloped roofs.
  • Drainage options for flat roofs include inner drains, scupper drains, gutters, downspouts, and siphonic drains, each serving specific purposes.
  • The selection of appropriate drains depends on factors such as roof size, expected rainfall, and local climate conditions.
  • Regular maintenance, including inspections, cleaning, and debris removal, is essential to ensure the longevity of flat roof drainage systems.

What is a Flat Roof Drain?

A flat roof drain is a vital component installed on flat or slightly pitched roofs to facilitate the removal of water, ensuring that water doesn’t accumulate and cause damage. These drains are specifically designed to work efficiently on surfaces that lack the natural drainage capabilities seen in steeply pitched roofs.

Types of Drainage System

There are several types of commercial flat roof drainage solutions utilized in commercial buildings, let’s cover them here:

1. Inner Drains (or Interior Drains)

  • Function: These are situated at strategic points on the roof, often at its lowest points. They channel water away through pipes that run through the building and dispel it underground.
  • Components: Inner drains typically comprise a drain bowl (to which the roof membrane is attached) and a dome or strainer on top to prevent debris from entering the drainage system.
  • Advantages: They are aesthetically pleasing as they hide pipes and gutters. They also prevent freezing as the pipes are inside the building.
A diagram illustrating the installation process of a commercial flat roof inner drain pipe.
Inner drains offer a clean look to a roof as all the piping runs internally in the building.

2. Scupper Drains

  • Function: Scuppers are openings in the sides of the parapet walls on flat roofs that allow water to drain off the side of the building into a downspout.
  • Placement: They can be placed at the deck level to allow water to flow out or can be elevated, acting as overflow outlets when water levels get too high.
  • Advantages: Scuppers are straightforward, making them less prone to blockages from debris. They’re especially useful when the roof has a considerable pitch or is designed with a slight tilt towards the walls.
A diagram showing how to install a commercial water heater on a flat roof.
Scuppers pass the water from the internal gutter through the wall and out into a downspout on the outside of the building.

3. Gutters and Downspouts

  • Function: These are channels fixed along the edge of the roof to collect and guide water that flows off the roof down to the ground or stormwater systems.
  • Material Variations: Can be made from a variety of materials including PVC, metal, and copper.
  • Advantages: They can handle a large volume of water, especially if sized correctly, and can be easily combined with other drainage systems like scuppers.

4. Siphonic Roof Drains

  • Function: Unlike traditional gravity-based systems, siphonic drains use a water-induced vacuum effect to move water off the roof more quickly.
  • Components: They consist of a specially designed drain with a baffle. When the system starts to fill, the baffle prevents air from entering the system, creating a siphonic action due to the change in atmospheric pressure.
  • Advantages: They require fewer roof penetrations and can use smaller diameter horizontal pipes, which can lead to cost savings in material and installation.
Comparing Syphonic Drainage to Conventional Drainage for Commercial Flat Roof Drains.
Siphonic drains offer a lot of benefits over a normal pipe.

5. Secondary (or Emergency) Drains

  • Function: As the name suggests, these are backup drainage systems. They come into play when primary systems fail or are blocked, preventing potential waterlogging or ponding on the roof.
  • Placement: Typically installed a bit higher than the main inner drains.
  • Advantages: They provide an extra layer of protection against water accumulation, especially during heavy rains or if the primary drainage system fails.

Choosing a Drain

When selecting a drain, consider the roof’s size, anticipated volume of rainwater, existing roofing materials, and local climate.

It’s also crucial to ensure that the chosen drain is compatible with the roof’s construction and meets local building codes.

The Importance of Proper Drainage

Proper drainage is crucial to:

  • Prevent water pooling, which can lead to leaks or structural damage.
  • Avoid premature deterioration of roofing materials.
  • Avert potential mold growth or damage to the building’s interior.

Factors Influencing Drain Placement

Several factors play a role in determining drain placement:

  • Roof Design: Larger roofs or those with more complex designs might require more drains.
  • Slope: Even slight slopes can guide water to specific locations.
  • Local Rainfall Patterns: Roofs in areas with heavy rainfall may need additional drainage capacity.


Maintaining flat roof drainage systems on a commercial roof is crucial to ensure efficient water runoff, prevent ponding, and extend the roof’s lifespan. Here’s a step-by-step guide on how to maintain commercial flat roof drains:

1. Regular Inspections

  • Schedule periodic inspections, especially before and after stormy seasons.
  • Check for signs of blockages, leaks, or deterioration in the drain system.

2. Clear Debris

  • Remove leaves, twigs, dirt, and any other debris from the drain openings, strainers, and surrounding areas.
  • Consider installing mesh guards or screens to prevent large debris from entering the drain.

3. Clean Strainers And Domes

  • Lift off the strainer or dome and clear out any collected debris.
  • Rinse strainers with a hose to ensure no small particles are clinging to them.

4. Flush The Drain

  • Using a hose, flush the drain with water to check the flow. Water should flow freely without any backing up.
  • If water isn’t draining well, there might be a blockage in the pipes.

5. Check For Blockages

  • If you suspect a blockage, you can use a plumber’s snake or auger to clear it. 
  • For stubborn clogs, consider using a high-pressure water jet.

6. Inspect The Drainage Pipes

  • Ensure that the pipes leading away from the drains are not damaged or obstructed.
  • If there are underground drain pipes, consider having them inspected using a camera to identify any blockages or issues.

7. Check For Proper Sealing

  • Ensure that the area around the drain is adequately sealed to prevent water from seeping into the building.
  • Over time, seals can deteriorate, so they might need to be replaced or reinforced.

8. Inspect Overflow Drains And Scuppers

  • Ensure that secondary or emergency drains are also clear and functional to deal with excess water.
  • Scuppers should be free of blockages and positioned correctly to direct water away from the building.

9. Examine Roof Condition

  • Look at the roof surface for signs of ponding or areas where water isn’t draining properly. This might indicate a drain placement issue or a problem with the roof’s slope.
  • Address any dips, sagging, or damage to the roofing material promptly.

10. Maintenance Records

  • Keep a log of all maintenance activities, inspections, and any issues identified. This can help in predicting future problems and ensuring timely preventative measures.

11. Consider Professional Help

  • Depending on the complexity of the drainage system and the size of the roof, consider hiring a professional roofing contractor for regular maintenance checks.

12. Educate Building Staff

  • Ensure that building maintenance staff are aware of the importance of drain maintenance and are trained to spot potential problems.

When to Replace the Drains?

Signs that drains may need replacement include:

  • Frequent clogging even after thorough cleaning.
  • Visible rust or corrosion.
  • Evidence of water damage around the drain area.
  • Age of the drain; older systems might be less efficient and might not meet current building codes.

Installation Challenges and Best Practices

When installing flat roof drains:

  • Ensure the roof area around the drain is slightly recessed or tapered to guide water into the drain.
  • Use durable sealing materials to prevent leaks around the drain.
  • Always follow manufacturer guidelines and local building codes.
  • Consider professional installation to avoid common pitfalls.

Cost Considerations

Several factors influence the cost of flat roof drains:

Type Of Drain

Some drains, especially technologically advanced systems, may have higher costs.

Installation Complexity

Roofs with intricate designs or those that require retrofitting may result in higher labor costs.

Maintenance And Repair

While a good drainage system can lead to savings by preventing damage, neglecting maintenance can result in costly repairs.


Ensuring efficient water drainage on a flat roof isn’t just about selecting the right type of drain, but it’s also about regular maintenance, strategic placement, and understanding the unique challenges presented by the building’s design and location.

By investing in a well-designed drainage system and committing to its upkeep, building owners can prevent costly water-related damages and extend the lifespan of their roofs. Whether you’re installing a new system or retrofitting an older building, being informed about the available options and best practices can make all the difference.

Frequently Asked Questions

What are the main types of commercial flat roof drains?

The primary types are inner drains, scuppers, and gutter systems. Inner drains sit at the low points of the roof, scuppers allow water to exit through the walls, and gutter systems are installed at the edges.

How often should commercial flat roof drains be inspected and cleaned?

Inspections and cleaning should occur at least twice a year, typically in spring and fall, to prevent blockages and ensure proper drainage.

What are the common problems associated with commercial flat roof drains?

Common issues include clogging from debris, improper installation leading to poor drainage, and damage or wear over time, which can cause leaks.

Can the placement of roof drains affect the performance of a flat roof?

Yes, the placement is crucial. Drains should be located at the lowest points of the roof to ensure efficient water removal and prevent ponding.

What materials are typically used for commercial flat roof drains?

Drains are usually made from durable materials like cast iron, PVC, or copper, chosen for their longevity and resistance to weather and corrosion.

A worker is working on a metal roof.

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