How Flat Roof Scuppers Work And Why Sometimes They Don’t – Part I

Flat Roof Scuppers – An Overview

Flat roof scuppers are architectural components designed to effectively manage rainwater by channeling it away from the flat roof surface and directing it through the building’s facade to ultimately be conducted away through downspouts or rain leaders. The primary function of scuppers is to prevent water accumulation on the flat roof, which can lead to structural issues, leaks, and other forms of damage.

The picture below looks up the front facade of a building towards the roof.  The collection box or conductor head at the top of the downspout catches the water that flows through the roof scupper.

flat roof scuppers

How Flat Roof Scuppers Function?

Scuppers typically consist of an opening or passage cut through the parapet wall or the edge of the roof. This passage serves as an exit point for rainwater that has collected on the flat roof. Gravity plays a significant role in this process, as water naturally flows to the lowest point on the roof, which is often near the scupper. When rainwater reaches the scupper, it is guided through a metal or plastic channel, which is often integrated into the scupper design, creating a path for water to exit the roof.

Common Problems With Roof Scuppers

Common problems associated with scuppers and the potential for leakage often stem from inadequate design, installation, or maintenance. One frequent issue is the reverse gradient of the flat roof near the scupper inlet. If the roof surface is not sloped correctly to ensure water flows toward the scupper, standing water can accumulate near the scupper rather than efficiently draining away. This standing water can increase the risk of leaks or even structural damage over time, as well as create an environment conducive to roof membrane deterioration.

The next picture below shows the side of a large commercial or industrial warehouse in the Washington DC metropolitan area.   This particular warehouse had leakage at both of the scuppers and downspouts shown in the picture collage below.

roof scuppers


Leakage at the scupper inlet can also result from poor sealing or improper installation of the scupper assembly. Water can infiltrate the building envelope through gaps or cracks in the scupper’s connections to the roof or parapet wall. These issues can lead to moisture intrusion and associated problems like mold growth, rot, and compromised insulation.

Debris And Blockages

Additionally, debris and blockages within the scupper itself can impede the flow of water and cause backup onto the flat roof. Leaves, twigs, or other foreign objects may accumulate in the scupper, hindering the proper drainage of rainwater. Regular maintenance and cleaning are essential to prevent these blockages and ensure that scuppers function effectively.

How Buildings Get Damaged Through Scuppers

The picture below shows the CMU wall of a commercial warehouse with a large flat roof system. The membrane above is an EPDM flat roof system, not as energy efficient or as strong as a TPO membrane, but somewhat strong nonetheless.   However, in this circumstance, the weak link which led to damage and deterioration of the internal components of the building came from the scupper outlet.   The defect or issue was caused by unattached flashing, but the issue was exacerbated by extended Is time ranges between maintenance cycles.

cmu wall damaged by scuppers

The picture below shows the view of the conductor head where it is attached to the facade of a building.   However, at this scupper, the flashing is loose where scupper outlet goes through the wall to the outside of the wall.   Also, this particular conductor head happens to be filled with leaves from the trees around the perimeter of the rooftop.  This highlights tge importance if proactive maintenance and upkeep.

blocked roof scupper

In summary, flat roof scuppers are engineered components designed to manage rainwater by directing it away from the roof and through the building facade. However, their effectiveness relies on proper design, installation, and maintenance to avoid common issues such as reverse grading, leaks, and blockages that can lead to damage and water infiltration into the building.

In an upcoming article, we will discuss additional details related to rooftop scuppers such as the material types, parts and components, and proactive management. Contact us for more information.

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