What You Need To Know About Your Flat Roof Drainage System

It is flat, but it isn’t. It is supposed to drain, but it doesn’t. Your company’s “flat roof” could be thought of as a system of interconnected parts, each waiting to fail. Without smoothly functioning gutters, internal drains, and scuppers, your building’s low-slope (it is never truly flat) roof will leak. 


To keep water channeling away from the building and its foundations, and to prevent water from dousing customers and employees entering your commercial property, industrial gutters usually ring a building. 

These conduct water to downspouts, which in turn empty into sewers, catch basins, or drain fields. Keeping a building’s foundations dry is key to preventing expensive structural damage.  

Internal Drains

At the lowest designed points of your low-slope roof, internal drains are openings (covered by strainers or leaf guards) that allow water to run through pipes to the outside, away from your building. 

Most internal drain systems also have backup drains, called overflow outlets, that take in standing water and expel it at a visible spot around your building perimeter. The discharging water serves as an alert that the main internal drains are clogged. 


Scuppers are holes in the parapet — the high wall around the roof perimeter — that are supposed to channel water off the low-slope roof into gutters and downspouts. We say “supposed to” because scuppers seem ideally sized to snag branches, leaves, and litter. That leads to clogs, which allow water to back up onto the roof surface. 


Clogs in scuppers, internal drains, overflow outlets or gutters can ruin a low-slope roof. Business-stopping issues arising from clogs are numerous:

  • Ponding — Ponding occurs when water sits on a roof’s exterior and puts water weight on the insulation, which sags and leaves a depression
  • Leaks — Standing water seeks its lowest level, thanks to gravity; open seams, corroded flashing, and all joints between unlike materials are prime suspects for expensive leaks
  • Structural sag — With sufficient water buildup, your roof deck can sag, making ponding worse
  • Moss, plants, and algae — Plant life can grow amid the water and organic debris

Low-slope or “flat” roofs are not designed to hold water; they are designed to speed water down and away. Work with A-1 Property Services to ensure your building’s flat roof is draining properly. We can inspect your roof and drainage system, diagnose your roof’s problems, and recommend strategies to improve rooftop drainage and decrease leaks.  Contact us today!