Understanding the Role of Drainage Systems in Flat Roofs

Flat roofs are a common feature on commercial buildings across Chicago and Northwest Indiana due to their cost efficiency and ease of installation. However, their design comes with specific maintenance needs, and one of the most critical components is the drainage system. Without a reliable system to move water off the roof, commercial properties are at risk for serious and costly damage.

Understanding how drainage works on flat roofs—and how to maintain it—is essential for property managers, facility teams, and business owners responsible for commercial spaces.

Why Drainage Matters on Commercial Flat Roofs

Unlike pitched roofs, flat roofs don’t allow water to naturally run off the surface. Instead, they are constructed with a slight slope to direct water toward designated drainage points. When these systems are not functioning correctly, water can begin to pool, especially after storms or snowmelt.

Standing water is a common issue on commercial roofs and often goes unnoticed until it causes damage. It adds weight to the roof structure and accelerates the deterioration of roofing membranes. In the Midwest, freeze-thaw cycles add another layer of concern. Water left on the surface can freeze overnight, expanding into small cracks or gaps. When it melts during the day, it seeps into the roofing system—only to refreeze again and expand further. This cycle weakens seams, flashing, and surface materials.

Key Drainage Components in Commercial Roofing

Internal Drains

Large commercial buildings often use internal drain systems, which are installed at low points of the roof and connect to pipes that run through the building. These systems are effective, but they must be kept clear. Debris, leaves, or ice can block the drains and create a backup, leading to overflow or roof damage.

Scuppers and Downspouts

Scuppers are openings in the perimeter wall or edge of the roof that allow water to flow out into downspouts or collector boxes. These are especially common on mid-sized commercial buildings. If scuppers become clogged, water can back up onto the roof or run down the building façade, causing damage to the exterior.

Gutters on Low-Slope Roofs

Some commercial properties with slightly sloped roofs use external gutters. While functional, these systems are more exposed and can become clogged with seasonal debris like leaves and twigs, especially in the fall. Routine cleaning is necessary to prevent overflow and water pooling near the building’s foundation.

What Property Managers Should Watch For

Facility teams should conduct regular inspections, especially in early spring and late fall. Look for signs of standing water, discoloration, or areas where debris tends to accumulate. Pay attention to whether drains, scuppers, and gutters are clear and functioning. Leaves and organic debris left on the roof surface tend to trap moisture, which breaks down the roof material over time and creates a higher risk of water intrusion.

Even if your drainage system appears to be working, small problems like slow drainage or minor ponding can point to deeper issues that need professional attention.

Trust Great Lakes Roofing with Your Commercial Roof Drainage

At Great Lakes Roofing, we’ve worked with commercial property owners, facility managers, and business operators throughout Chicago and Northwest Indiana for decades. We understand the specific drainage challenges flat commercial roofs face from severe weather to seasonal debris to aging systems. Whether you need seasonal cleaning, drain system repairs, or a complete roofing project, our team delivers dependable, tailored solutions. Contact Great Lakes Roofing today to schedule a commercial roof inspection and take the next step in protecting your building

Jesse Spain( President )

Jesse Spain has been with Great Lakes Roofing for over 16 years.  He has worked in all areas of commercial roofing including; laborer, estimator, project management, sales, and marketing.  Jesse has specified and negotiated over 30 million dollars in roofing projects.  Jesse holds a BA degree from Purdue University, West Lafayette, In.  and is currently enrolled in the National Roofing Contractors Association’s (NRCA’s) Future Executive Institute (FEI) Program.  The Future Executive Institute, through Northwestern’s Kellogg School of Management,  is a three-year program designed to develop the management and leadership skills of future and current roofing company leaders.