If you’ve ever looked up at your commercial building’s flat roof during or after a rainstorm, you may have wondered: “Where does all that water go?” It’s a common question, and one that deserves a clear answer. Poor drainage isn’t just inconvenient; it can lead to leaks, structural damage, and reduce the overall lifespan of your roof. 

At Weather Shield Roofing Systems, we’ve spent more than four decades helping property owners and facility managers understand how water affects their roofing systems—and more importantly, how to keep it from becoming a costly problem. Here’s what you need to know about how flat roofs handle rainwater and how you can make sure yours is draining the way it should. 

How do large buildings drain rainwater from the roof?

Even though we call them “flat roofs,” they’re not actually 100% flat. Most are built with a slight slope (usually about 1/4 inch per foot) to guide water toward drainage points. These drainage systems may include: 

• Internal drains: Located in the center or low points of the roof, these pipes run through the building to direct water away.
• Scuppers: Openings in the edge of the roof that allow water to flow into downspouts.
• Gutters and downspouts: Found more often on smaller commercial buildings, these channel water off the edge of the roof and safely away from the building. 

Each of these systems works to prevent water from pooling and damaging your commercial roof or building.

How do flat roofs handle rainwater?

Flat roofs handle rainwater by collecting and redirecting it using the drainage methods mentioned above. The key to their success is regular maintenance and proper design. When everything is working right, rainwater flows off the roof quickly and efficiently. 

But if drains are clogged, the roof membrane is damaged, or the slope isn’t adequate, water can pool—and that leads to problems. 

Are puddles on my flat roof a problem?

Yes, puddles—also known as ponding water or standing water—can be a serious issue. If water remains on your roof for more than 48 hours after a rain, it can: 

• Weaken roofing materials
• Lead to leaks or moisture intrusion
• Add extra weight to your roof deck
• Reduce the roof’s lifespan 

At Weather Shield, we often find that ponding water is an early warning sign of larger issues like clogged drains, sagging insulation, or poor roof design. Addressing it quickly can save you thousands in emergency roof repair costs down the road. 

How do you keep water from accumulating on a flat roof?

Preventing water accumulation starts with these steps: 

• Regular roof inspections: Catch issues early with scheduled inspections, ideally twice a year and after major storms. 
• Clean your drains: Debris like leaves, twigs, and dirt can clog drains and lead to standing water. Cleaning roof drains regularly is crucial.
• Ensure proper slope: If parts of the roof are sagging or weren’t built with the right pitch, water won’t flow where it needs to.
• Use tapered insulation: This can be added during roof installation or replacement to improve drainage.
• Maintain your roof membrane: Small cracks or holes can trap water underneath the surface, leading to hidden damage. 

Do flat roofs need to be drainable?

Absolutely. If a flat roof can’t drain, it becomes a bowl. And unlike a bowl in your kitchen, you don’t want to keep water sitting on your roof. 

Drainage is required by building codes and is essential for roof investment protection. If your commercial roofing system can’t efficiently remove water, you’re at a higher risk for leaks, mold, structural damage, and even full roof failure. 

How do you stop standing water on a flat roof?

If your flat roof has recurring ponding problems, here are ways to fix it: 

• Clear and maintain drains: Start by ensuring all existing drainage points are working.
• Add or improve slope: Tapered insulation or re-sloping the deck can guide water more effectively.
• Install additional drains or scuppers: Adding new drainage points can relieve problem areas.
• Use Max Life™ Roof Care: Our proactive maintenance program includes regular cleanings, inspections, and minor repairs to prevent major issues. 

How to clean roof drains?

Cleaning roof drains doesn’t have to be complicated, but it must be done safely: 

• Access the roof carefully: Always follow OSHA guidelines or hire a professional.
• Remove debris: Use gloves or a scoop to clear leaves, dirt, and other buildup from around the drain.
• Flush with water: Use a hose to ensure water flows freely through the pipes.
• Inspect for damage: Cracked drain covers or loose fittings should be repaired promptly. 

We recommend having professionals perform this work regularly. With Weather Shield’s Max Life™ Roof Care program, roof drain cleaning is part of our routine maintenance service. 

Can poor drainage cause leaks?

Yes, poor drainage is one of the top causes of roof leaks on commercial buildings. When water isn’t properly removed, it finds ways into seams, penetrations, or cracks in the membrane. It can also accelerate UV and heat damage, making the surface brittle over time. 

That’s why drainage is one of the first things we inspect during a commercial roof inspection. 

Key Takeaways

Flat roofs can be incredibly durable and cost-effective, but only if water is managed properly. If you’re seeing standing water, slow-draining areas, or suspect your drains aren’t doing their job, it’s time to take action. 

At Weather Shield Roofing Systems, we’re committed to helping commercial property owners and managers extend roof lifespan, prevent damage, and save money through smart maintenance and expert care. Need help with drainage or want a professional inspection? Contact us today to schedule a visit. 

Drew Palmer

Marketing Manager

Drew Palmer is the Marketing Manager at Weather Shield Roofing Systems, where he leads content creation and drives the company’s marketing strategy. With 10 years of experience in marketing and recruitment marketing, Drew brings a creative, enthusiastic approach to storytelling through video and digital media. An adventurous entrepreneur at heart, he enjoys the challenge and growth opportunities his role at Weather Shield provides.