Proper drainage for flat roof systems begins with an understanding that no commercial roof is truly flat—all are low-slope roofs. While they may appear flat, with no pitch to them, these commercial roofs must angle slightly to allow for drainage. Flat roof water drainage—more accurately, low-slope roof drainage—is a significant challenge for commercial property owners and managers. 


Water seeks its lowest level, pulled down off your commercial building by gravity. That means your drainage systems must be at the roof’s lowest points. You can use three different methods of draining a low-slope or “flat” roof:

  1. Internal drains—these are similar to bathtub drains (though significantly bigger) and appear in the center of your roofing field, with insulation shaped to slope the roof toward the drains.
  2. Scuppers—Circular or rectangular openings in your building’s roof parapet allow water to run directly off the roof; often, an oversized downspout beneath the scupper channels the water down and away from your building’s foundations.
  3. Gutters and downspouts—Though oversized for commercial use, these mimic the same system of residential gutters and downspouts, moving water off the roof and into catch basins, storm drains, or landscaping.

A typical commercial roof may depend on two or all three of these methods to provide adequate drainage even in the hardest downpours. Storms especially test the ability of a low-slope drainage system, which is one reason careful maintenance by your local commercial roofer is vital. 


Ponding is defined as water remaining on a low-slope or “flat” roof for more than 48 hours. After a storm, you or your facilities crew may be tempted to inspect the roof to check for ponding. While you can safely survey the roof from a roof hatch or entry stairs, do not climb out onto the roof. 

All roofs—low-slope, steep-slope, dry, wet—are dangerous, but after a heavy downpour, your commercial roof can be surprisingly slick. Leave the footwork to your trusted local roofing contractor, but you can visually check for ponding. 

Ponding is a vicious cycle where water sits on the roof, pushing down and compressing the insulation beneath the membrane or finished roof. Over time, more water gathers, and the depression increases in depth and area. This leads to more and more roof damage

Ponding can cause roof leaks, with water appearing in the interior of your commercial building days or even weeks after the roof appears dry. Ponding can ruin insulation and alter the natural direction of draining water. 

Ponding can only be fixed by your roofing contractor, who will pull up the membrane, remove and discard waterlogged insulation, shape new insulation for the correct slope, and re-apply the membrane. Every step must be performed correctly, or the ponding and water leaks will worsen. 

Roof Deck

Water is surprisingly heavy. One cubic foot weighs roughly 62.43 pounds. Ponding of only one inch of water across 500 square feet of commercial roof (an area only 10 feet x 50 feet) is 41.67 cubic feet of water. That is 2,601.46 pounds of water. More than one ton of water is pushing down on your roof deck. 

A sagging commercial roof is not only inconvenient, but it is also a huge safety hazard. You risk permanent damage to your building structure, destruction of support members within the roof deck itself, and the health and safety of anyone inside your building. 

Rooftop Garden?

Ponding worsens with time. It encourages mold, algae, flowering plants, and even trees to take root on your roof. Before you realize it, you have an unwanted rooftop garden. The flora attract fauna, like insects and small mammals (rodents, for example). Soon your poor drainage leads to roof leaks, pest invasion, and safety concerns. 

Avoiding Problems

Annual or semiannual inspections by your trusted commercial roofer can prevent damage caused by poor drainage. Your roofer will:

  • Visually and thermally inspect every square inch of your roof
  • Clear debris from the roof field and around drains
  • Inspect and ensure the integrity of internal drain screens and scupper linings
  • Clear gutters and downspouts of organic material and clogs
  • Test seams and flashing
  • Provide an inspection report with recommendations for improvements and repairs

Since your so-called “flat” roof is not safe territory for you or your facilities crew, your nearby commercial roofer is ideally suited to provide complete roof asset management. With frequent inspections, all drainage systems can operate efficiently to prevent clogs, ponding, leaks, and destruction by pests.

Ideal times to call on your commercial roofer for inspection and maintenance are in the spring and fall and after every significant weather event. If a severe storm moves through your area, contact your roofer for a peace-of-mind inspection. 

Vanguard Roofing provides complete commercial roofing services throughout the northeast United States. Connect with us today to find out more about commercial roof replacement, roofing energy options, low-slope roof repair, and more.