Your Guide to Commercial Flat Roof Drainage


One of the most important parts of your flat commercial roof is the drainage system. Flat roofs can be prone to ponding water, a phenomenon that leads to leaks, a short roof life, and the possibility of collapse. Understanding how your drains work and the required maintenance is a must. 

Common Drain Locations

Roof drains are typically located around the perimeter of the roof, in the center, or both. Perimeter-only drains are typically reserved for roofs with a smaller area and simple footprint. The drains may hook into a gutter system that routes water to the ground or to pipes that lead to underground stormwater drains. Very simple designs may be little more than spouts that allow water to flow freely off the roof.

Interior or center drains are always hooked to a drain pipe that runs through the roof or even through the building itself. This pipe then empties out the side of the building onto the ground or it is routed into a stormwater or sewage drain system.

Issues That Lead to Ponding Water 

Poor roof construction or minor collapses due to age are one cause of drain clogs, particularly with interior drains. The roof should slope slightly toward the drains so that water can easily flow off the roof. If the slope is nonexistent or too shallow, then water will pool up instead of flowing toward the drain.

Another issue is a clogged drain. Clogs can be a result of outside debris, such as leaves, garbage, or nesting materials from birds on the roof. Clogs can also grow in the drains, particularly in narrow drain lines. Algae and biofilms thrive in the moist drain and grow until they create an extensive clog that prevents water flow.

Repair Tactics

A repair is needed if the roof slope is a problem. A commercial roofer can buildup the roof to develop the proper slope. They will then install a new membrane. If building up the roof isn't an option, then they may recommend installing additional interior drains.

Clogged drains are an easier thing to repair in most instances. Drain lines can be cleaned out with high-pressure jets of water, which removes both debris and organic growth from the line. Your roofer may also treat the lines with an algaecide to prevent future growth. Gutters and drain screens will also be cleaned out. Your roofer may recommend installing cages over the top of the drains to cut down on future debris buildup.

Contact a commercial roof contractor if you have concerns about drainage on your roof.


4 February 2021

Roofing Fundamentals: What To Know Before Replacement

When it came time to install a new roof on my home, I had no idea where to start. In fact, I was exploring roofing options that were a really poor fit in my climate. It wasn't until I reached out to a local roofer that I got some lessons on how to choose the right roofing material for my home and the weather exposure it is subjected to. I built this site to share my journey, including the mistakes I made and the things that I learned along the way. If you are considering replacing your home's roof, I hope that the information here can help to guide you through the process.