Green Roof Retrofits Relieve Stormwater Systems (and Look Lovely)

by | Aug 22, 2017

As stormwater management – and growing stormwater fees – becomes an increasing concern for business, organizations may want to consider green roofs as a way of relieving underground stormwater systems, a new study says. Green roofs have the capacity to capture an average of 70% of rainfall over a given time, eventually releasing the water back into the atmosphere and thus reducing the load on underground stormwater systems, according to a study from the University of Toronto’s Green Roof Innovation Testing (GRIT) Lab (via

The study looked at a variety of green roof design variables, including planting type, soil substitute, watering schedule, and planting depth, studying how each of these four factors influenced water capture. It found that irrigation schedule had the biggest impact on retention capacity: roofs that are watered less frequently – only when their soil reaches a predetermined moisture level – have a greater capacity to capture stormwater.

For companies investigating the potential of green roofs, weight load is the most important consideration. The study found that a 10-centimeter planting depth (compared to a 15-centimeter planting depth) makes more companies eligible for a retrofit. Similarly, compost weighs significantly less than mineral planting materials, making it more suited to green roof retrofits.


Studies have shown that stormwater retention is the most important feature of modern extensive green roof systems. Typically 150,000 sf. of extensive green roofs can replace a stormwater retention basin, according to Green Roof Technology. In this regard, green roofs save property owners and municipalities valuable land and allow denser construction without expensive investments in increasing existing stormwater systems.

Additionally, green roofs protect the roof’s waterproofing membrane from external influences and prevents accelerated aging due to UV degredation. The combintion of the functional layers and the vegetation insultates the building against heat loss and creates potential energy savings during the intense air conditioning months of the summer.

Why Stormwater Management Is a Rising Concern

For many municipalities, stormwater management has become a top concern, for a number of reasons. First, infrastructure across the country is aging while population is growing. This means that a greater strain is being put on systems that are already under stress from age.

Additionally, urbanization has led to a continued decrease of the amount of permeable surfaces where water can freely drain. Existing stormwater and sewer systems are then forced to manage even more water.

So what does this mean for businesses? Stormwater fees have been increasing and are expected to continue to do so. (Case in point: Arlington Heights, in suburban Chicago, is implementing a new stormwater utility fee for businesses to help fund flood control projects.)

The GRIT Lab study concludes that retrofitting existing buildings with green roofs is a viable option for improving stormwater management, but cautions that “the proper research and knowledge is essential.”

These Folks Are Doing It Successfully…

Ford Motor Company told Environmental Leader in May about a complex swale system with a green roof over a million square feet that the company built to manage water runoff. Levi’s Stadium, home of the San Francisco 49ers, sports a 27,000-square-foot green roof. And Audi Field, a new stadium in Washington, DC, will provide storage for more than 55,000 cubic feet of stormwater onsite through green roofs, bioretention areas, and infiltration basins.



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