Green Roof Retrofits Relieve Stormwater Systems (and Look Lovely)

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.

Click here to read the full article - Sourced from Environmental Leader