Benefits of Green Roofs

Benefits of Green Roofs
Green roofs are booming in popularity because they bring multiple benefits to buildings and to communities.

These benefits include:

Reducing roof replacement costs. In Minnesota, a typical gravel-ballasted flat roof is expected to perform for 15-20 years before replacement is necessary. Roof membranes deteriorate when exposed to the hot and cold temperature extremes we experience in Minnesota, and also when exposed to UV radiation from sunlight. The components of a green rooftop protect a roof membrane from these aging factors. The membrane under a green rooftop can be expected to perform for 35 – 50 years before replacement is necessary. This means that a building can avoid 1 – 2 roof replacements over a 50-year lifespan. This reduces life cycle costs as well as significantly reducing the volume of roofing materials deposited in our landfills.

Reducing energy costs. Green roofs can reduce heating and cooling demands. During a Minnesota winter, the insulation layer and growing medium of a green roof can add r-value to a building’s roof. In the summer, the impact is more significant. Living plant material evaporating moisture from leaf surfaces will cool the rooftop surface, reducing cooling demand up to 25 percent.

Reducing stormwater management costs. An extensive green roof with four inches of growing medium can be expected to hold a one-inch rainfall event before any water runs off the roof surface. Nearly all the rainfall events we receive in Minnesota are less than one inch. This four-inch green roof can be expected to capture two-thirds of the rain that falls on its surface. With a deeper layer of growing medium or with rainwater or greywater harvesting systems, a green roof can be used to manage a significantly larger volume of water. As a part of a building’s stormwater management infrastructure, a green roof can reduce the cost of other stormwater management systems, and in some cities can reduce monthly stormwater management fees. The City of Minneapolis uses a stormwater utility fee structure that provides incentives for using green roofs to manage stormwater on-site.

Improving urban air quality.. Green space is good for air quality. Green rooftops provide opportunities to increase the amount of green space in densely developed urban areas where green space can be hard to find. A 1998 study by Green Roofs for Healthy Cities developed a model showing that, if six percent of the rooftop surfaces in the City of Toronto were replaced with green rooftops, the additional green space would remove 30 tons of airborne particulate pollution each year.

Mitigating Climate Change and the Urban Heat Island Effect. Urban areas like Minneapolis-Saint Paul are generally 2 – 5 degrees (f) warmer than surrounding rural areas. This is known as the Urban Heat Island effect. On top of that, the USEPA projects a 2 – 7 degree (f) increase in aggregate temperature for our region over the course of the 21st Century. It is projected that a three-degree aggregate temperature increase in Minneapolis would triple annual heat-related deaths from 60 to nearly 200. By cooling rooftop surfaces and improving air quality, a critical mass of green rooftops can mitigate all of these impacts.

Providing urban green space. Green roofs can provide opportunities for significant green space amenities in urban areas. East Village Apartments in the Elliot Park neighborhood of Minnepaolis features an at-grade green rooftop installed over underground parking. The green space includes picnic tables and provides an amenity to this affordable housing development. The green rooftop at Brit’s Pub on Nicollet Mall in Minneapolis can be used for lawn bowling and private parties. And pedestrians using the Loring Greenway in Minneapolis may not even realize that they are walking on green roof surfaces above occupied spaces. Hospital administrators know that patients who can see green space outside their windows can recover more quickly. Nursing home and elder care facilities can use green roofs as part of horticultural therapy programs.