Green Roofs Minimize Stormwater Runoff

The building in this photo is located in Boston’s Seaport District near the World Trade Center. As you can see from the photo the building in the center has a Green Roof. A green roof is a roof of a building that is partially or completely covered with vegetation and soil, or a growing medium, planted over a waterproofing membrane.

One of the many benefits to installing a green roof on a building is the ability to reduce stormwater runoff. Stormwater runoff from impervious surfaces such as building roofs, parking lots, roads and walkways create drainage and pollution problems. By using a green roof system a substantial amount of water that would normally run off the structure can be captured to sustain the plant life that’s growing on the roof.

The building in the photo has a daycare center at the visible layer with mercantile space below that and finally an underground parking structure. As you can see a good majority of the roof structure is green and covered with grass and trees. It appears that 60% to 70% of the structure is covered with a green roof. Therefore, this structure has probably reduced stormwater runoff by at least 50% or more.

Other Benefits of Green Roofs:

They can provide a more pleasant and usable outdoor space compared to a concrete patio or stone roof.
They can help reduce the urban heat island effect (typically a factor in urban areas).
Increased wildlife habitat (typically birds and insects).
Reduced heating and cooling loads on buildings from the additional mass and thermal resistance.
Heavy metals and contaminants can be filtered out of rain water.

The biggest disadvantage to these types of roofs is the additional loading placed on the structure. The soil, water and plants add a significant load to the roof. However, the benefits of green roofs certainly outweigh the small costs associated with the stronger roof.

Green roofs are just one way of helping to reduce stormwater runoff. As stormwater management rules become restrictive green roofs will become more popular and useful. I guess you could say green roofs are as green as they come!