Blue-Green Roof – The Positives and the Negatives

Anyone walking from Center City to University City in Philadelphia on Walnut or the opposite direction on Chestnut in the last year during rush hour must have hated the construction that is occurring around 30th street. The site is to be the new Cira Centre Tower 2 and 3. The building will be a mix of residential and commercial zoning that will redefine the skyline west of the Schuylkill River.


ImageOne unique aspect of large development projects is the need to manage stormwater in Philadelphia. For the construction site at 30th and Chestnut/Walnut, a large green roof above the parking structure will serve as the required stormwater management system. The green roof design is quite innovative. The contractor describe the roof as a Veneer Stormwater System. So what is a Veneer Stormwater System?

Veneer Stormwater System is not only a green roof but also a blue roof. It is a green-blue roof with both features. There are benefits as well as concerns of the design. A green-blue roof uses a horizontal water flow instead of a vertical water flow as in typical green roof. There is a shallow pocket of water and soil under the green which would be more design friendly for roofs. There is minimal infiltration with a focus on evapotranspiration.

On the negative perspective, an ineffective green-blue roof often lead to a dead green zone and an ineffective blue roof. A green-blue roof is hard to maintain and repairs are often complicated. There is the also the concern of how much water a green-blue roof can retain and its effectiveness during a large storm.

The Cira site will be a highlight to the city of Philadelphia upon completion. Three glass reflective buildings west of the River would signal a slow migration of center city west. Regardless of whether the green-blue roof will function or not, this is an innovative project and one that should be examined in detail during and after construction.


About t5huang

Masters in Environmental Science at UPenn Candidate
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