The suitability of New York City rooftops for greening and agricultural use

Background/Question/Methods

In the past, more green roof research has focused on the benefits provided to a single building than on how these benefits would scale up to a neighborhood or city level.  This is at least in part due to a lack of suitable locations where green roofs are concentrated enough to perform field observations.  Most of the claims about how green roofs benefits would be realized on larger scales have, therefore, been made through modeling or back of the envelope calculations.  Both of which require assumptions about how the benefits of green roofs scale up, the number of roofs that could be greened, and what types of existing buildings are suitable for greening.  More often than not, no justification for these assumptions is given.  A decision tree was created to guide analysis of rooftop suitability and enable examination of the assumptions being made.  This decision tree contains some of the numerous factors which go into determining the suitability of a roof for greening.  GIS analysis was then performed on the Newtown Creek watershed in Queens and Brooklyn, New York, to determine the suitability of existing rooftops for agricultural production using green roofs.

Results/Conclusions

A total of 80% of the buildings in the Newtown Creek watershed could be retrofitted into intensive green roofs.  An additional 13 % of the buildings could be retrofitted into either sedum mat or extensive green roofs. This compares very favorably with the percent of buildings to be greened used in many models or back of the envelope calculation, which ranged from 5 to 100 %.   Only 17 % of buildings in the Newtown Creek watershed could support agricultural green roofs.  Of these buildings, 91 % could support intensive green roofs and less than 2 % are suitable for large scale agriculture.  The biggest challenge in implementing the decision tree was a lack of knowledge about key building features.  Estimations of both the roof slope and usable roof area, for example, had to be made based on Google Map satellite photos, which was a very time consuming step in the analysis.  Greening all possible rooftops in the Newtown Creek watershed could result in runoff reduction of 0.004 km3, sequestration of 53 thousand tons of CO₂, emissions reductions of 13 thousand tons of CO₂, and the production of 4 thousand tons of tomatoes each year.