If the roofs had a return on investment of 6.2 years (as claimed above), why would developers not install them? A return on investment of 6.2 years would be a 16.1% return on their money (roughly). Even developers that "flip" the property would be interested in incorporating technology/designs that yield such a return. Future buyers would reward them for incorporating technology that reduces expenses and provides the yield claimed above. Zeppelin understands cap rates and understands that the spread between a 16.1% return (if it were true) and market cap rates (5% to 6.5%) would be enough to incentivize ALL developers to incorporate green roofs without a mandate. Therefore, I do not think the 6.2 year (16.1%) return on their investment is accurate. 

Will the authors of this article please provide the math they used to determine that the green roofs have a return on investment of 6.2 years?