The ROI claim of 5 years is great and quite an incentive, but not for most builders. If they sell the building the 5 year ROI is meaningless to them. 

I hate the idea of a mandate, but this should really be happening. It actually benefits the community and the city as a whole. 
Passing on the cost to the renters? Big deal. 

I wish there were a way to incentive the builders to do the right thing. 
Maybe Denver should offer rebates to builders for rooftop gardens.

A rooftop garden can also make a building more desirable, which means it is worth more money and higher rent.