Denver voters have passed a law mandating that all new structures of over 25,000 sq ft have provision up top to plant stuff and grow it. All older structures getting a roof replaced, must get the garden treatment as well, the alternative in both cases being to install solar panels.
A quick look shows that roofs in this area must be built to hold 25-30 pounds per square foot to account for snow loads in winter. Dirt is generally accounted to weigh about 100 pounds per cubic foot on the low end. I'm going to assume that it takes about 2 inches of dirt to support growing lawn grass based on digging up my own lawn. That mean a new roof on a commercial building would need to be built to support 41-46 PSF to handle the new loads. So getting a new roof now means getting structural upgrades to the underlying feature.
Oh and by the way, are the load bearing walls beneath the roof rated for the higher loads or will the property owner have to tear down the whole place to accommodate the green roof? Or is this just a back door to compel purchase of solar panels? Either way, the price of commercial real estate in Denver is going to go up substantially.
OTOH, this will make similar buildings in the outlying areas more attractive and help Denver deal with its traffic problems by funneling traffic to the 'burbs instead of down town. Imagine Denver as a ghost town surrounded by taller industrial and commercial buildings.