I was bidding on a green roof in lower Manhattan the other day and the architect threw out a price of $25-$35 per square foot as a typical price to install a green roof.
He is correct that a green roof can cost $25-35/sq.ft. I have even seen really large jobs cost less. There are always exceptions.
But that green roof usually has certain qualities:
It is a shallow green roof of 3-4″
It is a large green roof with economy of scale
It does not contain construction elements like a box frame
It contains plugs or cuttings that take a year or two to spread
Roof access is easy (elevator or for large roofs, crane)
Street Access is easy (easy parking, lots of unloading space and time)
However, that square foot price jumps when the following are added:
Soil depth (more soil to ship and get to the roof and the plants tend to be larger and thus costlier)
Smaller roofs – base costs remain the same regardless of roof size
Construction elements like boxes, seats …
Pregrown mats or trays (you are paying somebody else to grow them plus the proprietary trays come at a premium)
Difficult access (like a recently finished and clean private home, narrow streets, no parking)
Essentially a green roof price starts at $25 if you have the most basic large size green roof, easy access and find somebody willing to cut prices.
But I have seen green roofs as expensive as $500 a square foot. If you have a 100 square foot green roof in a fancy apartment building in Manhattan your life is going to be really difficult. Parking tickets are the least of it. Difficult doormen, building rules, and unwelcoming neighbors are all part of the mix that drive prices up.
Throw in a fancy pregrown tray system, some custom made edging and a top of the line irrigation system because they want it to appear lush year round, and you have a green roof that costs more than a a farm in Indiana. But boy is the green roof beautiful! There is something about a green roof on the new York concrete jungle that really can’t be beat.