Question

"People in Albany and New York City are using “green roofs” to improve the environment. A green roof can be added to many buildings that have large, flat roofs. Green roofs have three parts: a protective layer to separate plant roots from the roof of the building, an absorptive layer to catch and hold rainwater, and a layer of plants. Often, green roofs use Sedum, a short, desert plant, because it is efficient at storing water in its leaves and can withstand the colder climate.
A green roof saves energy, reduces carbon dioxide in the atmosphere, and prevents rainwater and melting snow from overloading sewer systems. It can also protect the roof of a building from damage. However, green roofs can be expensive to install, and require care and maintenance.

State one disadvantage of a green roof."

Answer 1

Basically, the disadvantages of a green roof are listed in the last sentence:

"However, green roofs can be expensive to install, and require care and maintenance"

The disadvantage of a green roof would be there for the owners: the cost and time needed to have such a roof.

An additional disadvantage would be the possibility of the plants on the roof mixing with and replacing the local vegetation, thus diminishing the biodiversity in this place.