An extensive green roof is a green roof that is less than 6″ thick. As opposed to an intensive green roof that is more than 6″ thick. Because of the thickness of the growing medium an extensive green roof can only support sedum and drought tolerant small plants. The reason for this is the limited root space and the small amount of water the soil can store.
Here is a good image of an extensive green roof structure.
Extensive green roofs are usually the most appropriate for Brooklyn brownstones because of the light weight load. Brownstones have brick or wood frame structural walls which certainly are strong but not as strong as formed concrete. In the roof structure, unless it has been reinforced a Brooklyn brownstone typically has 3″x8″ douglass fir joists which can usually hold a 4″ deep green roof without any problem.
As a Brooklyn green roof installer we’ve become very comfortable with the extensive green roof process and have moved away from the austere sedum only roof that green roof purists specify for extensive roofs like this one:
Of course sedum roofs are beautiful and I intentionally picked a really pathetic example of a sedum roof 
Here is a better example:
Sedum are really great because they are strong as hell. They withstand cold, drought and grow fast.
But we have found that you don’t have to stick with sedum if you want your brownstone roof to have a thicker foliage. There are a lot of cold and drought tolerant plants such as wild flowers, grasses and tough succulents. And if you are able to pass gray water onto the roof you really aren’t limited at all in your choice of plants.
For example check out our Brooklyn Green Show House roof below. The soil depth is 2″ to 5″ deep so it definitely falls into the extensive green roof category.
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