I recently wrote about intensive green roofs. On the other side of the green roof coin is the extensive green roof. This type provides many of the same benefits of the intensive system but does it with a lot less fuss.
These green roof systems are typically designed to be self-sustaining after the first year. They are constructed in a similar manner to an intensive green roof. However, the growing medium is much shallower, generally 2”-6”. This substantially reduces the weight down to a more manageable 16-35 pound per square foot. Unfortunately, this does not allow for trees, shrubs and much foot traffic.
What it does allow for, and what makes it self-sustaining, is the use of more drought tolerant plants such as sedums and grasses. Because of this, these roofs are not made to be walked on like an intensive system. Instead of delivering a park-like occupied space, this roof is more utilitarian. It still reduces rainwater run-off and helps lower heating and cooling costs. That isn’t to say that a properly designed extensive green roof still can’t have a nice shaded break area for the buildings’ users.
Extensive green roofs are great for retrofit applications as there are systems that are self-contained. These are can be purchased in small squares or rectangles which can easily be placed by 2 people. Each is placed individually making for a simple installation. If for some reason a section of the roof fails to thrive, that square can easily be taken out and replaced.
Finally, these systems are good for lower rainfall areas and when less maintenance is desired. The EPA building in Denver has an extensive green roof system where they are experimenting with various drought tolerant plants. This study designed specifically for green roof design geared towards the dryer western regions of the United States.