Coffman North Green Roof

What are Extensive Green Roofs?

February 2, 2017

By Jake Thering  - The other form of green roofs, other than intensive, that can be established are extensive green roofs. These are usually installed for engineering and performance goals, compared to the performance and recreational goals of intensive roofs. This type of green roof differs from intensive in a couple of other ways that might make it a more appealing option to invest in. 

Perhaps the biggest difference is that extensive systems have fewer options for the types of vegetation that they can choose to have on the roof. This is because of a couple factors. Extensive systems are going to be lighter in terms of pounds per square foot. They range anywhere from 15 to 50 pounds per square foot making it a more viable option for buildings that might not be able to support the heavier intensive green roofs. Also, the soil of an extensive system is only three to six inches and has less organic matter, only 10 to 20%, than the soil for an intensive system. Since there is less soil and the system as a whole has to be lighter, extensive green roofs are not able to support trees or shrubs like intensive systems. Instead, grasses and small plants are the available vegetation options for extensive green roofs. 

The challenge with extensive green roofs is making sure that the entire system is light weight. So, when deciding on the irrigation, drainage, and other components of the roof, the overall weight has to be put into consideration.

Here is an example of the way that an extensive green roof can be constructed:

The various layers of the green roof allow for the system to have a lighter weight so that the roof can support it. This design also ensures that the system will drain properly so that no erosion is present and the waterproofing membrane, asphalt, makes sure that there is no water damage to the building. 

Extensive green roofs are a great way to insulate a building and turn empty, useless space into an environmentally friendly component of a building. They are often times lighter than intensive systems and also do not have the option to plant trees or shrubs. The system implements a light weight design in all of the layers of the green roof that allow it to be efficient and not cause any structural damage to the building. Extensive green roofs are an attractive option if intensive green roofs are not possible and they give you some of the same benefits, like energy reduction and improvement in air quality, as intensive roofs.