Bureau of Watershed Protection
Designing and Building a Greenroof

The limiting factor in greenroof design is usually the weight bearing capacity of the roof. Because of this, greenroofs are divided into two main types, extensive and intensive, based on weight.

Extensive greenroofs are the lightest weight and lest expensive. They have a thin soil layer (1"-6") and must be planted with a specialized selection of low growing, drought resistant plants. Thin soil layers and low growing plants allow this type of greenroof to be planted on even steeply pitched roofs. Because of weight limitations, they are not intended to be accessible except for maintenance.

Intensive greenroofs can be built on roofs that support much greater weight loads. The soil depths are much greater on these roofs and can support a much broader range of plants, including shrubs and trees. Intensive greenroofs are usually intended to be used as recreation areas by people and often include paths, seating areas, and other amenities. These roofs generally have higher costs and higher maintenance requirements.

There are several ways of constructing both extensive and intensive greenroofs, depending on the pitch of the roof, the structural weight limits of the roof, and the climate, but they all share certain elements:
1. Waterproofing layer
• A good waterproofing layer is essential to the success of a greenroof
2. Root Barrier
• Prevents roots from penetrating the waterproofing layer. This may be a separate layer or incorporated into the drainage layer
3. Drainage layer
• Used to both retain water for the plants to use, and also help excess water flow to drains. Several manufacturers provide drainage layers of various materials, some with small water reservoirs and integral root barriers. Very small or steeply sloped roofs may not need a drainage layer, but generally recommended.
4. Filter Layer
• A thin fabric or mat layer that prevents soil mixture from clogging the drainage layer. This may be integral to the drainage layer.
5. Engineered soil mix
• Special mix of lightweight aggregate and small amount of organic material or compost designed to be very lightweight and provide good drainage.
6. Appropriate plantings
• Plants for greenroofs must be carefully selected as they have to withstand extremes in temperature and moisture. Most common plants are sedums (stonecrop) and Delosperma (hardy iceplant).

Further Greenroof Resources:
Greenroofs.com
Green Roofs for Healthy Cities, North America’s Green Roof Industry Association
Penn State University Green Roof Center
Michigan State University Green Roof Research


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