4.7 Vegetated Roofs

Vegetated roofs (also known as ecoroofs and green roofs) are thin layers of engineered soil and vegetation constructed on top of conventional flat or sloped roofs (WSU-PSP, 2012). Like other LID BMPs, vegetated roofs provide wide-ranging benefits, such as reducing the effective impervious area, promoting on-site retention and evapotranspiration of stormwater runoff, and potentially reducing the size of downstream stormwater flow control facilities needed. Air quality and habitat are also enhanced through the use of vegetated roofs.

Perhaps the two most compelling non-stormwater benefits of vegetated roofs are associated with energy savings and service life. The planting system of a vegetated roof creates a buffer between ambient air temperature and the roof insulation. The result is a minimization in fluctuation between high and low temperatures. These benefits can reduce the load on the building’s mechanical heating and cooling systems resulting in considerable energy savings.

Properly constructed vegetated roofs also last longer than most conventional roofing membranes. Vegetated roofs last longer because the waterproof membrane is protected from ultraviolet rays. The vegetation and substrate covers the membrane and protects it from thermal shock stresses that can result in excessive wear and cracking.

This section provides design guidance on extensive and intensive vegetated roofs. A brief description of both is provided below.

Extensive Applications

Extensive green roofs have a shallow (< 6” depth) growing media (see Figure 4.7.1). These roof designs are typically light-weight structures (approximately 10 to 35 pounds per square-foot when wet) that cover large expanses of rooftop and require minimal maintenance. Extensive vegetated roofs do not typically accommodate human use, except for maintenance access (Dunnett and Kingsbury, 2008). Their intent is to maximize the total vegetated area. These are particularly good for roof retrofits, in which the structural capacity of the roof cannot necessarily be improved.

Intensive Applications

Intensive vegetated roofs typically accommodate human recreational use in that they are used much like a typical garden (Dunnett and Kingsbury, 2008). Intensive vegetated roofs use deeper growing media (> 6”) and can include small trees and shrubs (see Figure 4.7.2a and 4.72b). They tend to be more expensive and their heavier weight on the roof (approximately 50 to 300 pounds per squarefoot when wet) must be considered during the design of the roof structure (University of Florida, 2008). They are often built in highly visible areas, such as outdoor roof terraces. Intensive designs are more likely to succeed in new construction where the load bearing capacity of the roof is designed concurrently with the vegetated roof.

The classification of extensive versus intensive roofs is used here to present the well-accepted vernacular of vegetated roof design. These typologies have been helpful in the past at indicating the kinds of plants and functions the vegetated roof could provide. The fact is, if done correctly, elements of one can be incorporated in the other. Vegetated roofs should be designed on a site-by-site, building-by-building basis, so all opportunities and constraints are comprehensively evaluated and used to guide the vegetated roof design.

Fig4-7-1_XFA_Tremco_St Mary-1
Figure 4.7.1 - Vegetated Roof - Extensive Design
St Mary Medical Center, Walla Walla, extensive vegetated roof. Source: Xero Flor America (XFA) and Tremco

Figure 4.7.2 -Vegetated Roof - IntensiveDesign
Bewewah Medical Center, Plummer ID, Intensive vegetated roof. Source: NAC Architecture