4.7.1 Applications & Limitations

Vegetated roofs can be an effective LID BMP in eastern Washington. However, freezing temperatures, heavy snowfall, strong winds, and hot, arid summers need to be considered when analyzing the use of a vegetated roof.

Vegetated roofs have become increasingly popular on office, industrial, and warehouse structures where large, flat roofs are typical of design. These types of uses usually have large expanses of impervious surface and a vegetated roof may offer a desirable relief from locally-adopted stormwater management fees calculated on impervious surface coverage.

The use of vegetated roofs on residential structures in the United States is less common. Many single-family residential roof structures were never designed to accommodate the wet soil loads associated with a vegetated roof. Consequently, the retrofit of residential structures for a vegetated roof often requires significant structural buttressing. Residential structures may also have steeply pitched roofs that make vegetated roofs either technically or economically infeasible.

While vegetated roofs can be installed on slopes up to 40 degrees, slopes between 5 and 20 degrees are most suitable and can provide natural drainage by gravity. Roofs with slopes steeper than 10 degrees require an analysis of engineered slope stability and those greater than 20 degrees require a structural reinforcement system and additional assemblies to hold the soil substrate and drainage aggregate in place (WSU-PSP, 2012).