MS4 fact sheet - Green Roofs

Green Roofs

The benefits of green roofs and examples of successful programs

Green roofs are becoming commonly accepted and installed across the Upper Midwest on buildings of all shapes and sizes. Green roofs are being utilized as a means to reduce costs associated with the life-cycle of conventional roofs, and heating and cooling. In addition, they are being used to address stormwater management and large green roofs are being used to create spaces for public benefit in urban settings. This fact sheet focuses on the benefits of green roofs and provides examples of municipal programs and resolutions for municipal buildings.

Cross-section of a typical green roof

Benefits / Pollution Reduction

Green roofs offer several benefits such as reduced runoff, increased evapo-transpiration, prolonged roof life, reduced roof temperature, decreased energy costs, reduction of the urban heat island, habitat for birds and insects, carbon sequestration, improved air quality, and enjoyment and increased productivity for adjacent building occupants. Green roofs also have aesthetic qualities which help to meet landscaping requirements, and they create additional living space if constructed properly. The possibilities of so many benefits, particularly in urban high-density environments such as downtown Minneapolis and St. Paul, have triggered the use of green roofs.

Program Development & Implementation

Types of Green Roofs

A green roof typically consists of the following components listed from the bottom upward: roof deck; a waterproof membrane to protect the building from leaks; a root barrier to prevent roots from penetrating the waterproof membrane; an insulation layer; a drainage layer, usually made of lightweight gravel or plastic; a geotextile or filter mat that allows water to soak through but prevents erosion of fine soil particles; a growing medium; plants; and, sometimes, an erosion control blanket.

There are two general types of green roofs: extensive and intensive. Structural load capacity, that is, how much weight the roof can hold, is a significant factor in determining whether an extensive or intensive green roof should be considered from a design and liability standpoint.

Extensive green roofs Comprise a lower-maintenance design with groundcover in shallow soil, which in relationship to an intensive green roof adds significantly less (14-35 lbs/ sq. ft) to the roof’s dead load (Dunnett and Kingsbury, 2004). This roof type is covered in engineered soil medium that is 70-90 percent inorganic and generally 3 to 6 inches deep.

Extensive green roofs support a limited palette of vegetation that is generally low-lying and designed for maximum groundcover, water retention, transpiration, and erosion protection. An extensive green roof is generally much less expensive to construct than an intensive green roof and requires less maintenance. Maintenance on extensive green roofs is usually performed in bulk – similar to a lawn. Extensive green roofs are generally designed to support limited traffic for building and green roof maintenance.

Entensive green roofs These are garden-like installations with deeper, more organic soils that can potentially support more diverse plantings. Intensive green roofs can increase dead loads from 59-199 lbs/ sq. ft. (Dunnett and Kingsbury 2004). The range of plantings is dependent on a combination of the soil type and depth, as well as the maintenance availability – including watering and fertilization. Native plants and several traditional garden plants are available in