What the Heck is a Green Roof?

Many of you have likely noticed an uptick in interest in sustainability initiatives. All of us at Magic Dirt, are pretty pleased about that. In fact, back in March, we participated in the City of Little Rock’s 8^th annual Sustainability Summit.

We donated two bags of Premium Potting Soil for use in the table top green roof demonstration. What’s a table top green roof? Glad you asked. Green roofs, which are now more appropriately being called vegetative roofs, are living roofs or vegetated roof covers, where growing media (Magic Dirt!) and plants take the place of a membrane, gravel ballast, shingles or tiles. The number of layers and the layer placement vary from system to system and green roof type. At the very least all green roofs include a single to multi-ply waterproofing layer, drainage, growing media and the plants, which cover the entire roof deck surface.

Commonly, the roof space and the intent of the green roof determines the design. For example, is the intent just to provide an ecological cover, or is the space intended recreation, vegetable gardening, etc.? Factors to take into consideration when designing a green roof include:

• Roof load capacity as determined by a structural engineer
• The slope of the roof
• The budget

Based on these aspects, the depth (i.e., the weight) of the growing medium will determine the types of plantings used.

What are the differences among the types of green roofs?

You’ll find three main types of green roofs – extensive, semi-intensive or intensive –  although a green roof can be designed with a combination of all three types. According to the experts at Greenroofs.com, the types of roofs are defined as:

Extensive – Also referred to as eco-roofs, or low-profile, these roofs have thinner and fewer numbers of layers, so they are lighter, less expensive and low maintenance. Extensive green roofs are built when the primary desire is to have an ecological roof cover that will receive limited human access. The minimum growing medium or soil substrate starts at about 2 1/2″ to5 or 6″ (approximately 13-15 cm) at most, although vegetative mats can have even less than 1″ of growth media. The soil media contains 70-80% inorganic or mineral material (or higher) to 20-30% organic (or less). Low-growing, horizontally spreading root ground covers with a general maximum plant heights of 16-24″ are ideal. Alpine-type plants are successful here because they are high drought, wind, frost, and heat tolerant, all necessary attributes for green roofs. Appropriate plants include sedums and other succulents, flowering herbs, and certain grasses and mosses. Fully saturated weights range from a low of about 10-50 lbs./sq. ft. Extensive green roofs can be constructed on slopes up to 30°, and steeper ones can be installed with raised grids or laths to hold plants and soil media in place.

Intensive – Also referred to as high-profile, these green roofs look like traditional roof gardens because a much wider variety of plant material can be included since growing media depths are increased. The growing media starts from about 12″ (30 cm) and can range up to 15′ or more, depending on the loading capacity of the roof and the architectural and plant features that the building owner desires. The engineered soil media usually contains about 45-50% organic material to 50-55% mineral, and fully saturated weights range from about 80-120 lbs./sq. ft. Architectural accents such as waterfalls, ponds, gazebos, etc. are possible here. These green roofs provide recreation spaces and encourage interaction between people and nature, so they must be flat. Maintenance requirements are also more intensive.

Semi-Intensive – A combination of an extensive green roof with areas of higher plant depths, the semi-intensive living roof will have both areas of lower than 6″ of growing media and higher, ranging from 8-12″ or 20-30 cm.

Green roofs provide many outstanding benefits to a building, its users, and the whole community. They are becoming increasingly more popular and the ease of installation is continually improving. We’re happy to have participated in this initiative!