Green Roofs Backgrounder for Landscape Contractors

Landscape professionals may want to see if the growing interest in green roofs can offer new opportunities for them. After all, many green roof manufacturers contract with landscaping companies to actually build the green roof on an existing roof.

Your best bet is to contact the major manufacturers of green roofing systems and find out how they handle their installations and outsource contracting. A listing of green roof corporate members is available at www.greenroofs.org. Do your homework and make sure the company you're looking to partner with is reputable and well-established.

You also want to do your homework on green roofs in general. This article should help you get a good head start.

TYPES OF GREEN ROOFS. To put it simply, green roofs are vegetated roof covers. Growing plants take the place of shingles and tiles. While the number of layers and layer placement vary from system to system and green roof type, all green roofs include a single to multi-ply waterproofing layer, drainage, growing media, and plants that cover the entire roof deck surface.

There are two main types of green roofs: extensive and intensive. However, a green roof is often designed with features of both, and are then referred to as semi-extensive or semi-intensive.

A growing trend is to install a modular green roof. With the modular system, soil and vegetation are pre-planted into recyclable modules of varying sizes at a local nursery, usually by landscaping professionals contracted with the manufacturer. They are then simply laid out, one after another, on top of the existing roof, which makes for quick and easy installation.

However, before jumping on the green roof bandwagon, landscape professionals should learn a bit more about the soil and vegetation used for a green roof system, along with the basic principles of green roof maintenance. For without making the proper selections with an understanding of the roof's care, the green roof may not survive.

PLANT SELECTION. The vegetation used for a green roof is usually referred to as plant media. The plants selected are typically sedums, grasses, perennials, native plants and other succulents. Plants native to the area are generally the best choice.

Additionally, the plant media must be hearty, able to withstand most weather conditions, as well as be drought-tolerant. They should also be low-maintenance plants requiring little or no irrigation. In most cases, green roofs are designed to just be "patrolled" on a regular basis. (See "Maintenance and care" later.)

"Sedum has proven to be the most tolerant plant media, especially in shallow soils," says Grace Koehler, sales manager for Midwest Groundcovers, a nursery in St. Charles, IL. "Even with extreme fluctuations in climate, it usually survives, if not thrives."

SOIL SELECTION. In many ways, the soil, or growth media, selected for a green roof is even more important than the plant media. This is because the growth media is the foundation of the green roof. Not only does it anchor the plants, but without the proper nutrients and other components, no matter what plants are selected, they may have a difficult time surviving.

"For instance, it is important that there is enough air space in the soil so that water and oxygen can move through the growth media," says Koehler. "In addition, it must include all the nutrients for the plants to survive the initial installation, as well as thrive."

This is why many green roof systems use engineered soil. This soil is specially prepared to include, among other components, a blend of macronutrients, which are organic nutrients that also have mineral materials.

Among these macronutrients are:

• Nitrogen
• Phosphorus
• Potassium
• Calcium
• Magnesium
• Sulfur

Along with the macronutrients, the engineered soil will also include micronutrients, such as iron, manganese, copper, boron and zinc. Although these are needed in just small amounts, Koehler says they are every bit as essential as the macronutrients for normal plant growth.