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Wow—That's Intense By Arden Miller, CZM When it comes to rooftops made with veggie toppings, there are two basic kinds: extensive and intensive. Both start with a waterproof membrane to seal the rooftop so that water cannot penetrate, and both include an irrigation layer and growth medium—also known as an overburden—made up of nutrients and soil. Where the rooftops differ is in the amount of growth medium, the filter and irrigation systems, and what's on top. Basically, anything from three to six inches of growth medium supports what's known as an extensive green roof. These are the rooftops that are not made to be publicly used spaces, but rather to insulate the building from cold and heat, catch stormwater runoff, and help reduce the urban heat island effect. (Other benefits to the extensive roof system can include creating a wildlife habitat area for the birds and the bees, providing insulation from overhead noise, and providing an aesthetically pleasing view.) Extensive green roofs usually weigh between 15 and 50 pounds per square foot and cost between $10 - 15.00 per square foot to install. It is generally recommended that the plants receive fertilizer and water regularly until they have grown in (usually, this happens within six months of planting them). Once established, unless there is a severe drought and they require watering, they are basically self maintaining. In some cases, professional installers will recommend that the rooftop system include a drip tube so that, in times of excess rain, the roots don't drown. When a roof has anywhere from six inches to three feet of overburden, it's called an intensive green roof. These weigh in up to 150 pounds for each square foot and can cost up to $75 per square foot to install. The intensive green roof has all the benefits of the extensive top, plus the ability to hold larger amounts of stormwater runoff, and it can support shrubs and trees in addition to ferns, flowers, and sedums. To ensure that the trees and plants have adequate room for roots, and ample drainage and water opportunities, intensive roofs have more construction layers. Depending on the types of plants selected, and the desired look, an intensive rooftop, like one's backyard or a public park, generally requires grooming, weeding, and fertilizing. These are solid roofs that can support human activity, and they are often designed to be used as outdoor park-like space. Most installation companies guarantee the integrity of the green roof's membrane, and will replace it if there are any leaks. An annual inspection, for drainage and leaks, is advisable. Be they intensive or extensive, or a combination of the two, with more than 400 types of drought-resistant plants and flowers available, landscaping possibilities are as vast one's imagination. N.B.: All growth systems vary, depending on the installation company, and these facts and figures are not meant as guidelines for installing your own green roofs, tempting as that may be. For information on professional roof contractors, see www.greenroofs.com/directory.php. | |||||||||||||||||||||||||||
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Boston, MA 02114