How to Grow Green Roofs
Green roofs have been common in many European cities for years, but they are just beginning to catch on in the United States. The roofs are such a plus for the environment in dense urban areas that some cities encourage and even underwrite them. A green roof lowers the temperature of the surrounding area and absorbs a large amount of storm runoff. It can save a homeowner money and supply a seasonal harvest, but it is an investment worth considering carefully and getting right. Look for a non-profit green roof organization in your community to help with planning. Does this Spark an idea?
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Things You'll Need
- Required zoning permits
- Structural weight-bearing evaluation
- Carpenter's level
- Tape measure
- Rolls or sheets of waterproof membrane or liquid waterproofing
- Box cutter or sharp scissors (for membrane)
- Airless sprayer or roller (for liquid waterproofing)
- Gravel, drainage rock or drainage mats
- Rake
- Lightweight, good draining growing medium
- Shovel
- Drought-tolerant, green roof-adapted plants
- Hand trowel
- Watering can or hose
Instructions
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Obtain all necessary clearances and permissions before beginning the project. Find out whether the structure is strong enough to support a green roof -- this may require the help of an architect or building inspector. Check to see if your community or homeowner's association allows green roofs, and whether your insurance policy will be affected. Explore grants for roof greening that may be available to offset your investment.
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Check the slope of your roof. Place one end of a 12-inch long carpenter's level on the roof. Level it. Use a tape measure to find the distance from the mid-air end of the level to the roof. This is the slope. Online calculators will turn slope into degrees for you. A green roof requires a maximum pitch between 20 and 45 degrees, according to the University of Florida Extension, although steeper pitches need barriers to slow rain runoff and keep plants from sliding off the roof or clumping together at a gutter. A flat roof needs a built-up, modest pitch to prevent puddles and muddy areas from forming. Plan a drainage system based on the pitch of the roof.
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Lay a waterproof membrane over the area to be planted. A membrane can be heavy-duty rubber sheeting, a composite material or a paint-on liquid that dries to a waterproof finish. Follow manufacturer's instructions for membrane installation.
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Spread a layer of drainage material. Gravel, crushed shale, lava rock and other stones are heavy, but provide permanent aeration that won't compact over time. Products like porous mats or channeled drains have been developed for green roofs. They may help to offset some of the weight of the soil and vegetation.
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Cover the drainage layer with planting medium to the suggested depth for the plants you have chosen. Use a lightweight soil as a growing medium. For maximum environmental sensitivity, use organic soil with added pumice, perlite or lava rock for extra drainage.
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Plant drought-tolerant species such as sedum and hardy succulents, for a green roof that is designed to lower the temperature of the home, insulate it and decrease storm runoff. Plants can be native species, grasses or other vegetation that has been tested or developed for green roofs in your growing zone. The ideal plant needs no irrigation beyond natural rainfall, and is low-maintenance with minimal weeding and pruning. Water the roof at first planting, and then monitor to see how well the plants are establishing roots and growing without artificial irrigation.
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Tips & Warnings
If you have planned a garden for the roof, limit it to an experimental section the first season, to gauge how well vegetables or flowers survive in the rooftop environment. Add raised beds to an existing green roof for growing herbs or vegetables. But calculate the extra weight and work required to farm your sky-high plot.
Use annuals as accent plants. They are relatively high manitenance and require periodic replenishing on a green roof.
Save time and effort -- but pay more -- by buying rolls of vegetative mat that comes with rooftop plants, growing medium amd backing, ready to lay down like sod over the substrate.
Be sure your roof is accessible and that supplies needed to create a green roof can be carried or delivered to it easily. Roof-access ladders, small trap doors or very tall buildings may be impediments to building a green roof.
References
Resources
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