Someday soon we’ll have rain again. In the meantime, you may be pondering installing a rain barrel or two to catch this precious substance from your roof in the next storm. Or maybe you’re thinking about building a beautiful rain garden to help slow and sink rainwater from racing into the streets and down the storm drains. But there’s a third way to capture rain and filter it before it reaches rain barrels or rain gardens-with a green, “eco,” or “living” roof.
For many years living roofs have been the rage in Portland, Oregon, but over the past few years, several have popped up in the Bay Area too: check out the roofs at San Francisco City College, California Academy of Sciences, Berkeley Animal Control, and West Elm in Emeryville, to name a few. But living roofs can be installed on a smaller scale too–on home or garage roofs–and help intercept runoff, cool and protect the buildings, reduce the urban heat island effect, and provide habitat for beneficial bugs, butterflies, and birds.
To build a very basic living roof, a waterproof membrane is put down to cover the roof, a 2-3 inch substrate of well-draining sandy loam mixed with compost (or other special soil mixture) is added as the soil layer, and then the roof is planted. Drought tolerant sedums, wildflowers, California natives, clovers, vetches, certain grasses, and even mosses and ferns can do well on living roofs. All living roofs will need to be watered through the first couple of years so they can become established. In an extended drought, the plants will also need to be watered every couple of weeks, but if you chose your plants wisely, they can survive with spring and winter rains and a little supplemental watering. When it rains, the water falls onto the plants, saturates the soil, and then reaches the waterproof membrane. From there it flows to the gutter, where it can then be directed into a rain barrel if desired. Some living roofs capture as much as 98 percent of the rainwater.
Landscape architect and stormwater guru Tom Liptan, who started the city of Portland’s ecoroof program several years ago, built one on his garage in 1996. He views his roof as a “microcosmos” that contributes to a more diverse urban environment through photosynthesis and evapotranspiration, microorganisms in the soil filtering pollutants, and birds, butterflies, and bees finding habitat there. In addition to its beauty, he says, the plants use the elements that destroy conventional roofs over time–sunlight, rain, and soil–to grow and provide us with cleaner air and cooler temps on hot days.