Did You Know That Green Roofs Can Capture Rainwater?

There is a good reason for this high-level, green infrastructure buildout. Over the past three decades, the District of Columbia’s rapid growth and the accompanying paving-over of pervious surfaces, led to a 34 percent increase in stormwater runoff. With sewer discharge and stormwater sharing the same large pipes in some areas, a rapid influx of water sometimes ends up sending sewage into nearby rivers.

Montgomery County and Prince George’s County in Maryland are experiencing similar strains on their stormwater systems.

To reduce that flow, D.C. adopted the Green Area Ratio Rule in 2016. It set a minimum requirement for water-retaining landscaping for every new construction and exterior renovation project on a commercial building. Since land in the District is expensive, the city allowed owners to install green roofs to meet this minimum.

“Green roof systems can capture and retain anywhere from 50 to 80 percent of the rainwater that falls on them,” explained Vanessa Keitges, president and CEO of Columbia Green Technologies, which designs and manufactures vegetative roof technologies. The other 20 to 50 percent of the rainfall is disbursed to the stormwater system over time and helps prevent flooding damage and combined sewage overflows.

The green roof is a cost-effective way for developers and owners to meet stormwater mandates. In an urban environment, a tank in the ground capable of holding that much water would take up valuable space that could be used for a basement or parking lot.

The Basics of Green Roofs

Green roofs vary in their design. They generally consist of several layers, from the bottom up, that include the deck, waterproofing, insulation, roof barrier, drainage, filter fabric, growing media and plants. Existing buildings that are being renovated may require reengineering and additional support to handle this additional load.

The growing media on a green roof is specially designed to reduce weight, ensure proper drainage and help the plants thrive in the environment. It is made from both organic and inorganic material; while it may contain soil, it often does not.

There are two major categories of green roofs. Extensive green roofs have less-growing media, often around 3 to 4 inches deep. They add 30 to 50 pounds per square feet to the roof load and are planted with low maintenance, drought-resistant plants. Intensive green roofs have deeper substrates (up to 12 inches) and increase the roof load by anywhere from 40 to 150 pounds per square foot. They feature a variety of plants and require more maintenance, including irrigation.

Because they are less complex, extensive green roofs are much less expensive to install than intensive roofs. About 80 percent of the green roofs constructed today are extensive.

Green roofs offer many benefits beyond stormwater management/water retention. Although they cannot be used in place of roof insulation, they do provide an extra layer of protection. They help reduce energy use by keeping warm air in the building during the cold months and absorbing heat from the sun in the summer. They also provide some sound insulation.

Green roofs last longer (40-plus years) than conventional roofs because the plantings protect the roofing materials from the sun’s UV rays and from temperature extremes. The plantings also reduce CO2 and increase oxygen production.

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