Green Roofs and Rain Gardens

Green Roofs

A green roof is a vegetative layer grown on a rooftop. Green roofs are not only important because they hold rain water, but they also reduce temperatures of the roof surface by removing heat from the air through evapo-transpiration. Green roofs can be ornamental and low profile or be used to grow vegetables and larger plant material on rooftops.

Green roofs need flat or nearly flat roofs to work. It is also impor-tant to know the weight bearing capacity of the roof. Often additional support is necessary. In most cases the planters or planting medium is separated from the roof by membranes that protect the roof. Check zoning codes that may limit the amount of roof area that can be cov-ered by a green roof. Extensive green roofs use as little as 2 inches of soil and grow mainly grasses and sedums. This type of roof will weigh much less than an intensive green roof but provide the same temperature reduction benefits and require much less maintenance.

An intensive green roof will have greater than 2 inches of soil and can support much larger plant material. Some green roofs are urban farms growing vegetables for market. Wet soil can weigh more than 120 pounds per cubic foot of soil but the climate of a rooftop can be hotter than at street level and more windy which will tend to dry out soil and desiccate plants.

A drip irrigation system can help insure that the soil does not dry out. There are commercially available systems for green roofs which supply any fabrics or membranes as well as grid and modular systems. An inexpensive green roof can be made with recycled planters.

Rain Gardens

A rain garden is a garden of native shrubs, perennials, and grasses which can withstand both drought and occasional flooding planted in a small depression, which is generally formed on a low point of a natural slope. Gravel or well drained soil is added to make the rain garden work more efficiently.

Rain gardens are designed to temporarily hold and soak in rain water runoff that flows from roofs, driveways, patios or lawns. The rain garden is dry most of the time and typically holds water only for the day following a rainfall event.

Look for any sort of depression or flooded ar-eas in the garden that will temporarily hold and rain water runoff that flowing from roofs, driveways, patios or lawns. Usually if these are present, there is a slope leading into it, allow-ing the water to flow towards the depression.

A rain garden can be any size but a typical rain garden is between 100 and 400 feet but could be smaller if that is all the space can accommodate. Rain gardens can be any shape but usually are shaped longer than they are wide and posi-tioned perpendicular to the slope of the land in order to catch the maximum amount of rainfall.

Rain gardens should be placed at least 10 feet away from building foundations and should not be located where water ponds for an ex-tended period of time (less than 0.5 in. per hour percolation rate). The soil in the interior, lower area of the rain garden should be amended with soil that drains well amended with sand and gravel to improve drainage.

There are specific plants that will thrive in both drought conditions and can withstand flooding. They are easy to construct, mostly requiring digging in the area in order to add the soil amendments or to increase the depth of the rain garden.