The recently completed City of Columbia Water Distribution and Wastewater Facility, or “City Water” as many call it, has already begun to draw a lot of attention from the residents in Columbia. It is the City’s first LEED registered project and the complex is expected to receive LEED Gold Certification by year end. However, friends and neighbors are asking about all of those plants on the roof… To answer two of the most common questions yes, they are supposed to be there and yes, they actually provide some really great benefits.

City Water Green Roof

View from Above: City Water Green Roof

Green Roofs began in Germany around 1960 and have recently gained momentum in the United States with current sustainability movements. Since green roofs slow rain water and lessen the impact on the storm water system which, in turn, reduces maintenance and construction of storm water lines and equipment, the application of a green roof to the City’s Water Distribution and Wastewater Administration building made perfect sense.

The application of the green roof was accomplished in several steps. The first item installed was a traditional single-ply roof membrane and insulation. In this case, we used a white single ply roofing that included a leak detection system. On top of the roof membrane, a capillary fabric was placed, then a 1.5 layer of light stone, together acting as a drainage layer. We then placed a landscape fabric and 3.5 inches of growth media (soil) with hydro mulch on top.

Growth Media Installation

Growth Media Installation

There are 26 varieties of plants on the roof, including Chrysanthemums and Black-eyed Susans — some of which can be seen from the ground. We installed drip line irrigation as well as a traditional sprinkler system. The sprinkler system will be used for the establishment period of the plants, and then it will be turned off permanently. The drip irrigation system includes a rain detection system, so it will only be used when natural rain fall does not sustain plant health. The system weighs 33 pounds per square foot when wet and a little more than 22 pounds per square foot when dry. The total system is 5 1/3 inches thick.


There are many benefits to a green roof; one of which is reducing the urban heat island effect. Green roofs can also lessen the impact on the storm water system in rain events — it slows the amount of time it takes for the rain water to make it to the storm sewer. The green roof will also naturally filter pollutants and carbon dioxide out of the air, which has been shown to lower disease rates such as asthma. Further, it will insulate the building for sound and thermally, improving the effectiveness of the roof and lowering energy costs. Green roofs have also been shown to extend the life of the roof by over 200 percent, mostly due to the membrane being completely covered by the green roof.

With all of these benefits in mind, the City approved the design and installation of the green roof. The roof was planted in early summer and has been going through the establishment period. Just like any planted area, the City will maintain the roof by pulling weeds and replanting. We are looking forward to seeing the roof change with the seasons and hope you do too!

Green Roof Close Up

Green roof during the establishment period

Submitted by Gene Bell, Associate Principal