Anatomy of a Green Building - Lesson 9 - Green Roof

Tuesday, November 08, 2011
By Matt Sutter, Chief Executive Officer
In Sustainability

The increasingly fashionable green roof. What exactly makes up a green roof? Are we talking about dumping dirt on the roof, then planting bushes and trees? Perhaps it is tall grasses, or perhaps short mowed grass? Will there be butterflies, or perhaps birds? What about stones? And why would you put a green roof all the way up on the roof, where no one can see it? What is the point of such a thing?

Many interesting questions to be answered. So here goes:

A green roof usually consists of different varieties of plants called sedum. These are small plants that are usually about 3" to 8" high and form a spongy growing layer above the soil. There are many different mixtures of sedum plants from which to choose. Some are very colorful, with flowering yellows, whites or blues. Some are closer to the shade of a pine tree and keep their green color through the winter. If properly selected, the plants should be native to the climate in which they will be installed. Trees or bushes as seen at your local Carrabas Restaurants are not part of a typical green roof because the soil needs to be relatively deep to support the plant's roots. Trees and bushes will also send out roots that could work their way into the seams of the roofing membrane.

The soil in which the sedums grow is usually 4" thick, but can be thicker if taller plants are used. The modern day green roof is grown off site in a nursery and delivered to the job site. Ideal lead time is 6 months from the time the seeds are planted until the baby seedlings are delivered to the job site. The plants come to the job site in plastic roof trays that are 12" wide and 24" long. The trays are taken off the truck and snapped into place like interlocking floor tile. The roof trays simply sit on an extra layer of loose laid roofing membrane and, over time, the plant material grows from one roof tray to the next and forms a very even vegetated surface.

The beauty of this system is that the vegetation has a chance to grow in ideal nursery conditions and comes to the site already green. If repairs need to be made to the roof, the roof trays are simply lifted up and moved until the roof is repaired. No shovels required. The installation of the system is so easy that even a bunch of architects can do it. Eight architects and one CM from SōL Harris/Day installed our roof in just 4 hours.

The benefits of the green roof are many. For the SōL Harris/Day building, the green roof sits at the same level of the second floor. The greenery is combined with pavers and is visible from inside the building. The patio area that is covered with pavers will be used on nice days for lunch and meetings, adding inexpensive space to the building. The pavers and greenery will keep the roof about 80 degrees cooler than a typical black roof on a summer day. This will reduce energy bills and help to prolong the life of the roof to 30 plus years. The soil and plants also absorb rainfall and keep water on-site, reducing the amount of water put into the county storm sewers.

So when you think of a green roof, think of small "growies" and roof trays, not trees and lots of soil.

Matt Sutter
SōL Harris/Day Architecture
msutter@harrisday.com
330.493.3722


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