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The Story of Our Green Roof: Part 2

Since we’ve constructed a green roof on part of our office building in late 2006, we have learned several things that we want to share:

Green Roof Type
Extensive
100, 2’x2’ trays with 4” of growing medium
Southern exposure
Plants
Sedum spurium ‘Red Carpet’
Sedum album L
Sedum kamtschaticum var. ellacombeanum
Sedum rupestre
Lessons Learned
Irrigation – We watered periodically during the first growing season to get the plants established.  We did use a drip irrigation system during that period which made it a little easier.  Since then, we have removed the drip irrigation and haven’t watered anything, except once or twice when one of the staff has recognized that it hadn’t rained in several weeks.  We recommend sticking to a strict watering schedule in which the plants are watered thoroughly every few days for the first week or two, and then once a week for the first growing season.  Long, deep watering encourages deeper rooting and stronger, healthier plants.  If the water just reaches the surface, so will the roots, which prevents the plants from gathering more water during droughts.
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Maintenance – During the first and second growing seasons, we weeded the green roof several times a year.  Since then, we have weeded once or twice a growing season.  Typically, an employee will take a break from their daily grind to spend five or ten minutes on a beautiful day weeding.

Plant Selection – All of the sedum have been doing well except Sedum spurium ‘Red Carpet’ which seems to get scale.  We recommend mixing a large variety of species in order get a higher survival rate and ensure that a disease won’t wipe out an entire area.  Sedums work well with shallow soils such as on our roof.  However, most are non-native.  If you want more native plants, six inches or more of growing medium is best.
Environmental Benefits – Our green roof retains up to 95 gallons of water during a rain storm.  That’s up to 95 gallons of rain water that doesn’t flow directly into the Combined Sewer system and into the Rivers.  It is also providing some refuge for several species of bugs.  We haven’t measured the air quality, but know that our green roof is producing more oxygen and absorbing air pollutants.
Cost Benefits – We are in the process of figuring out how much energy we save due to the green roof.  We already know that it is protecting the rubber membrane underneath from harmful UV rays and therefore extending its normal life span.
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Final Thoughts – If you are part of a small office building with just a few employees, constructing a green roof is possible with just a little bit of effort and money.  Sedums are the easiest to maintain.  Creating a maintenance schedule for the first two growing seasons will help ensure that your investment does not die.  Our office has a dishwasher and trash duty schedule in which each employee is responsible for about one month out of the year.  This can be adapted to a green roof maintenance schedule.  If you really want to know all of the specific benefits your green roof will have, try installing monitoring equipment or just look at your utility bills.  Finally, we couldn’t tell everyone that they should put green roofs on their buildings unless we also encourage them to first consult a structural engineer to determine if a green roof is feasible.

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