|Photos by Jeremiah Johnson.
|The sloped green roof research study installed. The green cloth is a shade cloth to ensure the seed germinates/establishes.
Nearly a decade ago, at a
new assembly plant in Dearborn, Mich., Ford Motor Company faced investing
millions of dollars in stormwater infrastructure. To help offset this cost,
as well as lower energy consumption within the building, Ford partnered
with Michigan State University to turn 10.4 acres of roof into green space.
Today, it is one of the largest extensive green roof expanses in the
Landscaping an area 20 feet (or higher) off the ground
can be seen as a real maintenance challenge. The reality, say green roof
experts, is that a properly installed green roof is fairly easy to
There are three types of green roofs: extensive,
intensive and semi-intensive. Extensive roofs are lower maintenance and are
most popular in areas where roof access is difficult or limited. This type
of green roof has soil substrate of 6 inches or less. Semi-intensive green
roofs have a variety of species and depths for roots. These systems could
have irrigation systems and include plants that require trimming. Intensive
systems most resemble traditional gardens and ground landscaping. The
root levels usually start at 8 inches and can go as deep as 15 feet,
allowing for shrubs and trees and can also include architectural additions
like gazebos or ponds. The amount of general maintenance for the green roof
will correspond directly with the type of system used. The extensive system
is set up for minimal long-term maintenance, while the intensive system
will need a great deal of care.
Maintenance also depends on how the roof is installed,
explains Jeremiah Johnson of Advanced Green Architecture: “Are you
coming in with 100 percent coverage? Are you going to seed it or plug it?
Maintenance is dependent on these choices. For plugs, I’d recommend
adding irrigation, but if it is 100 percent live growth, you can put it up
there and be done.”
Green roofs should be planted in either the fall or
the spring. Greg Raymond, managing member of EcoGardens, recommends spring
installation from late March/early April through June and fall planting
between mid-September through the end of October. These times of the year,
he says, allow the plant systems to best utilize natural rainfall and
|The green roof prior to the installation of the research study on the Old Mission Peninsula in Traverse City, Mich.
Before installing the green roof, Raymond suggests
doing a thorough inspection of the original roof. “We do an
inspection of the roof membrane, the exposed membrane, the counter
flashing, all the details that could deteriorate over time.” He adds
that this inspection is something that continues as part of the general
maintenance of the roof.
To prevent leaks, Sara Loveland, interim executive
director of DC Greenworks, recommends installing a new roof membrane as a
first step. Also consider adding gravel or large river rocks around the
drainage areas and borders. “That way, if there is a leak, you are
only moving rocks, not plants and dirt,” she says.
The type of topsoil is important, as well. According
to Greenroofs.com, green roof soils need to be lightweight to meet roof
loading weight restrictions and have good drainage while maintaining
sufficient rainwater retention. Regular garden soil can’t be used on
The first two years after installation are the most
critical. “That’s how long it takes the sedum to grow to 90
percent coverage,” says Loveland. “However, if you are using
pre-vegetated mats, it eliminates the more intensive maintenance
During those years, Loveland says, it is important to
assess the roof at least twice a year, checking for plant health
and drainage issues. “There might even be spotty
sun or shade issues that need to be kept in check,” she says.
Sedum is the most popular plant choice, because of its
ability to go for periods of time without water and for its shallow roots.
“Sedum does quite well across the vast majority
of the country,” says Raymond. “They are extremely drought
tolerant, they are efficient in terms of absorbing and getting rid of water
quickly, and most respond well to propagation through cuttings. If you want
to thicken your roof, you could snap off cuttings and redistribute them.
Sedum is diverse and durable and does a good job on roofs.”
Native plants with shallow roots also work well.
“Native plants tend to survive and thrive on their own, which is what
you want for green roofs,” says Loveland.
Plants that should be avoided are shrubs and ground
cover that are invasive. “You want to have a diverse culture,”
says Raymond. “Then, if you have a declining roof, it is just a
portion to worry about, not all of it.”
One major maintenance concern is keeping the roof
green during drought conditions. In general, roof irrigation systems are
expensive. Because of this, most green roof systems are designed to
withstand long periods of drought. The roofs are designed with good
drainage systems, and most of the plants used are drought tolerant.
“The roof might not look as good during a
drought period, but it would still be unusual if the plants die off, as
long as they are selected appropriately for the location,” Loveland
Loveland keeps a sedum in a pot in her backyard as a
control sample. “I use it as a gauge,” she says. When she goes
on site visits, she has a good idea of what the roof sedum should look
like, based on her backyard plant.
Once the green roof is established, the most critical
maintenance will be weeding. Weed pressure can be significant, says
Raymond, depending on issues like nearby open fields or bird population.
|Photos Courtesy of Dr. Brad Rowe.
The Ford Rouge green roof in Detroit
is roughly 500,000 square feet.
During the first two years, weeding should take place
at least four times. After that, a spring weeding and a fall weeding should
be enough. Johnson also recommends once a year trimming and general removal
of biomass that would be done in ground landscaping.
Experts are mixed on the use of fertilizer. Some feel
that fertilizer goes against the other eco benefits of a green roof, but
others believe fertilizer help the roof establish itself.
A green roof challenge is bird life. “Pigeons
are a huge issue,” says Raymond. “We try to deter them with
spikes, bio-safe chemicals or by introducing natural predators.”
Safety is always an issue with green roof maintenance.
The experts agree that building owners should hire professional landscapers
with green roof experience to do the basic maintenance.
“No green roof is maintenance-free,”
Raymond says. “When they mature, it becomes a little less, but
you’ll need at least one maintenance per season.”
The author is a freelance writer who writes for a
variety of trade and B2B publications, covering topics from sustainable
living to construction and landscaping/lawn care. She resides in State