Benefits of the Modern Roof Garden
The Hanging
Gardens of Babylon, the earliest recorded example of a roof garden in Western
history, was considered one of the Seven Wonders of the World. In the quest to
find economically feasible “green” building alternatives, cutting-edge
contemporary roofing technology is helping to bring an ancient and visually
desirable architectural practice back into vogue. Roof gardens, also known as
“green roofs,” “eco-roofs,” “living roofs” and “vegetative roofs” have been
widely adopted with positive results in Europe and are growing in popularity
here in North America.
Cost-Effective
Over Time
Savings from
heating and cooling are only part of what a garden roof can provide. Because of
the additional green space, it can allow a developer to maximize the use of the
property which could greatly increase the property’s value. Other savings come
from delaying water run-off from the building which helps lower sewer and
wastewater penalties. This was witnessed firsthand by John Thomas, product
development manager at Siplast Inc. Thomas’s company was responsible for
providing the roofing and waterproofing membrane materials for the 10-acre Ford
Motor Company garden roof – the largest living roof in the world – on the
automaker’s Dearborn Truck Plant. Between the amount of money saved in cooling
bills and avoiding the cost of modifying the River Rogue Treatment Plant
together with other potential pollution-related fines, Don Russell of Ford
Motor Company said, “The green roof had already paid for itself, and they
hadn’t even started producing cars yet.”
Research from Dr.
Karen Liu with the National Research Council of Canada (NRCC) shows that roof
gardens have the potential to extend the life of the roofing material beyond
its normal lifespan by shielding it from extreme weather, UV degradation and
temperature fluctuations that can cause damaging expansion and contraction of
the membrane. David Roodvoets, technical director of SPRI, agrees.
“There’s no
thermal stress, there’s no water running over the membrane,” he said. “We’ve
heard of a system in Europe that was built in 1938, and I was there in 2002 and
it was still functioning.”
If contractors
want to further enhance the high thermal performance of single-ply and
multi-ply roofing membranes, roof gardens may indeed be one way to achieve
this. Roof gardens lower the amount of heat absorbed by the building envelope
through sunlight, and while traditional roofs can reach temperatures of 180
degrees Fahrenheit, the transpiration process of the vegetation on a roof
garden and the mass of the growing medium can keep a roof’s temperature in the
70 degree Fahrenheit range.
This decrease in
heat build-up translates into lower air-conditioning bills and less initial
outlay for air-conditioning systems, for smaller units can handle the reduced
cooling requirements.
Increases Property
Value
Properties with
roof gardens typically experience an increase in property value, for many
reasons. Roof gardens are aesthetically pleasing, offering a lush, green and
often flowering expanse for the viewer’s pleasure. And if the underlying
building structure permits, roof gardens can be developed as extensive
recreational areas.
Insulation against
sound transmission is a by-product of roof gardens. Exterior sounds are muffled
and absorbed, while sound leakage produced by interior acoustics is reduced.
Ecologically- and
Environmentally-Friendly
The ecological
benefits of roof garden construction are manifold because they can dramatically
reduce storm water run-off and flow rate and, depending on rain amount and
garden depth, many rain events will result in no water draining off the roof.
Overall, a garden roof can retain as much as 70 per cent of rainfall, cutting
down on the risk of flooding.
In Orlando, the
University of Central Florida’s Stormwater Management Academy has done
extensive research into the flood-reduction aspect of roof gardens. Cities such
as Washington, DC, Portland, Seattle and Chicago are already investing in green
roofs as a crucial part of their storm water management programs.
Storm water
management is a key issue to the cities because combined flow from sanitation
and storm sewers can overload a treatment plant. This can cause raw sewage to
go into downstream aquifers or storm sewers that drain into small creeks and
the resulting ecological damage caused during large rain events. Roof gardens
delay or totally eliminate rainwater flow from the roof, giving the streets a
chance to clear. When the roof does become saturated, the runoff occurs at a slower
rate and over a longer period of time, stretching out the event with a lower
total volume sent to the sewer.
“The City of
Chicago is probably the biggest market for roof gardens in the United States,
and that’s because the city has been very proactive, installing garden roofs on
a lot of city buildings,” said Dick Gillenwater, manager of Advanced Project at
Carlisel SynTec Inc. “The city provides incentives for privately operated
commercial buildings that have installed roof gardens.
According to Jim
Rubenacker, director of Customer Operations at Sarnafil Inc., “When the City of
Chicago decided to have an intensive green roof installed on Chicago City Hall,
it effectively put public awareness of such systems in place immediately. Tours
of the rooftop educate the young and the not so young at the same time.”
Additional
research from NRCC explains “evapotranspiration,” which occurs when plants
secrete or “transpire” water through pores in their leaves – much like the way
people sweat. This cools the surrounding air by the evaporation of the water.
This phenomenon may help reduce the “Heat Island” effect common in cities or
other large communities where energy usage is concentrated. According to the
Heat Island Group, “a single, mature, properly-watered tree with a crown of 30
feet can ‘evapotranspire’ up to 40 gallons of water in a day, which is like
removing all the heat produced in four hours by a small electric space heater.”
In cities where
pollution is an issue, studies show that the gardens may provide relief by
capturing and filtering pollutants out of the air, and they can also help
neutralize acid rain.
We all know that
plants absorb carbon dioxide and emit oxygen back into the air, but according
to the Photosynthesis Centre at Arizona University, “An average hectare (1076
squares) of corn produces enough oxygen per hectare per day in mid-summer to
meet the respiratory needs of about 325 people. This means that the one million
or so hectares of corn grown in Ontario produce enough oxygen for the annual
respiratory needs of Ontario’s 10 million residents in about 11 days.”
Imagine the
benefits, in this respect, of even a small roof garden. Humans aren’t the only
beneficiaries – “green roofs” provide a natural habitat for animals and insects
that might otherwise be displaced by the rapid development of both rural and
urban areas.
Types of Roof
Gardens
In general, there
are two types of roof gardens – extensive and intensive. Extensive garden soil
media thickness runs from two inches up to eight inches, however, the typical
range is three- to four-inches offering a lighter weight of around 15 pounds
per square foot. Within extensive gardens, sedum and other shallow rooting
plants are commonly utilized, for they tolerate the shallow soil levels and are
sun and drought tolerant. Sedum require little maintenance for they are low
growing ground cover that does not need to be trimmed.
Even though this
type of roof garden can be installed over a variety of roofing membranes and
structures, a structural engineer should be consulted to determine any relevant
weight load limitations. Extensive roof gardens can, at full saturation, weigh
between 12 to 25 pounds per square foot.
Intensive gardens
use thicker soil depths (eight- to 48-inches) and more complex landscaping,
which may involve using planters, and include shrubs and trees that often
require additional anchorage. Other ornamentations such as ponds, fountains and
pavers for walkways, as well as garden furniture, may be featured in this type
of roof garden.
Given their
additional weight (starting at 50 pounds per square foot when fully saturated,
or more if larger trees are used), intensive roof gardens are commonly
installed over concrete decks. Again, a structural engineer should be consulted
to determine weight limits or necessary modifications to roof structures.
“Because of mixed
uses on the roof, a new category, semi-extensive (25- to 50-pounds per square
foot), has been created,” said Thomas. Typically, semi-extensive garden roofs
are defined as combination of both extensive and intensive roof gardens on the
same roof.
Construction of a
Roof Garden
The heart of any
roof garden, whether extensive or intensive, is the roofing membrane that lies
beneath it. SPRI members’ roofing systems, which consist of a variety of
single-ply membranes, have been successfully used with roof gardens, both
extensive and intensive.
A preference in
the type of membrane used may depend upon whether the application calls for a
ballasted or fully-adhered surface.
“In Europe, PVC is
probably one of the leading products used for roof gardens, but modified
bitumen is right in there with it,” Gillenwater said. “If a very large roof was
being done, EPDM would be my optimum choice because it is designed for large
ballasted-type applications, whereas if a smaller roof or mixing systems was to
be done, a heat-weldable product like TPO or PVC would be a good choice.”
On the other hand,
there are many who would prefer the use of modified bitumen roof membranes
because of the safety factor of having the benefit of redundant waterproofing
layers.
Rubenacker has a
slightly different view. “Because of the long and successful history of green
roof systems in Europe, using thick, reinforced PVC membranes has been the
roofing material of choice, because of the security and root resistance of
proper heat-welded seams,” he said. “Because of this I know I would choose PVC
or a thick TPO membrane with heat welded seams regardless of the project size.”
The thickness of
“single-ply” roofing membranes used with garden roofs is typically around 60 to
90 mils, while multi-ply modified bitumen systems can range from 180 to 250
mils or more. Some companies further modify their membrane specifically for use
with garden roofs.
“Normally, our
roofing product is about 170 to 180 mils, but we make a specific product for
green roof applications, a modified bitumen membrane that is constructed
differently than the one we use for roofing,” Thomas said. “At 250 mils, it is
a thicker, more heavily reinforced product, so it is a more durable sheet.”
Whatever membrane
you choose, the experts agree it is important to have a layer of insulation
over it with intensive gardens, as well as a drainage layer, an optional
root-resistant layer (often referred to as a “root barrier”), and the correct
growing medium for your garden.
The layers of the
garden roof in top-down order are generally the following: vegetation, growing
medium, filter layer, drainage layer, roof barrier and protection layer.
The growing medium
is an engineered soil, consisting mostly of light weight aggregate with some
sand and organic material. It is designed to be lightweight, able to fertilize
the vegetation and maintain bulk density. Within this layer, a fine mesh net
can also be incorporated to control and strengthen the root system.
Then comes the fun
part – choosing what plants your roof garden will display. Extensive gardens
require the least maintenance of all, while intensive gardens demand more time
and attention to reach their full potential. The extensive garden is typically
populated with sedum or other native drought resistant, self-propagating
vegetation that requires little watering and maintenance.
Popular choices
for intensive garden roofs include flowering plants and shrubs, evergreens for
year-round appeal, or seasonal stunners that offer colourful summer or fall
foliage. A variety of native plants can be used to create a low-maintenance
garden, while non-native plants may require additional care to survive.
Special
Considerations
It is important to
note that water leakage from drainage backups or root punctures can lead to
roof damage if the correct materials are not used, or if there is faulty
installation of these materials. Areas particularly vulnerable to leakage in
these circumstances include abutting vertical walls, roof vent pipers, outlets,
drains and air conditioning units.
Care should also
be taken to prevent pesticide leakage from roof materials, so that is does not
contaminate ground water. Additionally, the use of gardening tools capable of
puncturing roof surfaces needs to be carefully monitored – over-enthusiastic
gardener could end up causing a major leak. However, a thick layer of
insulation, as well as an additional root barrier layer, can act as an “early
warning system” to alert the gardener that the membrane is not far below,
Roodvoets said.
If roof traffic is
heavy, it may be necessary to create designated walking paths to take pressure
off the vegetation. Roofing slope may also affect design. To ensure drainage
capacity, the support to the waterproofing layer should have a slope of at
least 1.5 per cent.
On steeper slopes,
up to /45 degrees, a raised grid structure should be installed to secure the
soil. A layer of gravel or pebbles may be placed around the outside perimeter
of the roof to provide additional drainage and safe roof access. Excessive
drainage becomes a problem at angles greater than 25 degrees, but reservoir
boards can minimize this.
When a growing
medium is saturated with water, there has been some evidence that suggests
green roofs can slow the spread of fire to and from the building through the
roof. Still, if not properly bordered by a non-flammable material such as
paving stones, the plants themselves may become a fire hazard. But this can be
overcome with plantings that have a high water content such as Sedums. Another
tactic, dependant on the type of garden roof, is to connect the sprinkler
irrigation system to the fire alarm.
Garden Roofs vs.
Cool Roofs
Garden roofs have
their pluses and minuses in comparison to the other cool roof technologies
available. Liquid applied coatings, prefabricated membranes, metal panel roof
systems and other specialty product systems, such as clay, concrete and
metallic tiles round out the field of product categories available. One
comparison between garden roofs and other cool roof technologies is that the
cost for a garden roof is much higher than some other roofing technologies, but
garden roofs may have a life cycle much greater than other waterproofing
methods, which would offset some of the costs.
If a building team
decides on a garden roof to address sustainability issues, garden roofs can
qualify for credits toward green building certification through the LEED
(Leadership in Energy and Environmental Design) Green Building Rating System,
which was developed by the U.S. Green Building Council. This national standard
for green building certification awards credits in the category of Landscape
& Exterior Design to Reduce Heat Island/Roof (Credit 7) for covering at least
half of a roof with a garden, as well as regional green building standards set
by such entities as the Florida Green Building Coalition.
While the initial
cost of garden roofs are higher in comparison to other cool roof systems for
new construction and renovation projects, government incentives can offset
initial outlays or increase the rate of return on initial investment. Green
roof development in cities such as Portland, OR, is being encouraged through
development code incentives with such options as a Floor Area Ratio (FAR) that
encourages green roof development for the purpose of water runoff control.
A FAR bonus allows
the total area of building to be larger than it might be permitted otherwise,
if certain criteria are met. For example, if the total area of the green roof
is 10 to 30 per cent of the building’s foot print, each square foot of green
roof earns one square foot of additional floor space. The amount of additional
floor space then rises with a corresponding increase in size of the garden roof,
with a garden roof covering from 30 to 60 per cent of the building’s roof,
earning two additional square feet and roofs covering more than 60 per cent of
a building’s roof gaining three feet of additional floor space.
In general, garden
roofs and cools roofs have the potential, with proper design, to provide
improved efficiency of HVAC equipment and a reduction in the cooling demand of
the building. This could reduce the actual time that HVAC equipment operates.
This would not only reduce energy costs, but would be expected to increase the
service life of the equipment. Reduced operation time may also reduce on-going
maintenance and repair costs of the equipment.
Overall, if
potential obstacles are anticipated and overcome, the teamwork of
manufacturers, contractors, architects and engineers can now create a
financially profitable, ecologically sound, highly functional and visually
satisfying product – a showpiece for their clients to enjoy and display. A
little piece of ancient civilization, modified for the contemporary world.
