1. Can I use standard topsoil? Why not?
2. How do we address high wind conditions?
3. Extreme temperature changes? Variations in Weather?
4. What if my roof leaks?
5. How do I maintain this roof garden?
6. What are the maintenance requirements?
7. How much load does a roof garden put on my roof?
8. Do I need to mow it?
9. Do I need to worry about insects?
10. What are the benefits?
11. What kind of R-Value does a roof garden have?
12. What is the life expectancy of a roof garden?
13. Is erosion a problem?
14. Does it have a warranty?
15. Can it survive a drought?
16. Are roof gardens limited to low slope applications?
17. Water retention capabilities, weight implications?
18. Aesthetic benefits: Can the Roof Garden be walked upon, used as courtyard / common area?
19. What is the benefit of using native plants?
20. What kind of plants can be grown on a roof garden?
21. Can my roof be dead level / flat? Why not?
1. Can I use standard topsoil? Why not?
Regular soil is heavy, can contain pathogens, undesirable insects, and weeds.
Its high organic content would shrink over time, potentially leaving a 4” roof
garden with only 2” of soil after a few years, resulting in severely hampered
drainage. Good engineered growth mediums for roof garden use contains no more
than 8% organic matter and weigh half (or less) of what topsoil weighs when
saturated without sacrificing any water holding ability. A quality, engineered
product reduces the possibility of pathogens, hidden seeds, or insect eggs, and
it should conform to strict FLL-guidelines. Forschungsgesellschaft
Landschaftsentwicklung Landschaftsbau --Guideline for the Planning, Execution
and Upkeep of Green-Roof Sites)
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2. How do we address high wind
conditions?
After the establishment period (60-90 days), the plants’ roots tie together and
stabilize the growth media, reducing high wind concerns. As of this writing,
there are no common accepted wind standards for roof gardens in the United
States. It is expected that standards will be in place by the end of 2009.
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3. Extreme temperature changes?
Variations in Weather?
The growth media components are tested to survive dozens and dozens of
freeze-thaw cycles. In addition, the growth media stabilizes the temperature
fluctuations encountered by the layers below, lengthening their life span.
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4. What if my roof leaks?
A “belt-and-suspenders” approach is taken to roof garden design to ensure there
are no leaks. All roofs are leak-tested before application of the roof garden
components. However, In the rare instance of a roof garden assembly leak, the
leak can be found with basic diagnostic techniques or electronic vector mapping
when a TPO or PVC roof is used. Because Carlisle uses a fully-adhered roof
system, the leak usually occurs in very close proximity to where the water
enters the building, simplifying the search. In these rare instances, the roof
garden directly above and around the leak must be removed for roof repair.
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5. How do I maintain this roof garden?
It depends on the type of roof garden. Most extensive (shallow) gardens require
only minimal maintenance such as fertilization and weeding in the spring
followed by a fall check up and removal of excess debris. Large intensive (deep)
roof gardens could require as much maintenance as a typical estate garden, which
would include regular pruning, weeding, irrigation, etc. All-sedum roofs after
3-5 years should require no watering or fertilization.
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6. What are the maintenance
requirements?
Most roof gardens require some irrigation during the establishment period (60-90
days). Extensive (shallow) roof gardens tend to require less physical
maintenance but can dry out more quickly in hot environments. Once established,
most roof gardens should not require much, if any, water beyond what nature
provides.
At a minimum, there should be two maintenance events per year. In the Spring,
the roof garden should be weeded, fertilized, and drains inspected. In the Fall,
debris clean up and weeding should occur. If a permanent irrigation system is in
use, it should be disconnected and drained in the Fall, then reconnected and
tested in the Spring.
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7. How much load does a roof garden put
on my roof?
The typical 4” roof garden weighs between 20-25 pounds per square foot. Weight
can reach 100+ pounds per square foot for intensive gardens with shrubs and
trees. The typical 4” roof garden weighs between 20-25 pounds per square foot.
In cases where weight is a concern, Carlisle Roof Gardens can be engineered to
achieve a 15-17 pound per square foot saturated weight. Building owners should
consult a structural engineer to ensure that the structure is capable of
sustaining the load of the chosen Roof Garden design.
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8. Do I need to mow it?
Only if you want to. The vast majority of roof gardens use sedums or other
plants that do not require mowing or a high degree of maintenance. In certain
cases, building owners will opt for having a functional lawn on their roof,
which would require mowing in most cases..
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9. Do I need to worry about insects?
Unlikely. There will be more insects on a roof garden than on a typical membrane
roof. However, this is usually to the benefit of the newly formed rooftop
ecosystem. Where there is a higher concentration of plants, there tend to be
more insects. Where there is a higher concentration of insects, there is likely
to be a greater number of birds. If edible plants are being grown, there could
conceivably be issues with certain pests such as aphids or thrips. Rooftop
gardens tend to be more unforgiving to pest insects and animals than ground
level gardens, limiting the potential problem.
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10. What are the benefits?
The benefits are numerous: Greatly decreased storm water runoff with cleaner
water quality, lowered cooling and heating bills, reduced noise infiltration
into the living space, extended roof life, additional usable building space,
potential food production, carbon dioxide sequestration, air and particulate
filtration, and combating urban heat island effect to name a few. And it looks
wonderful!
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11. What kind of R-Value does a roof
garden have?
In a completely dry state, the R-Value of a typical 4” deep Carlisle roof garden
is approximately 6. However, the higher the moisture content of the assembly,
the lower the R-Value, as thermal conductivity increases. The massive cooling
savings realized from a roof garden are a result of the plants’
evapotranspiration.
Plants function as small water pumps operating at high pressure and low volume.
When materials experience a phase change from liquid to vapor, they absorb a
large of amount of heat energy from the surrounding environment. In the case of
water, every gallon transpired by the plants absorbs roughly 8,000 BTU’s of heat
energy. As a result, during hot summer days, the roof membrane temperature is
typically 5-10°F. cooler than the ambient air temperature.
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12. What is the life expectancy of a
roof garden?
While Carlisle warrants up to 20 years, the roof life is expected to increase by
100%. In Germany, there are several 50+ year old roof gardens which have never
been replaced and have never leaked. The waterproofing membrane beneath a roof
garden receives no ultraviolet light or massive temperature fluctuations,
contributing to a long life span.
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13. Is erosion a problem?
The majority of roof gardens are designed with a 1/4” in 12” slope which greatly
limits the amount of erosion that can occur. Once a roof garden is established,
the plants’ roots tie the assembly together, biologically tying together the
upper level of the roof assembly. In addition, Carlisle Growth Media is designed
to allow rapid drainage rates, even when saturated which helps to prevent
erosion issues.
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14. Does it have a warranty?
The traditional Carlisle warranty covers the products in the assembly, but the
building owner is responsible for removal of the overburden (plants, growth
media, drainage layer, etc.) in the event of a claim. For a nominal fee, an
overburden warranty can be purchased by the building owner. If the overburden
warranty is purchased, Carlisle would be responsible for locating the leak,
overburden removal, leak repair, and overburden replacement.
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15. Can it survive a drought?
Plants are chosen for the particular environment to have low water requirements.
The use of native plants and/or sedums halves the water requirement of the roof
garden.
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16. Are roof gardens limited to low
slope applications?
No. High slope roof gardens can be employed using various methods including
stepped gardens, erosion mats and rows of baffles to contain the growth media
against erosion. Carlisle even developed a Roof Garden to be used on a 43°
slope.
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17. Water retention capabilities,
weight implications?
On a year-round average, 55%-80% of all storm water that falls on a roof garden
is retained and not released into the drains. All weights reported in roof
garden assemblies reflect the saturated weight. As an example, if a growth media
weighs 35 pounds per cubic foot dry and 60 pounds per cubic foot saturated, a
12” deep garden would hold up to 25 pounds of water per square foot. This is
equivalent to 3 gallons of water or 4.8” of rainfall. Building owners should
consult a structural engineer to ensure that the structure is capable of
sustaining the additional load of a saturated Roof Garden.
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18. Aesthetic benefits: Can the Roof
Garden be walked upon, used as courtyard / common area?
Many building owners use pavers or other methods to achieve a courtyard effect.
Having an enjoyable outdoor space on the roof benefits all building users.
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19. What is the benefit of using
native plants?
Native plants are always recommended on roof gardens. Native plants, combined
with an organic fertilization regimen, tend to halve the water requirements of
the roof garden.
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20. What kind of plants can be grown
on a roof garden?
Although sedum is, by far, the most common plant used on roof gardens, one could
grow just about anything, provided the plants will work in your USDA zone and
the growth media on the roof is deep enough. Smaller alternate plants include
herbaceous perennials such as sage and creeping thyme. Some chefs in large urban
centers will grow their culinary herbs on the roof, which saves thousands of
dollars per year and provides the freshest possible product. Larger plants can
include native shrubs and even full-sized trees in extreme applications.
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21. Can my roof be dead level / flat?
Why not?
Roof gardens need minimal slope to prevent standing water and anaerobic
conditions. Anaerobic conditions can occur very quickly when it is warm and the
sun is shining, pushing the plants' photosynthetic factories at full tilt. The
first sign of inadequate oxygen at the root zone usually manifests itself as
wilting under conditions of high light intensity and warm temperatures. At this
point, the roots' permeability to water and nutrient uptake is retarded, closely
followed by an accumulation of toxins. The next stage of oxygen starvation is
the plants' production of ethylene (the stress hormone of the plant world). This
gas accumulates in the roots and collapses the root cells. When this occurs, the
root zone becomes a haven for pathogens such as Fusarium, which causes rotting,
wilting and die-off very quickly. With oxygen-starved roots on a hot and sunny
day, both stages described above can literally happen in less than 48 hours.
The water retention performed by roof gardens is not accomplished by physical
restraint, but rather by a process of absorption. A good roof garden growth
media at a depth of 4" should be able to retain a minimum of 60% of all rainfall
during the course of a year. With 6" of growth media, some roof gardens have
seen year-round retention levels of 85%. If retaining as much storm water as
possible is a primary goal, many people install cisterns or basins for capturing
what water does come off their roof garden. The benefit of this is that the
water is generally cleaner after making its way through the growth media than it
would have been had it come off a standard membrane roof.
In cases of a dead-level roof deck, Carlisle tapered insulation can be used
underneath the assembly to achieve a slight slope.
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