Roof Garden FAQ
Published: 09/10/2012

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)

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.

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.

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.

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 checkup 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. 

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.

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. 

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..

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. 

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!

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.

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.

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. 

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. 

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.

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. 

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. 

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. 

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.

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. 

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.

24/7 website and technology-related assistance.
  • Carlisle SynTec Systems
  • P.O. Box 7000, Carlisle, PA 17013
  • Ph: 800-479-6832
  • Fax: 717-245-7053
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