Frequently Asked Green Roof Questions

What are the major benefits of Vegetative Roofs?

• Improved Air Quality
• Aesthetic Improvement
• Maximization of unused roof space, i.e. creating a back yard in the sky
• Increased Energy Efficiency
• Increased Roofing Membrane Lifespan
• Improved Health and Well-Being
• Increased Biodiversity
• Food Production
• Local Job Creation
• Educational Opportunity

What are Vegetative Roofing Standards?

Germany’s Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau (commonly referred to as the FLL) is the only recognized vegetative roof standard. Hanging Gardens bases its vegetative roofing philosophies on these guidelines, which include industry standards for testing the weight, water holding capacity, nutrient content, and contains a specification for growth media. The American Standard Testing Methods (ASTM) has limited standards for vegetative roofing, containing only four standards. Hanging Gardens’ project partner, Charlie Miller is responsible for writing three of these standards.

How much does a Vegetative Roof cost?

It is almost impossible to pin point a price without knowing the exact requirements of a project. There are a variety of types, sizes and choices for vegetative roofs that can drastically impact the price. Varieties of vegetative roofs include: Extensive vegetative roofs (Less than 6″ deep), Semi-Intensive (6-12″ Deep) and Intensive (12″+). Beyond the variety of vegetative roofs, the size of a project can impact cost since there are economies of scale that drive price. All things being equal, smaller projects cost more per square foot than larger projects. Extensive vegetative roof pricing varies from as low as $5.50/ S.F. but can be as high as $20 or more. Semi-Intensive pricing typically ranges from about $8.50/S.F. to $15.00 but can be as high as $25 or more. Intensive projects range from $12.50/S.F. to $30.00; however, we have been involved in projects costing over $200/S.F. We recommend that you contact us with your specific project needs for a more accurate price estimate. We would be happy to fine tune budget pricing for your project. We do not promote specific systems or vendors. Instead, Hanging Gardens will always listen to your needs and find the best fit for your project that will maximize the value of your investment. If you are working with another vegetative roofing expert, you should ask them to do the same.

How much do Vegetative Roofs weigh?

As is true with the question, “How much does a vegetative roof cost?” there is no one answer to the question, “How much do vegetative roofs weigh?” Generally speaking, Extensive Roofs (Less than 6″ deep) weigh 22-28 lbs./S.F., Semi-Intensive Roofs (6-12″ Deep) weigh 35-75 lbs./S.F. and Intensive Roofs (12″+) weigh 75+ lbs./S.F. It is important to keep in mind that these numbers are based on common systems. These weights can vary dramatically depending on the plantings, specific media type and the stormwater holding capacity requirement. We recommend that you contact us with your project requirements for a more accurate estimation of weights. We will need to know the type of system you prefer, your building’s structural capacity, your desired plant mix and the stormwater holding capacity requirement. Based on that information we can determine exactly what your total saturated dead load would be for your vegetative roof. This weight estimate will indicate whether or not you can proceed with your project without any additional structural analysis.

Generally speaking, if the dead load of your vegetative roofing requirements exceeds your structural capacity, you will have to spend an exorbitant amount on structural reinforcement, reduce the scope of your project or consider your project cost prohibitive. However, Hanging Gardens has a potential solution for projects requiring cost prohibitive structural reinforcement. We have developed vegetative roofing systems that weigh approximately 40-50% less than commonly available vegetative roofing systems. Although these systems cost slightly more than the conventional systems, they are considerably less costly than structural reinforcement. Please contact Hanging Gardens to learn more about our Ultra-Lightweight vegetative roofing systems.

Is there a maximum slope for a Vegetative Roof?

The general rule of thumb is that any roof lower than a 2:12 pitch will not require slope stabilization and a vegetative roof can be easily installed. For roofs greater than a 2:12 pitch, Hanging Gardens provides slope stabilization systems that will allow you to have a vegetative roof. Since these slope stabilization systems are an additional expense, knowing the details regarding the slope of your system is important to proper design and pricing, since there are truly no limitations on slope. For example, our green wall systems are on an infinite slope. Please contact Hanging Gardens to learn more about the possibilities.

Is irrigation required on my Vegetative Roof?

Questions often surface regarding maintenance of vegetative roofs during a drought. Droughts and extended droughts do require additional maintenance. Vegetative roofs contain plants, and those plants need water. If you do not supply your vegetative roof with sufficient water, it will either die or at the least have a temporary appearance of scorched earth before slowly returning to its pre-drought appearance. It is important to beware of companies claiming that irrigation is not necessary. Dissatisfied owners who have been mislead by false claims regarding no need for irrigation now believe that vegetative roofs always fail and merely waste money.

At Hanging Gardens, we want to provide successful projects every time. If our clients are investing large amounts of capital in a product, we want to inform them of the best practices to ensure their investment remains lush and healthy. Hanging Gardens promotes only “smart” irrigation systems that can detect moisture and only water your vegetative roof as needed. Our irrigation systems will apply the correct amount of irrigation to ensure your vegetative roof does remain lush and healthy, so your investment is not compromised. Please contact Hanging Gardens to learn about our Smart Irrigation System packages.

What type of waterproof membrane should I use for a Vegetative Roof?

Various types of waterproofing membranes are used for vegetative roofs. Polyvinyl chloride (PVC), thermoplastic poly-olefin (TPO), ethylene propylene diene monomer (EPDM), hot applied rubberized asphalt and modified bituminous membranes are some of the most common membranes used in vegetative roof construction. So which is the best for a vegetative roof? The best choice of membrane for a vegetative roof depends on the roof material or deck.

At Hanging Gardens, we recommend PVC roof membranes for steel and wood decks because of its architectural compatibility with that form of construction. PVC is the most time tested waterproof membrane that is also root resistant. TPO has the same properties as PVC; however, the NRCA (National Roof Contractors Association) has stated that TPO is not an acceptable membrane for vegetative roofing. In our opinion, the reason for their rejection of TPO is because TPO is a relatively new product on the marketplace and it has had some issues relating to the high heat conditions of the deserts of the Southwest USA. In our opinion, if TPO is the choice of roof membrane that you would like to use, that Electric Field Vector Mapping is used in conjunction with that system. If that is done, there should be no major issues to be concerned about. EPDM, on the other hand, is not a recommended roof membrane by Hanging Gardens. EPDM is generally not a reinforced membrane and its seams are glued. The glue does not mix well with the constant moisture on a vegetative roof. Over time, it is believed that the moisture will cause the seams to separate and potentially cause a leak. Other downsides of EPDM are that it requires an additional root barrier and the common black EPDM does not work with Electric Field Vector Mapping (Otherwise known as EFVM – See below for more information on this). Regardless of the system that you choose, every roof membrane should be tested for water tightness prior to the vegetative roof installation.

For concrete decks, Hanging Gardens recommends hot rubberized asphalt waterproofing. This system will require a root barrier. Contrary to what may be thought, the additional cost of the extra layer of a root barrier to the hot rubberized asphalt, does not compromise the economic savings of the product. Hot rubberized asphalt with a root barrier is still generally the most cost effective option for concrete decks. Hot rubberized asphalt is directly placed on the concrete to create a 220 mil thick waterproofing barrier. This system rarely leaks unless there is an installer error or a manufacturer’s defect. Hanging Gardens only recommends the best installers and time tested manufacturers, so the likelihood of a leak is quite low. As added insurance, EFVM works well in conjunction with hot applied rubberized asphalt. Other asphaltic based systems like Cold Tar Pitch, Built-up and Modified Bitumen are also suitable roof membranes and work well with vegetative roofs, however, Hanging Gardens believes that these are redundant for a vegetative roofing system. In addition, PVC is compatible with concrete decks and, in limited situations, may be the best choice for a concrete. To be certain which roofing system is best for your project, please contact Hanging Gardens.

Is a water test sufficient to ensure watertightness of my Vegetated Roof?

One pertinent concern that surfaces regarding vegetative roofs is the likelihood of an eventual leak and, subsequently, how such a leak would be remedied. A water test conducted before the protection fabrics, drainage boards, growth medias, plantings or module systems are installed will determine that your roof is water tight at that point of time. However, if a puncture occurs during construction of the overburden or at sometime after the project is installed, detecting the leak is nearly impossible and your entire vegetative roofing system will need to be removed. To prevent such grief and expense, Hanging Gardens recommends a product called Electric Vector Field Mapping (EFVM) in lieu of a water test. EFVM is a stainless steel grid that is installed underneath or integral with the waterproof membrane (Note: this system does not work with black EPDM). The system is connected to conducting wires that are accessible above the roof membrane. After the installation of the roof membrane, Hanging Gardens’ project partner, International Leak Detection, will electrify the grid, allowing us to detect the source of leaks with pin point precision by following the water migration. The biggest benefit is that EFVM can be used at any time after the vegetative roof is installed. So, if sometime in the future your waterproof membrane starts leaking, a technician will be able to easily identify the source for the contractor to fix. The biggest difference between a water test and EFVM is that a water test cannot detect leaks after the system is installed, forcing you to potentially remove the entire vegetative roofing system, whereas EFVM will only require removal of approximately a 2′ x 2′ section. For information about the best insurance policy for a vegetative roof, EFVM, please contact Hanging Gardens.