Because of their contribution to sustainability, a solar-panel roof is considered a Green roof even though it does not include landscaping. This one is atop the Georgetown University Intercultural Center. Photo courtesy of J.G. Branch
Unless your specialty is roofing, you may be confused about what Green roofing really is. After all, there are many types of Green roofs. The term Green roofing refers to landscaping installed over a weatherproofing system, often pertaining to landscaped roofs, and it can also be applied to the use of Green technologies, such as photovoltaic energy cells installed over a roof system. While Green roofing is a seemingly new concept, the technologies used to Green a roof are not necessarily new.
Whatever type of Green roofing is being referred to, however, one thing is clear: Green roofing provides organizations with excellent opportunities to save energy, reduce environmental impact, and highlight the company’s concern for the environment to customers and the community at large.
One major concept with all Green technologies is to weigh reduced environmental impact during manufacturing and installation versus service life. That is, replacing a Green technology system after only a few years may not be very Green, due to the impact of the waste and manufacturing of more materials. With Green roofing, the impact of replacing a system is significant, because the overburden layer composed of growth materials and plantings must be completely removed to replace and repair the membrane system. Because the membrane systems are constantly exposed to moisture, conventional roof systems may not be an appropriate selection. Instead, plaza waterproofing systems, specifically designed and manufactured for constant moisture exposure, should be used.
Similarly, plaza waterproofing concepts and detailing must be implemented to ensure longevity of the system. Although there are many benefits to Green roofs, there are disadvantages that must be considered, such as increased cost, access to the membrane and unwanted wildlife and insects. Depending on the planting and growth medium and planting overburden system selected, a Green roof system can cost up to twice as much as a conventional roof system.
Membrane Access
If leakage does occur, the membrane system is buried below the planting/overburden systems, which must be removed to determine the cause of leakage and make repairs. As with any roofing, this is challenging because the interior leakage may not align with the breach in the membrane.
Unwanted Wildlife and Insects
The planting systems provide suitable habitat for birds, insects, and other wildlife, which may be unwanted. Additional protection may be required to prevent unwanted wildlife and insects from inhabiting the Green roof.
Maintenance
While conventional roofing membranes require periodic maintenance to repair membrane degradation, Green roofing requires periodic landscaping maintenance to remove unwanted weeds and other plants from overtaking and detracting from the Green roof plantings.
There are two general types of Green roof planting systems, extensive and intensive.
Extensive
- Exist for environmental benefit, not accessible roof-top gardens
- Shallow soil / media (two to six inches deep)
- Vegetation: shallow root structure plantings (sods and sedums), designed for harsh environments (“drought resistant” species)
- Irrigation: Little to none required, typically only during the “startup” period
- Weight: 15 to 50 lbs/sf
Intensive
- Mixed environmental benefit and accessibility
- Deep soil / media (9 to 18+ inches deep)
- Vegetation: Varying depth of root structures, including trees and shrubs
- Features: Architectural features such as paving, walkways, benches, etc.
- Irrigation: Required
- Weight: 80 to 150 lbs/sf
Based on the type of Green roofing desired, the rest of the system can be selected, including the waterproofing membrane and insulation type/location. It is important to remember that Green roof systems are covered and constantly wet, and that conventional roofing membranes modified for Green roof systems may not be suitable for Green roof systems.
Membranes time-tested for plaza waterproofing applications should be considered. The membrane must be capable of withstanding constant water exposure, based on this; Green roof systems should include plaza waterproofing membranes, which are designed for this type of exposure. The type and placement of the insulation is based on the type of Green roof plantings.
In general, the insulation is typically located above the membrane. This prevents the weight of the Green roofing from compressing the insulation and damaging the membrane. In some instances, lighter weight extensive Green roof systems can include the insulation below the membrane system.
Just like conventional roofing, the membrane of Green roof systems must be sloped to drain. Although the membrane systems are capable of withstanding constant water exposure, it is good practice to remove water promptly and away from details to reduce the potential for leakage. Depending on the type of structural deck, slope to drain can be provided by tapered insulation or tapered concrete.
Tapered concrete adds significant weight to the structure and is more expensive than tapered insulation, but provides adequate support of the membrane in any type of Green roof system and is more versatile than insulation. Tapered insulation weighs less than concrete, but the membrane system must be installed over the insulation, limiting the type of Green roofing planting types. Also, due to the installation process, tapered insulation is less versatile for non-symmetric roof plans.
A Green landscaped roof at the campus of Georgetown University. Photo courtesy of J.G. Branch
In addition to the primary components list above, the following secondary systems must be included in the system, when appropriate:
DRAINAGE COMPOSITE: Molded plastic sheet with adhered filter fabric. Allows water to migrate from the overburden layer and migrate to the drains within the composite.
ROOT BARRIER: When deep root structure plants are used (trees, shrubs, etc.), root barriers should be used between the plantings and waterproofing membrane to reduce the potential of roots reaching the membrane and causing deterioration.
VAPOR RETARDER: Depending on the waterproofing system and building conditions, a vapor retarder may be required to prevent condensation from forming within the roof system.
WATER RETENTION LAYER: Depending on the planting type, water retention layers or systems may be incorporated to pool limited amounts of water within the system to maintain moisture in the planting system between rain events.
As part of the selection of a suitable system, the system details must be considered, including:
BASE FLASHING: For conventional roofing, the top edge of the base flashing must be at least eight inches above the membrane surface. In Green roofing, the top edge of the flashing system should be at least eight inches above the top of the planting system.
PENETRATIONS: For all roof systems, penetrations must be minimized to reduce the potential for water leakage. Due to access constraints and challenges, this is more important in Green roofing.
GRAVEL: Gravel should be installed at all perimeters and around all other penetrations and details to allow water to promptly flow from these areas.
DRAINS: Dual-level drainage must be provided, ensuring that water can drain from the top surface of the overburden, as well as the waterproofing membrane layer.
PLANTERS: For deep root structure plants, like trees and shrubs, independent planter boxes should be provided to separate the roots from the waterproofing system.
Additional items to consider when selecting a Green roof system include the following:
WARRANTIES: Don’t rely on them. It’s much more important to ensure proper installation of a quality roofing system.
MANUFACTURER: Whenpossible, use manufacturers that supply all the components from the waterproofing membrane up to the planting system.
DESIGN AND INSTALLATION: Ensure proper design and installation by engaging a design professional.
FLOOD TESTING: Before installation of the overburden, a flood test of the system should be performed. Water should be at least two inches deep at all details. If a detail fails, it must be retested after repairs.
ELECTRONIC FIELD VECTOR MONITORING (EFVM): This is a technique using electrical charges to trace leakage paths. Although a suitable technology for existing membrane systems with leakage, they may not be as effective as flood testing for new systems. Flood testing backs up water against details, providing pressure which may cause improper detailing to fail and open. However, EFVM uses only a mist of water, and improper detailing may not fail and open until after the overburden is installed.
Green roofing offers significant benefits to the environment and the building owner.
However, to ensure longevity of the system, proper materials and detailing must be provided. Otherwise, the costs to repair or even replace a leaking Green roof system dramatically reduces the green-ness and energy cost savings of the system. FEJ
