Frequently asked green roof questions

Roofscapes Inc.

FAQs....

Concise Answers to Frequently Asked Questions

Please note: The information above is provided solely for general educational purposes. It is not intended to serve as a design guide. Roofscapes, Inc., takes responsibility only for green roofs designed, installed, and maintained through the Roofscapes, Inc. Network.

What are green roofs?

Green roofs, also called �vegetated roof covers,� �living roofs,� and �eco-roofs� are thin layers of living plants that are installed on top of conventional roofs. Properly designed, they are stable, living ecosystems that replicate many of the processes found in nature.

What are the major advantages of green roofs?

Green roofs provide many ecological and aesthetic benefits, including:

� Controlling stormwater runoff, erosion and pollution,
� Improving water quality,
� Mitigating urban heat-island effects, cooling and cleaning the air,
� More than doubling the service life of the roof,
� Conserving energy,
� Reducing sound reflection and transmission,
� Creating wildlife habitat, and
� Improving the aesthetic environment in both work and home settings.

For a German in-depth case study in which green roofs are an integral part of urban stormwater control, see Study of Extensive Green Roofs in Berlin, by Manfred K�hler and Marco Schmidt.

Are there proven, scientific standards for green roof design?

Presently, the only widely-accepted, established standards for green roof construction are those developed in Germany by the Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau. e.V. (FLL) These standards and guidelines are comprehensive, and include industry standard tests for the weight, moisture, nutrient content, and grain-size distribution of growing media. FLL also certifies laboratories to conduct critical tests, such as the root penetration resistance of waterproofing membranes. These guidelines are available in English translation directly from the FLL or through Roofscapes, Inc.

Within the past few years, the American Standard Testing Methods (ASTM) convened a Green Roof Task Force to create green roof standards. Charlie Miller, P.E., President and founder of Roofscapes, Inc., has been a very active member of this Task Force from the beginning and has been instrumental in drafting these standards.

What are the most important factors in designing a green roof?

There are many complex, interactive factors that green roof design engineer takes into account, balancing many considerations for optimal performance, including:

� Climate, especially temperature and rainfall patterns
� Strength of the supporting structure
� Size, slope, height, and directional orientation of the roof
� Type of underlying waterproofing
� Drainage elements, such as drains, scuppers, and drainage conduits
� Accessibility and intended use
� Visibility, fit with architecture, and owner's aesthetic preferences
� Fit with other 'green' systems, such as solar panels
� Costs of materials and labor

What is the difference between an 'extensive' and an 'intensive' green roof?

There is no strict demarcation. Generally speaking, 'extensive' green roofs are less than 6 inches deep, and, depending on depth, may support a range of plants, such as Sedums (low-growing succulents), herbs, meadow grasses, and perennials. Deeper 'intensive' systems can sustain complex landscapes, including small trees and even small ornamental ponds and fountains.

How much does a green roof weigh?

Green roofs vary greatly in weight, depending on their depth and the material components. The important measurement is 'wet' weight -- fully-saturated fabrics and plants. For example, Roofmeadow� systems, engineered to be both lightweight and efficient, generally weigh about 6.75 pounds per square foot for each inch of depth. Thus, the saturated weight of a 2-inch deep system is about 13 pounds per square foot, including a mature plant cover. Mid-range, 5-inch, systems weigh approximately 34 pounds per square foot and are compatible with wood or steel decks. 'Intensive' systems weigh 36 pounds or more, and generally require a concrete supporting deck. The ASTM (Standards E2397 and E2399) has published specific protocols for determining the weight and dead load of green roofs.

Be cautious of systems that are marketed to be lighter in weight; do your homework. Very lightweight media often contains high amounts of organic content (i.e., pine bark, coir) resulting in media that is inconsistent with the industry standard. Specifically, organic content in media supporting a thin extensive sedum green roof should not exceed 4% by mass, primarily for the purpose of maintaining adequate structural and bedding strength and of minimizing long-term compression. Compression should not exceed 10% of the nominal depth.

How does a green roof affect the conventional roof below it?

Based on German experience, a green roof can be expected to double or triple the life of the underlying conventional roof by protecting from human activity and wind-blown debris; shielding from UV radiation; and buffering temperature extremes, thereby minimizing damage from daily expansion and contraction.

What kind of waterproofing do I need?

Many types of waterproofing are compatible with green roofs. World-wide, polyvinyl chloride (PVC) and polymer modified bituminous membranes are the most common. Many of these installations have now been in place for over 30 years and continue to perform as designed. PVC, EPDM and thermal polyolefin (TPO) are, in most cases, inherently root-resistant; other common waterproofing materials require a root barrier between the waterproofing materials and the vegetated cover.

What about modular systems?

In modular systems, the green roof system is installed inside trays. While modules or trays have valid applications in intensive green roof designs, we do not advocate the use of modules or trays in extensive green roofs. Modular systems require the same attention to basic green roof design principles as all other green roof systems, including: quality and durability of the waterproofing system, performance of the media, minimization of irrigation requirements, appropriate plant selection, etc.

Additionally, clients should be sure to inquire about the long-term potential for compression or bio-degradation of the media supplied with green roof modules. A substantial decrease in the volume of the green roof media over time will increase the likelihood that the module edges will become exposed, impacting the overall aesthetic of the green roof.

From our perspective, modules do not offer any advantages with regard to accessibility of the roofing membrane for inspection and repair. Current electric leak detection methods (e.g., EFVM) have made the location and repair of leaks on extensive green roofs almost trivial. Potential problems can be identified within an area of less than one square foot without any disruption to the green roof. With conventional green roofs (planted in-place), the membrane can be exposed by simply rolling back about a square foot of the thin layer of the vegetated cover system at the effective area. In about 20 minutes the membrane can be repaired and the system restored with no heavy lifting involved. With modules, however, interlocking trays weighing 55 to over 200 pounds (depending on the type of modular system) must be detached and removed in order to repair a leak. Moreover, to our knowledge, it has yet to be shown whether green roof modules are compatible with the EFVM technology.

One of most important benefits of green roofs is protection for the underlying waterproofing membrane. As a result, greened roofs can be expected to last much longer than exposed roofs. The �conventional' approach to green roof design focuses on covering 100% of the waterproofing materials. In particular, conventional green roof detailing focuses on covering and protecting the most vulnerable parts of the roofing system: its flashings, penetrations, terminations and drains. As one example, we advocate the use of enclosed drain access chambers to protect drains and scuppers and to eliminate the potential for clogging by blowing trash or vegetation. Some modular green roof layouts leave an exposed margin next to parapets, penetrations and drains. This approach diminishes the protective function of these green roofs. Therefore, we strongly recommend that modular designs, like conventional green roofs, provide 100% protection by incorporating vegetated or stone margins, drain access chambers, etc.

Modular systems can be useful when designing small, intensive gardens on residential property or on terraced commercial roofs. Owners who wish to engage in active roof gardening will also find modules a convenient and safe way to indulge in their hobby. Roofscapes, Inc. offers rugged modular green roof systems to our clients under such special circumstances.

Are leaks under a conventional green roof a problem?

Surprisingly, leaks in the waterproofing layer are less likely when it is protected from the elements by a green roof. If a leak does occur, it can readily be located through new electronic technology, such as electric field vector mapping (EFVM), which can rapidly and accurately pinpoint even minute holes. This system is more reliable than the older flood testing method; need not be installed in advance, and can even be used on steeply-sloped surfaces. Repairs to the waterproofing are quick, and disturbance of the green roof is minimal.

Is attaching all elements of the green roof to the underlying structure important?

Based on 35 years of German experience, usually not. Plant roots bind the layers of media and fabrics to create a unified cover, and the plants themselves create enough surface wind turbulence to foil potential uplift. (It's the converse of what happens on an airplane wing.) If your green roof will be located in an unusually high wind area, such as a high rise building or coastal area, discuss your concerns with your green roof design engineer, who will specify appropriate ballasts. Roofscapes, Inc. has guaranteed the green roof on Boston harbor's World Trade Center for up to 94 mph winds using concealed ballasts.

How steeply pitched can the roof be?

The maximum slope recommended for conventional green roofs is 30 degrees, or a 7:12 pitch. A variety of methods utilizing meshes, slope stabilization panels, cribbing, or battens can be used to secure the green roof and prevent media shifting and erosion. Roofscapes, Inc. has used all of these methods on various sloped roof projects.

Roofs with pitches greater than 7:12 can be greened; however, special techniques are required. Roofscapes, Inc. designed a 54-degree pitch green roof for a demonstration at the Ariel Rios courtyard at the EPA Headquarters in Washington, DC, using advanced technologies.

In what climates do green roofs work?

Green roofs have been built most widely in temperate climates, but special techniques allow them to thrive in semi-arid, tropical, and even windy coastal areas. Roofscapes, Inc. has designed and built green roofs across the continent, and from hot, moist Florida to the cold, windy Boston harbor.

Is irrigation required? If so, what kind?

With thoughtful engineering, irrigation is rarely necessary. When irrigation is required, the water should be delivered deep under the surface�where the roots will seek it and it will not be lost to evaporation. Surface irrigation systems (drip or spray systems) are wasteful and require more maintenance than subsurface irrigation methods.

Why is drainage important?

Proper drainage ensures that the growing medium will be maintained in an aerated condition suited to health plant growth. Basal drainage must also be designed with large rainfall events in mind. The goal is for all rainfall to percolate to the base of the system. The portion that is not absorbed should move 'underground' toward roof drains or scuppers. During very large storms, brief episodes of surface runoff may occur. In a Roofmeadow� system, such surface runoff can enter the roof drains at gravel surfaced areas that surround the drain access chambers. Surface ponding, even during large storms, is evidence of a poorly-designed green roof.

How does green roof media differ from 'soil'?

Conventional green roof media is mostly light-weight mineral material, with a minimum of organic material. It should be designed to meet established FLL guidelines for both water retention and drainage, as green roof systems with poor water retention will require irrigation to compensate.

One the most important criteria for media is that it remain a viable growing and water-control substrate for decades. We have found that one of the most effective ways to reduce long-term maintenance-related costs is to use media that will not degrade or compress over time. Media should be accompanied by third-party laboratory data confirming its essential properties and the provider's warranty should expressly offer to replace defective or degraded media.

What kinds of plants should be used on a green roof?

In frost zones 4 through 8, at least half of the plants installed on an extensive green roof should be varieties of Sedum. In colder climates grass-dominated covers are recommended. Reliable tropical plant lists are not currently available. However, in cooperation with the Florida Department of Environmental Protection, Roofscapes, Inc. has installed three green roof prototypes to evaluate candidate plants for un-irrigated extensive systems.

Many other plants can be incorporated to provide habitat value or visual interest. Appropriate auxiliary plants will vary by region, but generally, the deeper the green roof, the more complex the plant community that can be sustained. They range from Sedum and herbs on the thinnest systems, to meadow grasses and perennials on mid-weight roofs, to turf and small trees on the deep, 'intensive' roofs.

Hundreds of different species have been used by Roofscapes, Inc. in American projects. The experience gained from these projects allows us to confidently recommend plant communities for green roofs. Their selections will depend on many factors: depth of growing media, seasonal temperature ranges, average rainfall, and the aesthetic preferences of the owner.

For optimal plant growth and health, the plants should be selected, installed, and maintained by experienced horticulturists or landscape contractors who understand the local environment and climate, in consultation with the design engineer.

Are any green roof plants invasive non-natives that could be spread by wind?

The only Sedum that might be considered invasive is Sedum sarmentosum, also known as Star Sedum,Gold Moss, Stringy Stonecrop, or Graveyard Moss. It has green spearhead-shaped leaves, lemon yellow flowers, and long tendrils. We do not recommend using this species.

How are green roof plants started?

There are several methods for starting green roof plantings. From April through October, Sedums may be established from fresh cuttings. Broadcast on the growing media, these cuttings will quickly form roots and affix themselves. Alternatively, Sedums may be started in May-June or September-October from 72-cell plugs, planted a foot apart. Perennials can be seeded, except during the summer months. A bio-or photo-degradable wind barrier fabric or hydromulch is required while the plants are being established.

How long until the roof will be fully covered?

Generally, a green roof that is planted correctly will reach maturity in two growing season, or less. Occasionally, reduced sun or rain may slow the process. During maturation, it is especially important that the roof be properly maintained with weeding and fertilization. If more rapid maturity is desired, pre-grown Sedum mats, can be employed.

Is there a danger of dry plants catching and spreading fire?

Not if the green roof is designed correctly. In Germany, green roofs actually have a better fire rating than conventional roofs because the mineral media layers cannot burn. The extensive use of Sedums (a type of succulent) and gravel borders and breaks also protect against fire. In an arid, fire-prone region, require your designer to engineer the system with fire-retardant features, paying special attention to mineral content, gravel layers, and plant selection.

What effect will the winter freeze and thaw cycles have on the roof?

Not all green roofs are resistant to freeze and thaw cycles. Roofscapes, Inc. uses mineral-based media that stand up well against multiple freeze/thaw cycles and engineers infrastructure elements to accommodate thermal contraction and expansion. The freeze and thaw cycles have no effect on Roofscapes, Inc.'s green roof systems.

Is there erosion in the winter when the vegetation dies back?

The design of a green roof system should include a significant percentage of non-deciduous plants, so that the mature cover is never be bare of foliage and the potential for erosion is minimized. The risk for erosion is greatest just after the initial planting. During this early phase, the media should be anchored with either an erosion control mat or hydromulch.

Can I plant a flower or vegetable garden on my roof?

Generally, green roof systems are not deep enough for growing vegetables and the tools of traditional gardening would damage the green roof and the underlying waterproofing. However, incorporating container gardens or modular elements can accommodate an owner's horticultural or agricultural desires.

Will I be able to walk on my green roof? Can it be used for recreation?

If the roof is accessible from the building and the roof will support the weight, decks, patios, and walkway pavers can be integrated into the design of the green roof. Among the available plant covers, only turf grass will stand up to regular foot traffic. Turf green roofs will require a minimum of 6 - 9 inches of media depth and irrigation is mandatory (i.e., an intensive green roof). As an alternative, stepping stones or walkways can be used to provide access through more fragile extensive green roofs.

What kind of maintenance is required?

Generally, after the plants have become established, most extensive green roofs only need weeding and occasional infill transplanting twice a year. Maintenance visits should be timed to intercept any weeds before they go to seed. This level of maintenance is sufficient to maintain the health of the plants and protect the underlying roofing materials. Some owners enjoy greater involvement and choose to 'garden' in their green roof more frequently. Intensive green roofs will require the same care and maintenance as a similar garden situated on the ground.

Can green roofs be combined with other green features?

Certainly. For example, green roofs can be designed in conjunction with solar panels. In fact, Roofscapes, Inc., in partnership with Cahill Associates, has developed the Green Solar Canopy� for use over parking lots and garages, solving several urban problems at once. Green roofs also work very well in combination with other 'low-impact' development measures, such as infiltration beds, rain gardens, bio-retention systems, cisterns and rain barrels. In Germany, large developments often have zero runoff discharge. In these developments, rainfall is captured on the green roofs, returned to ground water through infiltration, and re-used for irrigation, toilets, etc. (See Study of Extensive 'Green Roofs' in Berlin, by Manfred K�hler and Marco Schmidt.)

Can recycled materials be used in constructing green roofs?

Reducing the use of virgin materials is an important goal if the green roof industry is to be sustainable in the long run. When using recycled materials on green roofs, there is a three-way interaction among quantity, cost, and performance. Currently components containing recycled materials are often more expensive than those made from virgin materials. In some instances, accommodating materials with recycled content would compromise performance.

The problem is more acute for media than for synthetic components. Durable recycled media materials (e.g., calcined ash, crushed brick) tend to be relatively heavy, more difficult to obtain, and more expensive than virgin mineral media. Synthetic components (fabrics, membranes, etc.) with recycled content are more readily available and do not compromise function. Unfortunately, because many of these products are made in Europe, shipping costs and exchange rates make recycled synthetic components relatively expensive.

Usually �reducing� better preserves our natural resources than �recycling,� because of the energy required to transport, reclaim, and transform waste materials. Green roof systems that minimize the use of synthetic infrastructure altogether reduce the consumption of both virgin and recycled synthetics � and the energy expenditures of their processing as well. For example, the Roofmeadow� Type III: Savannah system, utilizes only thin fabrics and natural growth and drainage media. The savings in materials and processing compares favorably with that of other systems using recycled, but higher mass, components, such as plastic trays or water retention �egg cartons.�

How much does a green roof cost?

Cost per square foot depends on many factors: the size and slope of the roof, depth and complexity of the system, height and accessibility from the ground, cost of labor, and need for specialized elements, such as drains, railings, pavers, slope stabilization measures, etc. To determine if a green roof will fit your budget, we suggest that you submit a Green Roof Worksheet in order to determine which system will meet your needs and how much it may cost.

What financial benefits do green roofs provide?

Thirty-five years of experience with green roofs in Germany have demonstrated that a roof assembly that is covered with a green roof can be expected to outlast a comparable roof without a green roof by a factor of at least two, and often three. Although modern green roof systems have not yet been in place longer than 35 years, many researchers expect that these installations will last 50 years and longer before they require significant repair or replacement. For a building owner with a long-term investment in the roofing system, this benefit factor goes a long-way toward paying back the initial investment in a green roof.

Will a green roof save energy?

Yes, although not due to insulation or "r-factors." A green roof acts as a thermal mass or heat sink, slowly absorbing and holding energy from sunlight and releasing it when the ambient air cools. In this way, it acts as a heat �storage battery� and reduces the heating and cooling demands within the building. Energy savings will be greatest in low buildings, due to the high ratio of roof area to the total of exposed building skin. The benefits will also be greater in warm climates, where cooling is the principal energy cost. Roofscapes, Inc. can facilitate a simplified building envelope analysis that can help set some reliable benchmarks for energy-related savings associated with a green roof design.

Are there tax incentives or grants available for green roofs?

At present, there are relatively few incentives offered for green roofs in the United States. However, Portland, OR, stands out as municipality that promotes green roofs through its Floor Area Ratio Bonus for new downtown buildings with green roofs, and as well as its Community Watershed Stewardship Grant Program. If present trends continue, similar programs can be expected in other major metropolitan areas that are plagued by urban flooding, water pollution, or 'urban heat island' impacts. For a summary of current tax incentives and government and private foundation grants supporting green roofs, consult the chart prepared by the City of Chicago Department of Planning and Development.

Are there other incentives for using green roofs?

Yes, a green roof can be an important element in LEED (Leadership in Energy & Environmental Design) green building certification, which signifies that the owner is an ecologically-responsible community partner and adds to the prestige of the building.

IS THERE ANYTHING I SHOULD AVOID IN BUILDING A GREEN ROOF?

Yes! To build and maintain a successful green roof, there are several design and installation problems to avoid:

Avoid any design that is not properly drained.

Inadequate drainage can result in:
    (1) Dead weight that the roof may not be able to sustain;
    (2) Plant mortality; and
    (3) Degeneration of the planting media
In particular, we counsel against the use of gel packs.

Avoid using herbicides to prevent root penetration of the waterproofing.

Herbicides will eventually leach out of materials, escaping into the environment and diminishing the root-inhibiting function of the root-barrier. When supplemental root-barriers are required, we recommend using thermoplastic membrane. Copper films are recent development in the green roof industry. Until a proven track record is available, we warn against using copper films as root-barriers.

Avoid using surface irrigation, if at all possible.

Surface drip and spray systems are expensive to maintain, do not deliver water efficiently to the roots, where it is most needed � and waste enormous amounts of water through evaporation. Also, on surface-irrigated green roofs plant roots tend to develop nearer the surface. This makes the covers increasingly dependent on irrigation and vulnerable to lapses in the watering schedule.

Avoid providers who know only one tune.

No one system is suited to all roofs�there are too many variations in pitch, drainage, climate, and exposure, to name but a few engineering considerations. 'One-size-fits-all' serves the aims of the manufacturer, not the needs of the consumer.

Avoid un-integrated design and installation.

A trained, experienced green roof design engineer makes many educated decisions about materials and how they are to be used on your project. Many factors are held in delicate balance. An installer who has not been part of the design process may�for reasons of cost, convenience, or unfamiliarity with the process�make changes that undermine the integrity of the system. This is especially common with landscape contractors who have had no or little green roof experience. Also, there are often aspects of a project that only become apparent on-site and that if your designer does not supervise the installation, s/he cannot make appropriate adjustments.

Finally, avoid false economies.

Your green roof is a long-range investment. The highest return will come from thoughtful, responsive design engineering and stringent standards of installation and maintenance, giving you a trouble-free green roof for years to come.