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In my enthusiasm for the potential benefits of greenroofs, I would certainly be remiss if the potential problems of greenroofs were not also explored.  A discussion of some of the possible physical, environmental, and aesthetic issues follow:

PHYSICAL ISSUES:

Waterproofing Integrity

Every reputable roofing company will guarantee and provide a warranty for the waterproofing integrity of their membrane(s), including greenroofing providers.  Water leakage from drainage backups or possible root puncture could lead to interior damage if the correct waterproofing membrane system, root barrier, and drainage layer are not selected.  Of course, when choosing a greenroof system and/or contractor, it is advisable to check references on completed projects for waterproofing success.  Vulnerable areas where leakage is possible include abutting vertical walls, roof vent pipes, outlets, air conditioning units, perimeter areas, etc.  A thorough water flood test needs to be conducted for leaks after installation of the waterproofing membrane to ensure quality control, certainly before the other layers are applied.

Pesticide Leakage from Roof Materials

Recently, an environmental science magazine began testing drainage from greenroofs in an effort to measure any pesticide runoff.  The potential does exist for certain elements, such as iron and aluminum, to seep out and infiltrate our ground water.  Care must be exercised in selecting thick membranes to ensure no release of pollutants, and the materials used in foundations and pathways on a greenroof should not leach carbonates.  This would also be a good opportunity to choose greenroofing companies who use environmentally friendly roofing components. 

Additional Support Considerations

For extensive and intensive greenroofs with projected live loads of higher than 17 pounds per square foot,  consultation with a structural engineer is a requirement.  Additional growing media depths, large plants such as trees, walkways, seating areas, parking areas, etc. will command greater structural support, and a greater layer build-up of the greenroof system.  For example, deeper planting beds can be constructed over internal columns and walls to provide a higher loading capacity.  Considerable costs can be associated with these high interactive projects, especially with renovation or retrofitting projects.

Unwelcome Wildlife Problems

I have had people ask be about the possibility of attracting rats, raccoons, squirrels, spiders and the like with greenroofs.  Because of their watertight quality, I would suspect it would be extremely difficult (or impossible) for these larger animals to enter a home through the roof.  But because a natural habitat is created, perhaps some undesirable critters would be invited, and then their proximity might put off some folks.  I actually haven't read or heard anything regarding this issue; please share any thoughts or experiences.

ENVIRONMENTAL ISSUES:

Certain aspects of greenroof technology could be challenged in terms of their positive environmental characteristics and sustainability factors.  While examining these issues, perhaps the ultimate answer lies in the varying degrees to which a concept is considered environmentally sound and sustainable.  In other words, there may not be black and white, or cut and dry responses to these important concerns.

Native vs. Cultivated Plants

For example, is it environmentally correct to use plants other than natives on greenroofs?  Purists would probably say that only natives should be used in most  landscaping projects, including greenroofs.  A true representation of the land would, in fact, aim to replicate its original natural habitat, and therefore native plants would be considered essential to the design.

However, several factors come into play here.  First of all, these are very planned landscapes, and we are trying to replicate the physical conditions of plants living in the ground, but many feet above.  Soil substrates definitely aren't native soils (too heavy, easily compacted, etc.) - so certainly this isn't a natural design to begin with.  We are already limited somewhat with the greenroof plant palette due to their exposure to harsher climatic conditions. (For the most part, the reference here is to extensive greenroofs, as plant selection is much greater for intensive greenroofs because of their higher soil depths, etc.)  Secondly, we are importing an established European technology with proven plant species, and many of these are readily cultivated and available here.  Also, if we self-limit our plant selection, the greenroof environment will be lower in bio-diversity.

Greenroofs will only function if the vegetation is successful, so initially it may be necessary to use proven adaptive species, and then introduce and promote our own native plants as they are proven successful.  Unless a specific design program calls for an exotic look, I believe that ultimately, it will be in our own best environmental interest to establish natives as the primary plant material for greenroofs.

Irrigation Requirements

Upon reviewing my first draft, UGA Professor Darrell Morrison asked me to consider a couple of possible criticisms, the first being "In a hot, dry climate, is it environmentally sound to have a green roof if it requires irrigation to establish/maintain it as a green roof?"

First of all, a hot and dry climate would dictate a distinct list of plants that are extremely drought and heat tolerant, especially succulents.  Many species that grow in full sun have developed mechanisms to reduce leaf evaporation, e.g., wax coated and spiked leaves.

Of course, any landscape requiring regular irrigation would be high maintenance and expensive in terms of supplemental water usage and cost.  If you design a golf course on a greenroof, then it would not be very environmentally sound because of these factors, in addition to the negative aspects of the monoculture of turfgrass.  However, as seen from the Optima and ZinCo examples in the Greenroof Components section, techniques have been developed to automatically capture and release stored rainwater within the greenroof system.  Although these details would increase the initial capital investment cost of the greenroof, they would quickly pay for themselves in terms of irrigation savings as well as providing an argument for sustainability.

If you compare all the natural processes that are mimicked by a greenroof environment to its potential irrigation requirements, I believe the greenroof would still be considered environmentally sound, although not necessarily self-sustainable.

AESTHETIC VALUES:

Professor Morrison also asked, "Are the aesthetics of green roofs always positive? Can the good points about green roofs be sufficient to 'lose' the aesthetic quality?"  The examples offered throughout were selected to reflect the wide variety of greenroof styles chosen by the owners and designers.  Some are more natural, some very designed, but certainly all are organic in that there are plants on the roof! If not planned correctly, an unsightly, overgrown appearance could result.  Of course, if you are trying to recreate a meadow habitat designed to attract wildlife, this may actually be your goal.

Also, the organic nature of plants needs to be acknowledged in that seasonal fluctuations and periods of severe drought and heat will be reflected in the look of the plants themselves. Therefore, colors, heights, and plant density will most likely change with the seasons.

Some people may feel organic architecture is inappropriate or "unnatural" for any building, and that is their own aesthetic value.  If no or low maintenance is desired, then the design must dictate the correct choice and placement of plant material.  A hands-off program may result in a wild and overgrown look.  Beauty is in the eye of the beholder, but surely the many arguments for the ecological and economic benefits of greenroofs can outweigh aesthetically weak designs.  Below are some examples of less structured organic design which some may find either objectionable or lovely because of its wildness:

ReNatur Extensive Greenroof Optigrun Garage Greenroof
Photo Source:  ReNatur, www.re-natur.de

Photo Source: Optigrun, www.optigruen.de

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