Strategy for Implementation and Conclusions
6. STRATEGY FOR IMPLEMENTATION AND CONCLUSIONS
6.5 D ENSITY B ONUS
Portland has been a leader in promoting the use of eco-roofs with growing medium depths between 50 mm and 150 mm, and has been actively involved in several pilot projects. An eco-roof density bonus option is provided in the Central City:
Where the total area of eco-roof is at least 10% but less than 30% of the building’s footprint, each square metre of eco-roof earns one square metre of additional floor area.
Where the total area of eco-roof is at least 30% but less than 60% of the building’s footprint, each square metre of eco-roof earns two square metres of additional floor area.
Where the total area of eco-roof is at least 60% of the building’s footprint, each square metre of eco-roof earns three square metres of additional floor area.
The City of Portland defines roof gardens as landscape areas over slab with growing medium depths of 200 mm or more. Both eco-roof and roof gardens are treated as pervious area in the City’s stormwater management calculations.
7. SUMMARY
The variety of social, environmental, and downstream infrastructure benefits that would accrue from widespread implementation of green roofs in the Metro Vancouver region create a tantalizing vision of a cleaner, greener and more sustainable dense urban area. While green roofs, in one form or another, have been installed in this area for over 35 years, they are not yet prevalent, and it may be up to the regional districts and municipalities to create an encouraging climate for increased green roof implementation. Particularly given the increasing public and governmental concerns regarding urban impacts, environmental degradation, and global climate change, the situation appears to be favourable for increasing green roof coverage. Research has shown that the benefits of green roof construction include:
Stormwater Management
Energy Efficiency
Urban Heat Island Mitigation
Increased Roof Membrane Lifespan
Air Quality Improvements
Improved Urban Aesthetics
Improved Property Values
Urban Agriculture
Increased Biodiversity and Habitat Preservation
Noise Mitigation
And yet even with all the research that has been done on green roofs, there remain challenges for implementation in the Metro Vancouver region, including:
Misconception associating green roofs with “leaky condo syndrome”
Difficulty of repairs and maintenance
Structural concerns
Availability of expertise
Limited local scientific research
Fear of the “unknown”/ lack of local market data
Cost
Lack of standards
Insurance coverage
Fire hazard and uplift
Aesthetics of flat roofs and “messy” look
For many of the above challenges, the best solution is or involves education, including education of municipal staff and building officials, designers, contractors and installers, and the general public. Though a long-term effort, education will help ameliorate many, though not all, of these concerns. In addition, various organizations are currently working on these issues such as regional and national building standards for green roofs,
insurance guidelines to assist insurers in evaluating green roofs and more scientific research projects to further define the design parameters and benefits of green roofs in this region.
According to developers, the single most critical factor in deciding whether or not to put
a green roof on a structure is cost.131 A green roof will always cost more than a standard
roof installation. In this report, it has been demonstrated that a significant portion of the cost difference can be allocated to stormwater benefits that are realized downstream of a green roof installation. This exercise shows that the estimated cost savings benefits of green roofs do not currently out-weight their estimated costs, but as the case study on the Silva building indicates, net costs can be minimal. Furthermore, by more accurately tracking the true costs of the damage to the environment and by either increasing DCCs or strengthening stormwater bylaws and allowing the economies of scale to lower construction costs, the business case for green roofs on concrete structures will likely be met.
The design of green roofs is evolving as more research in the Pacific Northwest helps us determine how best to achieve the stormwater management benefits required – especially during the high volume/saturated winter months. Specifically, more research is required on varying the composition and depth of substrate material and plant species to mitigate the seasonal variations in discharge. Current designs, from a stormwater management perspective, focus on volume reduction rather than peak flow reduction or water quality. A design needs to be capable of addressing all these components to effectively control runoff since stormwater DCCs are currently based on the size of downstream piping systems. In addition, for an individual project, energy and other benefits such as visual appeal should also be accounted for in considering the costs of a green roof.
It is clear that green roofs represent a more expensive solution to stormwater management than comparable surface-installed systems. Because of this, their application is better suited to higher density areas where there is little room for ground surface stormwater mitigation measures. Further, since the costs to support the weight of green roofs can be considerable, concrete structures represent the most cost-efficient building type for green roof installation. Based on the above considerations, the most promising opportunity for green roofs appears to be multi-storey concrete buildings in higher density areas. This situation minimizes the cost, and maximizes the benefits, of the green roof due to urban density. Through the evolution of support and incentive programs in the Metro Vancouver region, the number and visibility of installations will expand. As this happens, developers, installers, insurers, and the public will become more informed about and comfortable with green roofs, and eventually, costs are expected to decrease as the local base of green roof knowledge and experience grows.
131
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