Green Roof Resource

While my interest is in the model development side of green roof research, there were several interesting talks on plant performance beyond the characteristics of depth and slope . Kristin Getter from that other large university in Michigan presented results from sun and shade studies of sedum. She noted that while species dominance changed depending on whether the plants were shaded or received full sun exposure, the absolute cover did not change. However, there were differences in biomass production with a shaded roof clearly producing higher amounts of biomass. For climates similar to Michigan, she recommends s. acre, a. cernuum, s. kamschaticum, s. spurium in shade, and a. cernuum, T. calycinum in sun. While this information is useful to many, I am eagerly anticipating Getter’s results on a green roof’s CO2 sequestration abilities. Too bad we have to wait until next year.

Manfred Kohler spoke on studies evaluating installation using vegetated turf mats versus cuttings. His study also showed effects from sun or shade exposure although this was not the focus. Cuttings of sedum outperformed cuttings of grasses due to the slower growth rate of grasses. Grasses became established with the second year. While mats outperform cuttings in the first year, cuttings provide a greater diversity of plant species. It was observed that the grasses did better on the north facing roof while sedum did better on the south facing roof.

In a separate session, strategies were discussed for creating and maintaining successful green roofs in practice. Unfortunately, the talk was not included in the conference program, and I did not catch the speaker’s name although she’s worked on a number of green roof projects in the DC-Baltimore area. She showed a variety of roof “failures” and her investigations to determine the source of the failure. Wind affected several of her projects through scouring and increasing rates of evaporation. Excessive sun and shade also proved disastrous when plant selection and irrigation schedules were not compatible with the soil media depth and roof’s orientation. While this certainly is a fair challenge for the landscape architect or horticulturist, I also feel that there is a role for the engineer. Of course speaking from the hammer’s perspective, things can only improve upon hammering. Regardless of my bias, engineers can determine from location and building orientation the effects on the rate of evapotranspiration from wind and sun/shade exposure, which would assist in plant selection.

This year’s Greening Rooftops for Sustainable Communities conference was held April 29th through May 2nd in Baltimore. Treehugger summarized the award winning projects here. Green Roofs for Healthy Cities has provided detailed descriptions of the winners.

The sessions were held on Thursday and Friday, and I focused my attention on the policy and research tracks. On Thursday morning, Dr. Hamid Karimi from DC’s Department of Environment presented the District’s efforts to encourage green roofs and other green infrastructure. While San Francisco has received a lot of press concerning a new ordinance that would require most new commercial and residential buildings to be LEED certified, DC’s Green Building Act of 2006 is the first major US city to require LEED for private projects. By 2009 publicly financed buildings within the District must achieve LEED Silver Certification, and by 2012 privately owned buildings must also achieve LEED Silver Certification.

In addition to establishing green building standards, the District is also tackling water quality and erosion issues. For soil erosion and sediment control, sites must retain a 0.5 inch in 24 hours storm event onsite, and those sites along the Anacostia River must retain 1.0 inch in 24 hours storm event. The Department of Environment is currently revising stormwater fees to provide financial incentives for low impact development (LID) technologies. The current fee is associated with water usage while the new system should focus on impervious surface area. This fee structure will aid incentives such as the green roof grant within the municipal separate storm sewer system (MS4) permits. Expedited permit reviews for green projects are also under consideration.

A different approach to encourage green roofs is under development in New York State. Amy Norquist of Greensulate LLC spoke of a green roof tax abatement for $6.75 per square foot of green roof. The New York State Senate bill S07745 refers to a credit of 55% of expenditures up to $5000. The New York State Assembly bill A10234 provides greater detail. These have yet to be approved but are a clear example of states beginning to direct cities toward innovative LID technologies.

The federal government is not silent on this issue either. Dov Weitman spoke about EPA’s efforts to promote green infrastructure within the framework of the National Pollutant Discharge Elimination System (NPDES) program. EPA’s green infrastructure site provides a wealth of information describing various technologies (including green roofs), research activities, policies, and case studies.