Green Roof Resource

With the opening of the Olympic Games a few days away, air quality in the city is increasingly a concern. It is now possible to view air quality around the city thanks to a joint effort by Mapufacture, Inc. and Fortius One. The map shows values from air quality sampling stations around Beijing, China.

The air pollution index (API) indicates air quality for SO2, NO2, and PM10 (particulate matter that is 10 micrometers in diameter or less). The API cannot be directly compared to other air quality indices from other locations. The USA and Canada both use AQI, air quality index, which is similar to API but the indices are set according to different formulas based upon concentration. Hong Kong uses a different API, which can cause confusion when comparing the air quality of the two cities. While the indices cannot be compared, each index is used to evaluate when the air quality is safe or dangerous to public health according the location’s guidelines. The API for Beijing has the following ratings:

Generally, an AQI or API is set such that ratings of 100 or below are safe while ratings above 100 may be dangerous to at least some of the population. However, the formulas vary, and the specific concentration of that threshold varies according to the country or location where the index is used.

The World Health Organization (WHO) has established guidelines according to concentration, which is much more straightforward. Using concentration allows for comparison of air quality across borders. However, even the standards issued by the WHO are not standardized across pollutants (in time) as you can see below.

Source: WHO. 2006. WHO Air Quality Guidelines for particulate matter, ozone, nitrogen dioxide, and sulfur dioxide. WHO/SDE/PHE/OEH/06.02

While we can now view daily air quality in Beijing, where within the city the data is taken, and the proximity of the sampling stations to the Olympic venues, we are not able to compare it to WHO standards. Nevertheless, this is still pretty neat.

Here’s an update on NYC green roof policy incentives as discussed previously. According to an article in Greener Buildings, bill A 11226 passed the state assembly on June 23rd. The senate had previously passed the bill on June 11th. For buildings that green at least half of the roof surface, they may receive a one-time tax credit of $4.50 per square foot (up to $100,000) of green roof.

So how does $4.50 per square foot compare with the cost of a green roof? In our study on the costs and benefits of extensive green roof systems, which included costs to install the conventional components in addition to the green components, we found that 4 inch systems were on average $22 per square foot. An intensive system would certainly cost more, and there are certainly variations in this price. Assuming this price per square foot, the incentive covers about 20% of the cost. By reducing the upfront cost with an incentive, the time required for a return on investment (ROI) is reduced considerably. Considering only energy savings and a reduced stormwater fee, typically the average green roof takes 20 years before ROI can be reached. With the additional incentive, this can reduce the time required for an average roof to 12 years. Not a bad deal.

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.

After four years, Green Roof Resource is getting a new look. I am currently updating the site for better readability and organization.

While on the topic of renewing resources, this week I came across the International Stormwater BMP Database. The resource provides data from over 300 BMP (best management practice) studies evaluating performance of these systems. While the database was initiated in 1996, I stumbled across it after reading an article in the May issue of Stormwater. “Can Stormwater BMPs reduce bacteria?” reports that bacteria counts in BMP outlets vary widely, but in general retention ponds and media filters have the potential to reduce bacteria. There is concern that vegetative swales and other similar BMPs may attract wildlife and domestic animals increasing the concentration of fecal coliform in stormwater runoff from these systems. The concentration often exceeds the standard for recreational primary contact in waterways.

It was exciting to see that green roof runoff was included in the evaluation. However, with data being obtained from only one site (Hamilton Ecoroof in Portland, Oregon), analysis of the data was limited.

It would be interesting to evaluated BMPs in combination. For example, if retention ponds are effective at reducing fecal coliform concentration, can bioswales or green roofs reduce the size of a downstream retention pond to enable use of the BMP in areas where land is limited? Or is the apparent reduction simply an artifact of where these systems are typically used? For example, are retention ponds able to reduce fecal coliform concentration in the effluent simply because there is less in the influent? Perhaps in areas where fecal coliform is a greater problem, more conventional methods are appropriate. While fecal coliform concentration is a health concern and is contribute to beach and water closings access after rain events, one must not forget about the volume of storm events. BMPs are effective at reducing total volume of runoff entering waterways, and one should also consider the total load of contaminants exiting these systems in addition to the concentration of contaminants.