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.

In natural, undisturbed systems, approximately thirty percent of the rainwater reaches shallow aquifers that feed plants, another thirty percent percolates and nourishes deeper aquifers, and approximately forty percent returns to the atmosphere through evaporation and plant transpiration [1]. In urbanized areas, seventy-five percent or more becomes stormwater runoff. It is well known that green roofs are one type of porous surface that can correct this rainwater distribution reducing the demand on municiple sewer systems.

Depending on the climate of a region, green roofs can retain as much as fifty to seventy percent of water that falls onto a roof. Retention varies according to climate. Hutchinson et al (2003) reported that a 10 to 12 cm vegetated roof in Portland, Oregon retained 69% of the total rainfall with peak flow reductions of 80% during a 15-month monitoring period [2]. Investigations in East Lansing, Michigan compared a gravel roof and a vegetated roof with the mean percent rainfall retention ranging from 48.7% for a gravel roof to 82.8% for a vegetated roof [3]. A similar range was seen on two roofs of different slopes in Goldsboro, North Carolina. Total rainfall retention was 55% for the sloped roof and 63% for the flat roof [4]. Peak flow was reduced by 57% for the sloped roof and 87% for the flat roof [4].

Other studies have investigated the dependence of stormwater retention on slope. VanWoert et al (2005) evaluated the effect of roof slope and substrate depth on stormwater retention. Two slopes were tested, 2 percent and 6.5 percent at 3 green roof substrate depths (2.5, 4.0, and 6.0 cm) [3]. The greatest retention occurred on a 2% slope roof with a media depth of 4 cm [3]. Further studies on the effect of slope were conducted for the 4 cm roof by Villarreal and Bengtsson (2005). The slopes evaluated were 2, 4, 8, 14 degree under dry conditions and wet conditions (field capacity) located in Lund, Sweden [5]. They observed no effect on runoff from slope variation, but did observe an effect on runoff by the moisture content [5]. Under dry conditions the green roof retained 6 to 12mm of rainfall prior to runoff initiation while under wet conditions the green roof initiated runoff toward the beginning of a rainfall event [5].

Evidence suggests that both slope and moisture content affect retention and peak flow reduction. However, due to the variability of climates, further modeling of the hydrology of the green roof system is needed to better understand and anticipate performance.

References:
[1] Scholz-Barth, Katrin. 2001.
[2] Hutchinson et al. 2003.
[3] VanWoert et al, 2005.
[4] Moran et al. 2005.
[5] Villarreal and Bengtsson. 2005.

The energy literature on green roofs developed in the 1990s. The majority of research has focused on the insulative abilities of green roofs in summer. In summer months, green roofs behave as high quality insulation reducing the flux of solar radiation in a building [1,2]. Insulation layers may retard heat flux in situations that are undesirable. While insulation reduces cooling load for hours when outside temperature is higher than inside temperature, the insulation retards heat loss for those hours the internal temperature exceeds external temperatures [3]. For a dark roof, the negative impact on AC usage during hours where external temperatures were below internal temperatures was greater than for a white roof suggesting that both roof insulation and roof reflectivity should be assessed to minimize energy use in buildings.

A recent study on the surface heat budget on a green roof and high reflectivity roofs revealed that the sensible heat flux is small compared to concrete roof surface on both a highly reflective white paint surface and a green roof [4]. The heat flux is small on the white roof due to the low net radiation. In contrast, the green roof had a large net radiation. The small sensible heat flux for the green roof was attributed to the large latent heat flux by evaporation [4].

The two main parameters that influence the solar radiation that reaches the roof deck are leaf foliage and soil thickness. The leaf area index (LAI) and leaf angle affect shadows [1]. The larger the foliage development of a particular plant, the smaller the heat flux through the roof [1,2,5]. Roof surface temperatures also decrease according to increasing LAI [6].

Soil thickness also plays an important role in heat transfer. Thick soil layers reduced cooling needs during summer months while thin substrate layers resulted in little to no cooling benefit [5]. Roofs with substrates between 7.5 cm and 10 cm reduced the average daily heat flow throughout the year although greater in summer months [7]. Generally, heat transfer is greater on roof surfaces that are not vegetated [2,6] although the vegetated roof should not serve as a replacement for insulation [8].

Sources:
[1] Del Barrio, E. 1998. Energy and Buildings, 27:179-193.
[2] Niachou, A; Papakonstantinou, K; Santamouris, M; Tsangrassoulis, A; Mihalakakou, G. 2001. Energy and Buildings, 33:719-729.
[3] Akbari, H. 2003. Energy, 28:9:953-967.
[4] Takebayashi, H and M Moriyama. Building and Environment, Volume 42, Number 8, p.2971-2979.
[5] Theodosiou, T G. 2003. Energy and Buildings, 35:909-917.
[6] Wong, N; Chen, Y; Ong, C; and A Sia. 2003. Building and Environment, 38:261-270.
[7] Liu, K; Minor, J. Proceedings for 3rd Annual Greening Rooftops for Sustainable Communities. Washington, DC.
[8] Eumorfopoulou, E and D Aravantinos. 1998. Energy and Buildings, 27:1:29-36.

As green roofs become more established within the US building community, the need for standardization of good building practices increases. Green Roofs for Healthy Cities is establishing an accreditation program to provide proper training to professionals. The National Roofing Contractor’s Association (NRCA) has recently published a green roof guide. The American Society for Testing and Materials (ASTM) has published four Green Roof Performance Standards and one standard guide. Two additional standards are under consideration. What follows is a brief overview of the ASTM standards. Links to the summaries of the standards as provided by ASTM are below.

ASTM E2396-05 Saturated Water Permeability of Granular Drainage Media
This standard details the procedure for assessing the permeability of granular materials used in the drainage layer. The permeability under low-head, horizontal flow conditions that exist on green roofs is also addressed. This method for determining water permeability assists in determination of the dead load in the ASTM E2397-05.

ASTM E2397-05 Determination of Dead Loads and Live Loads Associated with Green Roof Systems
The dead load and live load determination standard provides a procedure for predicting the system weight of a green roof system. The weight assessment accounts for components that are typically encountered in green roof systems. The weight is determined under two conditions. Dead load is the weight of the system under drained conditions and the weight of retained water or other precipitation. The second scenario assesses the weight when precipitation is actively occurring and the drainage layer is saturated. The difference in weight between the first (dead load) and second conditions is considered a live load.

ASTM E2398-05 Water Capture and Media Retention Standards of Geocomposite Drain Layers for Green Roof Systems
To determine the saturated weight, the standard method for water capture and media retention of the drainage layer would be used. This standard is applicable to geocomposite drains layers that retain water and media in cup-like receptacles on their upper surface (e.g. shaped plastic membranes).

ASTM E2399-05 Maximum Media Density for Dead Load Analysis of Green Roof Systems
The standard method for maximum media density assists determining the dead load by providing a measure of the moisture content and the water permeability measured at the maximum media density.

ASTM E2400-06 Standard Guide for Selection, Installation, and Maintenance of Plants for Green Roofs
The plant selection, installation, and maintenance guideline is applicable to both extensive and intensive green roof systems.

Two additional standards are currently under development. ASTM WK575 Practice for Assessment of Green Roofs will include technical requirements and sustainable development considerations. Assessment of some technical requirements may refer to existing green roof standards. Fire safety will likely need additional standards developed for proper assessment. Assessment for sustainability considerations may include energy efficiency, water management, and biodiversity.

The other standard under development is the ASTM WK7319 Standard Guide for Use of Expanded Shale, Clay, or Slate (ESCS) as a Mineral Component in Growing Media for Green Roof. This standard describes the characteristics of the material to be a mineral amendment and covers the sampling appropriate for the procedure.