Green roof technology is not new.  Archaeological evidence suggests that sod roofs were being built in Scotland as early as 3000 B.C.  The Hanging Gardens of Babylon (700-800 B.C.) also had a version of a green roof.  During the 19th and 20th century a variety of European and North American building projects dabbled with green roofs, but the modern green roof movement found it roots in German-speaking European countries in the 1960s to early 1980s.  Today, Portland is recognized as a leader in the green roof movement with nearly 80 green roofs scattered throughout the city.

A green roof is simply a green space created on top of a building.  At a minimum, green roofs will consist of a vegetation layer and a substrate layer.  Most contemporary green roof systems will also include an impermeable liner to protect the building below and a drainage layer. Green roofs can be classified into two categories: intensive and extensive.  Intensive roofs consist of a relatively deep substrate (usually greater than 6 inches) and are analogous to rooftop gardens where people can visit and recreate.  They are relatively heavy and require significant structural integrity for support. Extensive roofs are characterized by shallower substrate (usually less than 6 inches), lower growing, drought tolerant plants with fibrous root systems, and are less likely to be directly accessible to people.

The benefits of green roofs are many.  Some benefits can be considered private (beneficial to the roof owner) other are more public (beneficial to the community).  Green roofs have proven to:

• extend the life of waterproof liners by eliminating damage due to UV radiation and severe temperature fluctuation.
• reduce temperature fluctuation within buildings by providing an insulating layer therefore red
• reduce the urban heat island effect
• reduce noise pollution
• reduce and/or slow down storm water runoff
• increase biodiversity in the urban environment
• reduce urban air pollution
• provide wildlife habitat
• be more pleasing than the typical gray or black roofing materials and may therefore increase property values
• provide an opportunity to grow local food in urban settings

Here in the Department of Horticulture, David Sandrock and Erin Shroll are conducting two studies related to green roofs in the Pacific Northwest.

Project 1
The first project is located at OSU’s Oak Creek Center for Urban Horticulture in Corvallis.  The objectives of the project are to:

• trial potential and recommended green roof plants for the Pacific Northwest
• determine the role of irrigation in survival of green roof plants
• determine the effectiveness of green roofs to mitigate peak-flow of storm water
• compare surface temperature on the planted roofs, the un-planted control roofs, and the traditional liner roofs

Twenty-four 4' by 8' green roof prototypes have been constructed in accordance with commercial green roof standards.  Each prototype has a uniform, lengthwise 2% slope. Twenty-one roofs were outfitted with a standard drainage mat. To simulate conditions of a conventional commercial roof, the remaining three roofs (experimental controls) were lined with the waterproof membrane only. An appropriate green roof substrate was selected and uniformly applied to prototypes at a five-inch depth. Throughout the experiment, nine prototypes are not planted and contain substrate only.

All prototypes are being irrigated uniformly for establishment during the first year. In the second year, three different irrigation treatments will be implemented: (1) irrigated according to the Hunter Smart System, (2) irrigated monthly, and (3) not irrigated.

Each roof has a collection unit for runoff from irrigation and rainfall events. Runoff volumes are measured after each irrigation and periodically after significant rain events to determine peak flow reduction of the three roof conditions (planted, fallow, and liner only).

On 25 June, 2007, 12 roofs were planted with eight species in a randomized complete block design.  Each block contains a single-plant replicate of each plant taxon for a total of 48 single-plant replicates of each taxon. Plants will be observed and evaluated over two years. Plant growth is measured monthly and plant aesthetics is rated on a scale of 1 to 5 (five being the best) by a panel of five participants. Bloom duration, drought tolerance, plant decline, and plant mortality are also recorded.  At termination of the experiment, the plants will be harvested, dried, and weighed to determine final biomass.

Throughout the two-year research project, ambient air and surface temperatures will be taken on the green roofs, controls, and liner-only roofs. 

Project 2
The second project is located on the rooftop of the 15-story Portland Building in downtown Portland, Oregon.  The objectives of this project are to:

• Create moisture, temperature, and light maps of the Portland Building's ecoroof]
• Correlate plant performance of Sedum spathulifolium with the recorded environmental conditions

Thirty, 1-square-meter planting groups containing Sedum spathulifolium have been located and mapped on the Portland Building ecoroof. Data loggers were placed, central to each identified planting, on the media surface (to record air temperature and light) and at a 4-inch depth (to record substrate temperature). In order to construct a more complete map of the entire ecoroof surface, pairs of additional data loggers are placed in any areas not already represented by a selected planting.

Light and temperature data from the data loggers are downloaded every 30 days. At each download event, plant width and coverage within the related cluster are measured using a point intercept system on a square meter grid, and digital photos are taken. Temperature and light maps of the entire roof will be created from the downloaded data, and plant growth of the Sedum spathulifolium will be correlated with environmental conditions.