by Jason King
Last year, we prepared some graphics for the BPA 905 Building Green Roof project, to submit it for a couple of awards. While unfortunately, we didn’t get an award, the graphics are definitely worth a second look – as they tell the story of the project and some of its unique features. Here’s part one of a two part post showing the graphics and final product.
Overview The rooftop terraces in this project are part of a larger, multiple phase effort for overall building energy efficiency upgrades as part of the American Recovery and Reinvestment Act (ARRA). The owner of the facility, the General Services Administration, along with their tenant, the Bonneville Power Administration (BPA) upgraded mechanical and building systems and replaced roofing for the entire building, allowing for the retrofit of four rooftop terraces that step down along the north portion of the building.
The project consists of four separate roofs stepping up from Level 4 to Level 7 along the north façade of the building. The total area is 9,000 square feet, with 5,800 of vegetated area.
This unique retrofit project started with new roofing that replaced the existing membrane and provided a significant increase in insulation depth for energy performance that would hit the target of R-50. Atop the new insulation, areas of non-vegetated roofing are installed with cool roofing membranes to minimize heat gain. Vegetated portions were specifically designed to meet the following client-specified goals:
- Increase the insulation value of the roof
- Capture rainwater to maintain the sites natural hydrologic cycle
- Provide habitat for beneficial insects and animals
- Reduce urban heat island effect
- Provide visual beauty for employees that look down on the roof areas
- Meet applicable federal standards for green roof construction
An enlarged view of the Level 4 rooftop, showing the configuration of pedestrian areas, seating, and access from the interior of the building. The sculptural berms and ‘river’ connects the four roof levels and reflects the different loading characteristics of the roof.
The existing terraces were predominately paved, with large planters that previously held ornamental varieties that were irrigated with domestic water supply. The spaces were described as ‘desolate’ and were used sporadically by building users, but not considered an amenity. Through the design process, the team worked with building tenants to identify the aesthetic preferences, necessary amount of seating, and overall range of planned uses for the spaces. This involvement was also expanded to include educational information, located at building entries and key areas like the cafeteria, regarding a range of vegetated roofing options, trickling down through all of the users of the building. The distinctions between extensive green roof, semi-intensive roofing, and intensive roof terrace were vital to understanding the type of project that was being planned, and managing the overall level of expectations, which led to significant support from throughout the building.
Exploded diagram showing the built up layers from roofing, insulation, paving, and green roof components on a typical level. The cisterns are fed by roof drains and pumped back up to the green roof for irrigation that requires zero potable water.
Results The project includes over 9,000 square feet of roof area over 4 different levels. Paved terraces surround sculptural berms planted with semi-intensive vegetated roof, encompassing a total of 5,800 square feet of planted area. The rooftops are oriented for views from within the building, as well as to be viewed from above. Starting at the upper level (7th Floor) a stone ‘stream’ winds through the adjacent sculptural ‘hills’, referencing the connection of the BPA to the indigenous waterways of the Pacific Northwest. The similar size of the upper roofs (5,6,7) are less uniform due to the berming and flowing nature of these elements, giving each floor a unique experience. This unifying element connects each level down to the larger (4th Floor) where it terminates into a fanned delta. The overlapping berms and waves of vegetation provide a separation of the space to avoid the fishbowl effect and allow for adjacent access to the plantings.
The dynamic quality of seasonal planting variation is captured in flowing waves of color and texture that overlap and change over the entire year.
Over thirty species of plants were included to achieve a mosaic, including small coniferous and deciduous trees, a range of shrubs, grasses, and perennials, rounded out with a mix of succulent groundcover cuttings to quickly provide soil coverage. Plantings were selected for drought tolerance, color, texture, scent, and seasonal variation. Rainwater is captured in 35,000 gallon cisterns and pumped up to each floor, giving the project, once established, net-zero potable water usage. High efficiency irrigation heads with evapotranspiration-enabled smart controllers and moisture sensors provide additional water savings by adapting to local conditions to provide only necessary supplemental water through seasonal summer drought.
A rendering of the proposed project shows the potential usable space along with the design intent. The graphics were essential for education, as well as to manage expectations of the end result that was not an extensive roof, but not a full roof garden.
Go to Part II: Implementation
