World’s largest green roof under challenge

by GEOFF WILSON

The Ford Truck Assembly Plant at Dearborn, in Michigan, United States, has the world’s largest green roof. But it may be under challenge soon from Australia.
Ford’s green roof was recognized as the world’s largest in 2004 by the Guinness World Records. The Ford green roof covers 454,000 square feet or just on 10.4 acres ( about 41,000 square metres, or 4.1 hectares),
This massive green roof sits atop Ford’s truck assembly plant and is part of a revitalising of the historic Ford Rouge Centre complex as a model for 21st Century sustainable manufacturing.
The green roof is also a significant component of a site-wide 600-acre stormwater management system. Other design objectives include:
• Establishment of habitat at roof level.
• Reduction in ambient temperatures.
• Protection of the roof membrane.
The green roof is key to Ford’s visitor education program — highlighting environmentally beneficial site and building strategies.
A Xeroflor system was chosen, primarily for being extremely lightweight and easy to install. Lightness was a factor due to structural spans of 50 feet (15,25 metres). Ease of installation was a necessity due to the roof size (requiring a crane and large staging areas) and coordination with remaining construction.
Availability of plant material became a significant issue due to the vast size of the roof. Ford placed orders a year in advance of the intended installation, so that sedum cuttings (‘Fulda Glow’ and “Diffusum’) could be cultivated during a full growing season. Vegetation was then pre-planted on blankets staged in Dearborn at the Allen Park Clay Mine in Spring 2002; the blankets were then installed the next autumn.
The growing medium depth is approximately one inch deep and consists of 7-9 mm of porous stone, sand, and organic material with a total saturated weight of 10 pounds / square foot. This calculation includes a mineral wool fleece material that absorbs rainwater. Roots penetrate this one inch water retention layer.
Researchers at Michigan State University tested a variety of plants under different soil depths. They investigated drought and freeze resistance, density of growth, weed control, fertilization, and irrigation requirements. A mix of nine sedum varieties was specifically created to thrive in the upper Midwest climate and should require little or no maintenance or irrigation.
The drainage layer is manufactured by Colbond and is a three quarters inch (2 cm) thick nylon mesh with a geotextile fabric bonded to one side. The 100% recyclable nylon filaments are installed face down, thereby creating an airspace through which drainage occurs.
The rigidity of the mesh prevents its collapse and allows water to flow unimpeded. Another layer of this material, placed in an inverted position, serves as the vegetation carrier. When used as the medium/vegetation carrier, the spaces between the filaments contain planting medium. Seed and cuttings are then applied to the surface. For the Ford Truck Plant installation, the vegetation was pre-cultivated on the ground for over 12 weeks, after which the carrier and vegetation were cut into 3.28 feet x 6.56 feet pieces, palletized and transported to the roof by crane.
An irrigation system was installed with the intent that it be used only while the vegetation acclimates and becomes established. The system is installed above the green roof surface. Liquid fertilization will be applied once during the initial year via the sprinkler system, using a product called Rosasoil that is 100% organic.
The waterproofing was provided by Siplast and consists of two layers, a modified bitumen product called Paradiene 20 TG and a Terranap cap sheet, installed on top of the areas that received the vegetation. The cap sheet is made of a non-woven polyester mat, impregnated and coated with SBS-modified bitumen. It has a root-inhibiting agent. XeroFlor also supplied a 20-mm high-density polyethylene sheet to provide an additional root barrier above the membrane.
Ford’s mechanical engineers are energy modeling to determine the required cooling load. It may then be possible to compare the performance of this building to others of similar use. Current predictions anticipate a seven percent decrease in energy use due to the green roof.
Current predictions are that the membrane life will be doubled from 25 to 50 years. It is anticipated that the green roof will also retain 447,000 gallons per year, amounting to 50% of the annual rainfall in Wayne County over the green roof area, or approximately one gallon/square foot/year.
Ford identified this function as a saving (in the tens of millions of dollars) due to the avoidance of a water treatment facility anticipated under new EPA regulations. Excess rainwater travels through a series of swales and wetland ponds where it undergoes natural treatment before returning to the Rouge River.
Over its 90-year history, the Ford Rouge complex was denuded of vegetation. By 1999, the building site only consisted of rail lines and parking lots. A principal goal of the revitalization project was to attract wildlife (primarily birds and insects) back to the site. Dr. H.J. Liesecke of the FLL in Germany concluded that the Ford roof would provide 25% of the productive habitat of an undisturbed green site; a 25% improvement over existing conditions. The assembly plant’s roof is also expected to improve air quality above the roof by 40%, in terms of dust absorption and the decomposition of hydrocarbons.
The proximity of the green roof to the Visitor Center’s observation tower was a primary design consideration, and the plant’s air houses, substations, and other rooftop elements were organized to present an orderly arrangement that framed the planted area. Ford has also established a bee apiary on the Visitor Center site adjacent to the Truck Assembly Plant. Honey produced by the bees is being collected and bottled. Honeybees have been positively identified gathering nectar from the sedum blossoms.
The Ford project demonstrates that even large industrial plants, denuded industrial sites and the surrounding area can benefit significantly from well-designed green roof infrastructure.