Green Roof History
Mineral wool usage is an important part of green roof history. From the late 1970s to the mid 1980s, the modern green roof was developed in Germany, differing from historic vegetation-on-structure efforts and rooftop gardens, in that extensive green roofs perform a wide variety of functions efficiently and reliably in a very thin profile.
One inventor who was instrumental in the development of the first extensive green roofs is Wolfgang Behrens; after developing innovative green roof technologies for the German government in the 1970’s, Behrens founded Xero Flor, now a division of Bonar. From the 1980s until recent years, Behrens obtained numerous patents for green roof systems that take advantage of the properties of mineral wool. Mineral wool is a lightweight material with a very high volumetric water retention capacity and high compressive strength, characteristics which Behrens capitalized upon. As the German green roof industry developed through the 1980s, a few other companies explored using mineral wool in their green roof assemblies, but some of Behrens’ early patents were so broad as to virtually preclude any competitors from using mineral wool in green roofs.
A 20 year old rock wool living roof installed on a bus station in Oldenburg, Germany
Sedum based green railroad tracks installed in Germany
Introduction to Mineral Wool
Mineral wool can be described as “cotton candy rock” as the material is formed of molten rock – commonly basalt or slag – spun into thin fibers resembling cotton candy. The spun fibers are typically mixed with a binding agent, compressed to a given density, and cured in a furnace. Due to a very high air void ratio, mineral wool has excellent insulative properties, and thus its most common usage is insulation, and soundproofing. Mineral wool is naturally hydrophilic, but its fibers may be coated with oil to render the material hydroscopic (water-repelling) for use as insulation or sound attenuation. Mineral wool possesses excellent fire-proofing properties as either a hydroscopic or hydrophilic/hydroponic material. A video explaining more about mineral wool manufacturing can be found here
Slag wool, a form of mineral wool produced from slag, was first discovered in 1840 in Wales. After several production refinements, mineral wool was first produced commercially in Germany in 1871, and became a very common and high-performing insulation material, and was also used in horticulture.
Diversification and Standardization of the Green Roof Industry
Possessing excellent properties that make it an important component in several green roof systems, mineral wool has an excellent 3-decade track record of success in green roofs throughout Europe, and more recently in North America and Asia.
As the German green roof industry grew, Xero Flor and other companies – notably Zinco and Optigreen – developed different types of green roof systems. This type of industry diversification is often beneficial to the consumer, as a wider range of choices is available to suit different needs. Extensive green roof profiles that were developed and are used today generally fall within a few broad classifications:
- Lightweight aggregate over a composite drainage course (called “single course” by the FLL),
- Lightweight aggregate over a drainage aggregate course (called “multiple course” by the FLL or commonly “dual media” in the US), and
- Mineral wool which might or might not be used in combination with a lower composite drainage layer and upper media layer (classified by the FLL as “single course”).
For purposes of this website and the Mineral Wool white paper, the first two types listed are referred to as “aggregate-based” green roofs.
In 1975, the German FLL Guidelines were developed. This milestone of green roof history reflects industry diversification; these guidelines are regularly updated to address quality control and best practices for a wide range of assembly and material types. The FLL classifies mineral wool as a “substrate board” (page 69 of 2002 English version).
Introduction of the Green Roof to North America
Extensive green roofs first arrived in the US around 1995-1997, another milestone in green roof history. However, adoption was slow, and prior to the late-2000s, extensive green roofs were fairly rare in most North American cities, but are becoming increasingly more common throughout the mid-Atlantic, Toronto, Vancouver, Chicago, and in many other urban areas as critical stormwater management tools.
A Furbish EcoCline living roof covered with a variety of low-growing Sedums
Young Sedum album (white stonecrop) flowering on an EcoCline R+2 living roof
Early pioneers of the North American green roof such as Charlie Miller of Roofmeadow gravitated toward an extensive green roof profile of lightweight aggregates over either an aggregate drainage course or a composite drainage sheet. Those profiles are also preferred by Optigreen and Zinco, German companies who competed against Xero Flor, and did not utilize mineral wool. Both Zinco and Optigreen began doing business in North America in the late 1990’s. Xero Flor did not join the North American market until the mid-2000s. So the North American green roof industry began without any parties utilizing mineral wool in green roof systems. Xero Flor is currently using mineral wool in Canada, China, and Europe, and some locations within the US.
Green Roof Market Growth and Diversification within the US
As the US green roof industry has grown, its path has been different from its European predecessors. Unlike in Europe, the US green roof industry is dominated by roofing and waterproofing manufacturers, most of whom are using variations of systems espoused by Roofmeadow, Optigreen, or Zinco – i.e. systems that achieve water retention primarily via aggregate media.
However, US industry diversification is beginning to occur, primarily in response to recent stormwater regulations. Whereas most extensive green roofs easily hold a 1-inch rainfall, now green roofs are being asked to retain 2- and even 3-inch rainfalls, which is not easily accomplished using aggregate-based green roof systems common in the US. In 2012, Baltimore-based Furbish introduced EcoCline, which utilizes mineral wool. Around the same time, Vegetal ID, a French green roof company, responded to the North American stormwater market with their high efficiency Stock-and-Flow green roof system which utilizes a plastic storage reservoir at the base of a modular green roof assembly. Both EcoCline and Stock-and-Flow adapt and leverage established European technologies to better respond to North American needs, and other innovations are sure to follow, continuing to change the course of green roof history. The North American green roof market is now beginning to mature and diversify, benefitting owners and specifiers with greater options; the use of mineral wool is simply one example of that diversification.
Current Usage of Mineral Wool in Green Roofs
Mineral wool is used in European green roof assemblies by Xero Flor, Knauf, Nophadrain, De Boer, and likely others. In the North American market mineral wool is currently used in Furbish’s EcoCline and also available in green roof assemblies by Xero Flor and Urbanscape.
