Filter Media at the Roots of a Natural Skyline
Many landmark buildings around the world are now beautified by green roofing structures, which are heavily reliant on nonwoven-based filtration, separation, growth nurturing, and drainage systems.
Beyond simple aesthetics, the installation of grass, plants, and even trees on a building’s roof works to absorb rainwater and release it slowly back into the atmosphere through evaporation or, if there’s too much build-up, allow the water to trickle slowly into drainage systems to ensure they don’t become overwhelmed.
There are other benefits, too, including insulation and cooling properties for buildings and the promotion of biodiversity in built-up areas.
This has seen green roofs become important elements in many countries. In addition to being highly visible, they also very effectively provide a distinctive image to buildings and development projects.
One leader in the installation of these systems is Optigrün, based in Krauchenwies-Göggingen in southern Germany. Together with around 140 partner companies in its network, Optigrün is involved in over 10,000 green roof and rainwater management projects annually, equating to over five million square meters of substrate materials laid and subsequently cultivated.
St. Pauli’s Bunker
A flagship Optigrün project is the roof of the converted World War 2 bunker that has dominated the landscape of the St. Pauli district of Hamburg in Germany for over eighty years. Built on the orders of Adolf Hitler to provide shelter for up to 25,000 people in response to the bombing of the city, it has now been converted into a five-story leisure center that comprises restaurants, event spaces, and a hotel, with 7,600 square meters of roof-top gardens across its different levels. A 560-metre walkway encircles the exterior of the building, and the freely accessible roof gardens consist of almost 23,000 plants maintained by a highly sophisticated rainwater management system.
The plants provide relief for the entire building, acting as a natural air conditioning system by insulating it in winter and protecting against heat in summer, which saves money on energy costs. In addition, with assistance from appropriate backing materials, the plants filter fine dust from the air and significantly reduce sound reflection due to their mass.
Protecting the roof surface from extreme temperature fluctuations and mechanical damage can also almost double the roof’s service life, according to Optigrün.
Heat Islands
In a recent interview with Deutsche Bank, which has backed the company for many years, Optigrün CEO Uwe Harzmann explained that sealed surfaces in Hamburg’s city center often turn into heat islands, especially in the summer months.
“The city heats up and causes air temperatures to rise sharply, and the hot air then rises rapidly, dragging moist air from the surrounding area with it,” he said. “In higher air layers, this moist air cools down quickly, leading to sudden and heavy downpours. These water masses overload the sewer system and frequently result in flooding.”
In response to this problem, the St. Pauli Bunker is equipped with retention roofs, which act as sponges, absorbing a large part of the precipitation and preventing rainwater from running directly into the sewers.
Containers under the nonwoven substrates collect the water, filtering only as much as necessary. If imminent rain were to overload the roof, the water would be drained into underground storage tanks or the sewer system. Meanwhile, sensors collect data and monitor factors such as heat storage and evaporation processes to supply a personalized weather forecast for the building.
Enka Drain
In May 2020, Freudenberg, a leading nonwovens manufacturer headquartered in Weinheim, Germany, acquired Low and Bonar, a London, UK-based company. This merger brought together two of the leading companies with extensive experience in green roofing and a wide portfolio of substrates.
For foundational layers, for example, Freudenberg’s EnkaDrain consists of a durable synthetic drainage core of fused, entangled filaments and a nonwoven fabric thermally bonded to one side. For maximum strength and continuous flow even under high loads, the entangled filaments are molded into a square waffle pattern or a U-groove configuration. The filter fabric is a spunlaid nonwoven with excellent filtering performance through its uniform open structure. It is dimensionally and thermally stable and exhibits high tensile strength per unit weight. It is also thin and flexible yet possesses excellent tear and puncture resistance. For seamless installation, the nonwoven fabric overlaps the drainage core along one edge of the roll.
EnkaRetain&Drain
As an all-in-one drainage and water retention layer meanwhile, EnkaRetain&Drain combines EnkaDrain with a highly absorbent lightweight water-retention fabric. This fabric is made of recycled synthetic fibers and holds more than 7.5 times its weight in water. The geocomposite comes in a convenient roll format and is water-vapor permeable.
EnkaDrain does not need aggregate, reducing the total roof weight. The matting is installed with the nonwoven filter layer uppermost.
The fabric of EnkaRetain&Drain absorbs water for continuous hydration of plant roots. It is in direct contact with the growing medium and, therefore, readily accessible by the plants. At the same time, excess water passes through the drainage core below.
New York
Through its Xeroflor subsidiary, Low and Bonar was involved in many green roofing projects even before becoming part of Freudenberg.
These include two New York landmarks – the Jacob K. Javits Convention Center in Manhattan and, most iconic of all, the Empire State Building.
Xeroflor systems are employed to stunning effect on the 6.75-acre green roof of the Javits Center, one of the largest in the USA and a wildlife sanctuary for dozens of local and migratory bird species, several bat species, and thousands of insects.
Research published by Drexel University and Cooper Union has demonstrated that over a year, the roof retains more than 75% of the rain that hits it, which mitigates nearly seven million gallons of stormwater runoff annually.
First installed in 2014, the center was expanded in 2021, resulting in the creation of a one-acre roof-top working farm and a 10,000-square-foot orchard of apple and pear trees. The expansion was built to hold at least one million pounds of soil in an 18-inch-deep bed.
Lightweight Xeroflor green roof systems were also installed on the 21st, 25th, and 30th floors of the Empire State Building as part of a $550 million ‘green’ retrofit, reducing its energy consumption by some 38% annually.
Low Maintenance
Xeroflor’s lightweight, nonwoven green roof systems range from weights of only eight to 12 pounds per square foot to applications of 31 pounds per square foot. Extensive green roofs generally use low-maintenance vegetation such as sedums or short grasses and, are less than six inches deep and are typically two to four inches in depth. Their light weight makes them ideal for retrofit projects on older buildings or any structure with limits on the total weight load. Generally, extensive green roofs require minimal maintenance and can thrive in most temperate climate zones.
A typical five-layer Xeroflor construction consists of a pre-vegetated sedum mat on top of the growing medium, water retention layers, drainage mats, and root barriers.
This layered system is placed above the conventional roofing membranes.
Bosco Verticale
Freudenberg also supplied glass-fiber reinforced polyester nonwovens made from recycled PET bottles for the landmark twin towers of the Bosco Verticale in Milan, Italy, with its 900 trees and over 2,000 plants.
These materials are employed in green roof structures not only as drainage and storage layers of filter media and carriers for bituminous membranes, but, in many cases, as the nutrient substrate.
For projects such as the Bosco Verticale, materials supplied by Freudenberg were a natural extension of the company’s long experience in the fields of both roofing membranes and geotextiles, for which Freudenberg now recycles around seven million PET bottles a day at its European facilities in France and Italy.
Combining the flexibility of polyester with the stability of glass creates excellent runnability, particularly at higher temperatures and for use in bitumen production lines. In addition, the bitumen membrane creates excellent long-term dimensional stability and endurance. The use of glass reinforcement also eliminates the phenomenon of thermal memory – once installed on a roof, the membrane will no longer shrink when temperatures fluctuate.
Bitumen roof membranes last more than 20 years on average; then, the damaged roofs are completely replaced, or the damaged points repaired. This is an important advantage of this technology compared to other sealing systems, particularly in maintenance work. Manufacturers can also recycle waste from bitumen roof membranes by grinding them into a powder, which can then be used as a raw material, further improving the product’s sustainability.
M-Trays
Of course, not all buildings are quite as grand as those mentioned so far, but greening the roofs of standard houses can still be beneficial and economical through products such as the M-Tray, developed by Wallbarn, based in South Croydon, UK.
Such modular green roof systems consist simply of an initial nonwoven separation and filtration layer on top of which are placed trays pre-planted with a diverse selection of fully established flowering sedum.
Engineered to seamlessly click together and create a continuous, lush green surface, M-Trays are designed with carefully spaced drainage holes to optimize water retention, allowing plants to absorb more rainfall effectively. This enhances the growth and health of the vegetation and contributes to the attenuation of stormwater, reducing runoff.
Wallbarn says that the initial investment in the M-Tray system can yield long-term savings through reduced energy costs and extended roof lifespan. One of its most significant advantages is its ability to provide immediate green coverage upon installation.
The pre-grown modules are delivered with a minimum of 90% vegetative cover, ensuring that a lush, verdant surface is created as soon as the modules click together. This instant aesthetic appeal is in stark contrast to traditional green roof systems, which may take months or even years to establish a fully covered and visually appealing green space.
