Designing and choosing a sustainable roofing system is not a new idea to architects. Whoever said that once your designs hit the roof, you must forgo innovative concepts? Garden roofing systems give architects another avenue of design while providing building owners the best of all worlds - beauty, innovation and energy efficiency.
Inspired by the ancient Hanging Gardens of Babylon, garden roofing systems have been around for centuries. Common sod roofs of the 18th century originated in Scandinavia, and in the 1960s Germany developed garden roofs that resemble the systems of today, which gained mass popularity in countries such as Switzerland, the Netherlands, Italy, Sweden and England. This trend took off in the United States nearly two decades ago and has been gaining momentum in the green movement because of the increased interest in environmentally friendly products.
Types of Garden RoofsThere are three varieties of garden roofs - intensive, semi-intensive and extensive systems. A few differences between the three systems are the amounts of soil and growth medium used and plant types.
Intensive systems can require 10 inches or more of soil and growth medium. These systems are usually directly applied over the entire roof deck and consist of many layers: root barriers, water retention liners, complex irrigation systems, soil, fabric and vegetation. These systems can accommodate full gardens with trees, shrubs and other voluminous plants, therefore requiring irrigation and the ability to hold a large reservoir. Since there are usually larger plants on intensive roofs, these systems require more watering and monitoring so there are no stray seedlings growing where they shouldn’t be.
Semi-intensive systems require between 6 to 10 inches of soil and growth medium. This type usually contains a reservoir layer larger than extensive systems, but less comprehensive than the complicated irrigation and reservoirs of intensive systems. The vegetation this system can accommodate includes grasses, bulbs, perennials, small shrubs and smaller varieties of evergreens.
An extensive system requires less than 6 inches of soil and growth medium and is the least dense option of the three systems. The types of plants typically used for extensive systems are hardy plants, small shrubs and mosses that are drought resistant and can stand up to high winds and other harsh weather. Some extensive garden roofs are designed to only need once-a-year weeding and an application of time-release fertilizers. Some extensive systems on the market are cultivated in modular garden trays with preassembled layers of soil, simple irrigation, and plants, which allows you to allocate the vegetated trays into any layout you desire.
When choosing a garden roof, the building’s weight-bearing capacity is the primary consideration. As such, lightweight EPDM and TPO membranes fully adhered to a high-density cover board can serve as a durable, reliable foundation to protect the roof from high foot traffic often associated with garden systems. A single-ply membrane will also create an all-important watertight barrier for the rooftop under the garden system.
Intensive systems are heavier, but with more soil you are afforded more options when it comes to plant variety. Semi-intensive systems are often seen as the compromise of the two ends of the garden roofing spectrum by supplying more planting options than extensive systems, while not being as heavy and complicated as intensive systems.
Extensive systems are low weight bearing and are available in tray systems. Transporting lightweight extensive modular garden trays is simpler than transporting numerous materials to compose a traditional built-in garden system. If you specify extensive modular garden tray systems, it is best to seek out trays that are pre-vegetated, grown off-site and monitored until maturity. Development of built-in sprinkler systems providing consistent watering to grow vegetation on-site until maturity are in the works, but pre-vegetated, grown off-site modules are the best current available option.
No matter the system, it is best to choose mature vegetation whenever possible because mature vegetation that covers about 95 percent of soil will require less maintenance in the long run. Remember whatever system you install, choose plants that will acclimate to the environment and help maintain the desired soil coverage on your rooftop garden.
As with all roof systems, rooftop access by maintenance personnel remains a concern. Using extensive modular garden trays with painted pavers that blend with the landscaping is an easy and functional way to disguise footpaths and give the illusion of a seamless vegetated roof. Also, if any roofing substrate issues arise, modular garden trays can be easily removed to allow maintenance workers to gain access immediately.
Green BenefitsGarden roofing is not a fleeting trend, but rather a growing market in the sustainable roofing industry. Some important green benefits of garden roof systems are energy savings, reducing noise filtration, filtering storm water runoff and bettering the surrounding environment.
Installation of vegetated roofing for your project, especially in an urban setting, can increase energy savings. Garden roofs act as insulation to help lower building temperatures in the warmer months by providing natural shade and increase heat retention in the colder months. Decreasing building temperatures can lead to reduced cooling costs, with energy savings depending on building size, climate and type of green roof you choose. According to Environment Canada, a department funded by the Canadian federal government that monitors conservation of their natural and renewable resources and coordinates environmental policies and programs, found that a garden roof with about 10 cm (3.9 inches) of growth would reduce cooling needs by 25 percent. In addition, garden roofs can increase a building’s thermal efficiency. Research by the School of Construction and the Environment at British Columbia Institute of Technology conducted field experiments which established that a 6-inch extensive green roof can prevent heat losses by 26 percent compared to a traditional roofing membrane.
Garden roofs not only provide benefits to the building owner and occupants, but to the surrounding environment as well. Many state and local governments offer building certification credits incentives through LEED Green Building Rating System to reduce Urban Heat Island Effect (UHIE) and to encourage green roof projects. According to the Environmental Protection Agency, UHIE commonly occurs in major metropolitan areas because they replace natural land cover with pavement, buildings, and other infrastructures, which can cause temperatures to increase as much as 10 degrees Fahrenheit above the natural land surroundings. An example of this is the city of Los Angeles, where stone and cement buildings, concrete parking lots and asphalt roads are the biggest cause of heat temperature rise. Vegetative roofing can also reinstate the natural cooling effects of shading and evaporation in an urban environment.
Another way garden roofs benefit the surrounding community is by re-oxygenating the air. According to the city of Chicago’s “Green Roof Fact Sheet,” one square meter (10.76 square feet) of vegetated roof can remove about 0.2 kilogram of particles from the air every year and improve air quality.
Whether the structure is a hospital, an office building, school or library, a vegetated roof offers a visual of live greenery while minimizing noise filtration from the surroundings. This is especially beneficial in a congested urban setting. The Los Angeles Environmental Affairs Department’s “Green Roofs - Cooling Los Angeles: A Resource Guide” states green roofs with a 12 cm (4.7 inches) growing medium have been shown to reduce noise filtration by 40 decibels - the denser the substrate layer, the more it filters sound.
A major challenge when choosing roofing materials for any location is how to handle storm water runoff. For example, buildings in urban settings tend to collect standing water due to the impermeable surfaces of city streets and other facades. Harmful standing water can become a breeding ground for insects and disease, while carrying chemicals, debris and other pollutants into the sewer systems to be discharged into the local water supplies, such as rivers, lakes, streams and other bodies of water.
Garden roofs can help purify and reduce storm water runoff because the plant roots’ bacteria and fungi trigger an ecological process that employs natural filtering methods while absorbing excess rainwater. Intensive systems tend to have higher water retention and filtration because of large amounts of soil. Extensive systems retain less water because they have less soil and do not require intricate irrigation systems.
Cities such as Chicago, Washington, Seattle and Portland utilize garden roofing as storm water management systems. The city of Chicago “Green Roof Fact Sheet” states that garden roofs with 3 inches of growth can typically reduce annual storm water runoff by more than 50 percent.
Cost AdvantagesGarden systems do initially cost more per square foot than other traditional roofing systems. However, if maintained properly, the vegetation can help extend the roof life expectancy up to twice as long as traditional roofs without garden systems.
Vegetated garden roofs can reduce thermal and ultraviolet (UV) degradation, which protects the roofing membrane and lengthens the life of the roofing system. What may cost you more up front will be a big cost savings in the long term.
Where cost or warranty considerations are a concern, check with the roofing membrane manufacturer on options for installing a suitable substrate now that will allow the owner the flexibility to upgrade to a garden roof system as funds become available and without any warranty disruption.
Some have steered clear of garden roofing options because past systems had a reputation of showing unattractive grid-like patterns, but they have now been redeemed by the next generation of live modular garden tray systems. Some extensive modular garden trays permit plants to grow taller so that the trays themselves are virtually hidden. Plus, some companies “green-novate” every feature of their garden roofing products by making the modular trays out of recyclable materials as well. An extensive tray system allows for seamless installation and presents a unique landscape upon the roof.
A vegetated system can be installed alongside pavers to create a cohesive garden patio that can be an amenity to your building. Rooftop gardens provide building owners with additional usable square footage; this is especially important when space is limited in cities. Multi-tiered buildings with vegetative roofing can offer many occupants and viewers aesthetic beauty in an urban environment enclosed in concrete.
Garden roofing is now at the forefront of cool roofing advancements. Aesthetic beauty, long-term capital savings and environmental benefits make garden roofing systems a great choice for nearly any building.