When Charlie Miller, a stormwater engineer built his first green roof in 1998, extensive green roofs were practically unknown in North America. Miller built a simple, low-maintenance 3000 square foot green roof on the roof of the Fencing Academy of Philadelphia, that stands up to this day. Today, green roofs are not considered a novelty they used to be in 1998, but represent a growing trend in green urban design that can effectively be combined with solar panels and wind turbines.
Green Roof Systems
What is a green roof? Green roof is a concept of using plants to improve a roof’s performance and esthetical appearance. They may also be known as living roofs, eco-roofs and planted roofs. Green roofs fall into one of two wide categories: extensive and intensive.
We’ll look more closely at the extensive green roofs in this article as they are easy to build yet bring notable economic and ecological benefits.
Extensive green roof is characterized by simple and thin profile. Several elements constitute extensive green roof: growing medium (soil), mixed succulent plants in a simple assembly, drainage and a root barrier. The depth of the soil normally does not exceed 6 inches. The growing medium is usually planted with drought-resistant, robust and low-growing plants. Plants that develop deep root systems are incompatible with the shallow depth of extensive green roof.
Green Roof Benefits
Extensive green roofs provide residents with multiple benefits including stormwater management. Speaking in economic and ecological terms, extensive green roofs produce the greatest return on investment as they are easier and non-expensive to design and construct. Their main advantage is significant energy savings plus environmental benefits that include improvement in the quality of urban life. Importantly, they positively impact the public health of urban areas, as they reduce urban heat island effect.
In the New York city there are more than 21,000 acres of flat roof areas that could be easily retrofitted to accommodate extensive green roof systems. The cooler temperatures of green roofs in hot weather makes them helpful for people who live and work in the building below. While simple extensive green roofs are not always designed to be easily accessible to visitors, a sedum roof on a building can be designed to incorporate a stone path and a patio area.
Simply put, intensive green roofs are conventional roof gardens. They have deep, more organic growing soil that can support a wide variety of plants together with shrubs and small trees. These roofs are accessible for everyday use by people who live or work in the building.
When intensive and extensive green roofs are compared, extensive roofs win on the account that they produce the greatest return on investment in both economic and ecological terms—being easier and less expensive to design and build.
Green Roof Design
The design of a green roof may depend on several factors, such as purpose and function. Inevitably, the design will depend on the structural requirements as well as the capacity of the rooftop. While the structure for new buildings can be designed to support the envisioned green roof, in case of the existing buildings the structural engineer will have to evaluate whether green roof design will depend on the limitations of the structure. That’s done by conducting a structural analysis of the roof, what is also the most demanding phase of a design: integration of a vision with structural ability.
Here are some vital issues that need to be resolved before the building of the green roof :
- The dry weight and the saturated weight of some the roof components must be evaluated,
- Loading of the structure must be calculated based on the weight of the green roof system at the point of saturation.
- “Dead load” must be calculated. The assembly, with plants and the water are all elements of the dead load.
- “Life load” must be calculated as well. It represents the excess water that flows through the saturated green roof system.
On the basis of the above and other important findings the engineer may either approve the green roof design for the structure or prescribe necessary enhancements to the structure. These enhancements may be costly at times, causing the initial design to be abandoned. The engineer then may propose the modification of the initial design of the green roof in order to reduce its weight.
What are the US Incentives For Green Roofs?
In the United States, policies of this kind have been complex to implement on the federal level.
One reason is that the green roof industry, encompassing a range of local experts from roofers to designers, does not possess the organization, homogeneity and methodical lobbying effort akin to that of corporate players on behalf of solar industry. Hence, there hasn’t been a consequential drive for incentives at the national level like those available for wind and solar. The good news is that there are plenty of local incentives throughout North America.
The U.S. federal government is providing low-key but potentially yielding initiatives on green roofs. The stronger stormwater management and other regulations, promulgated under the Clean Water Act might eventually make green roofs more widespread. Also, many agencies promote that new buildings conform to LEED standards that gives points for green roofs. Furthermore, the EPA has supported the use of low-impact development tools including green roofs. More green roofs are also likely to come thanks to the 2007 Energy Independence and Security Act.
Green roofs deserve attention of urban homeowners and renters as a more humane and eco-friendly alternative to the dominance of asphalt, black tar and gravel. Here are some useful resources to find further information and inspiration on green roofs:
- The Green Roof Manual
- Planting Green Roofs and Living Walls
- Roof Gardens: History, Design, Construction
- Green Roof Construction and Maintenance