September 1, 2009

FeatureCity Roofs Go Green

Green roofs have their roots in ancient history. Many people point to one of the Seven Wonders of the Ancient World as an example of one of the first green roofs. The Hanging Gardens of Babylon were built by King Nebuchadnezzar II around 600 B.C. as a gift to his wife. Though the Hanging Gardens of Babylon were built primarily for their aesthetic value, modern-day green roofs provide much more than a pleasant view.

What is a Green Roof?

A green roof refers to a layer of landscaping placed over the top of traditional roofing materials. A green roof consists of seven different layers (listed from the bottom up):

• roof deck
• waterproof membrane
• root barrier
• drainage layer
• filter cloth
• growing materials
• plants

There are two main types of green roofs: extensive and intensive. Extensive green roofs contain a low diversity of plants, require little maintenance, are low weight, and are relatively low cost. Intensive green roofs contain a high diversity of plants, require more maintenance, are higher weight due to deeper soil, and have a higher cost. Determining which type of green roof to install depends on factors including budget and the structural capacity of the roof.

Green Roof Design

Intensive green roofs are typically meant for public use and are modeled after a garden you would find on the ground. They may contain a variety of plants and functional items such as tables, chairs, and pathways. These types of green roofs are most suitable for flat roofs.

Extensive green roofs are not meant for public use. These green roofs feature soils and plants able to withstand hot and dry rooftop conditions as well as enough filtration to handle brief deluges of rain from storm events. Extensive green roofs may be installed on many roof types, though they are most suitable on roofs that have a five to twenty degree slope.

Benefits of a Green Roof

The concept of green roofing has been long-known to residents in Iceland, Scandinavia, the United Kingdom, Newfoundland, and Nova Scotia. In these areas, sod was typically used as a roof covering to help insulate homes. The first modern green roof was installed in Germany in the 1970s. Today, many green roofs are being installed atop buildings in urban areas. One of the major benefits of green roofs is their ability to reduce the “urban heat island effect.” According to the U.S. Environmental Protection Agency (EPA), the annual average daytime temperatures in cities containing one million or more people can be 1.8 to 5.4 degrees Celsius higher than surrounding areas. The urban heat island effect is even more dramatic at night, when average evening temperatures can differ by 12 degrees Celsius compared to surrounding areas.

Due to this increased air temperature, urban heat islands contribute to pollution and increased energy consumption, especially during peak hours. Urban heat islands are also associated with an increase in heat-related illnesses and mortality rates. Green roofs combat these effects in a number of ways. The EPA lists five different benefits of green roofing. These benefits include:

• reduced energy use
• reduced air pollution and greenhouse gas emissions
• improved human health and comfort
• enhanced stormwater management and water quality
• improved quality of life

Drawbacks of a Green Roof

One of the major drawbacks of a green roof is its cost. Not surprisingly, installing a green roof is more expensive than just installing a traditional roof. In fact, green roofs tend to cost 30 to 60 percent more than traditional roofs. An extensive green roof typically costs $10 per square foot. Intensive green roofs typically cost $25 per square foot. Maintenance costs, such as watering and fertilizing the plants, is an additional cost to consider. Proponents of green roofs argue that though they are more expensive than traditional roofs at the outset, the green roofs are less expensive in the long run due to reduced energy requirements from the building itself, reduced costs for stormwater management, and a longer life of the roof in general.

Green Roofs Gain in Popularity

Green roofs have been popping up on rooftops in a number of metropolitan areas throughout North America. One such city is Chicago, Illinois, which is home to more than 200 green roofs, which accounts for 2.5 million square feet of landscaped roofing. Even Chicago’s City Hall features a 20,000 square foot green roof made up of of 20,000 plants that encompass 158 different varieties. The 24.5-acre Millennium Park, built over Millennium Park Garage, is considered by many to be the world’s largest intensive green roof, regardless of its ground-level location.

Given the benefits of green roofing to urban areas, many cities are investing in this special type of urban landscaping. Several cities, including Chicago, Cincinnati, Ohio, and Portland, Oregon, offer grant programs to help fund green roof building projects. Much of this money originates from funding provided by the EPA.

More to Explore

• Green Roofs
• Green Roofs in Urban Landscapes
• Chicago Green Roofs
• Green Roofs for Health Cities

Last updated September 1, 2009 | Comments (10) | Permalink | Feature