From Sod To Sedum: The Elements Of Green Roof Construction

by Owner on June 22, 2011

Wouldn’t it be wonderful if a bird’s eye view of our cities and suburbs was of greenery rather than asphalt, tiles and concrete? Green roofs are getting us there by converting sterile rooftops into organic habitats. Not to be confused with rooftop container gardening, green roof construction is a system of layers that incorporates the plants and growing medium into the design of the roof.

Benefits of green roofs

There are numerous benefits to green roof systems. First, green roofs absorb rainwater and therefore reduce storm water runoff. As the water is filtering through, the plants also extract heavy metals and some toxins from the water and therefore act as bio-filters.

Green roof systems are excellent insulators. They reduce heating in the winter and cooling in the summer, thus reducing the amount of energy that goes towards heating and cooling systems.

The living plants used in green roof construction create much needed habitat in otherwise sterile urban landscapes. Green roofs also work to filter air pollutants and produce oxygen.

They help to lower urban air temperatures through evapotranspiration, which reduces the temperature of the roof and therefore of the air around it. This further helps to reduce cooling costs for the building.

Last but not least, there is also an aesthetic appeal to green roofs that can’t be denied.

Green roof construction: How is it done?

The basic design elements of green roof systems are fairly consistent. There are two types of green roofs: extensive and intensive. Extensive roofs are a common, light design and are covered in a thin layer of smaller vegetation. Intensive roofs require more support and can support a wide variety of plants, including trees and shrubs. Here are the steps:

  • The foundation of the whole project is the support structure. Most commercial buildings will use a concrete slab and intensive roofs will obviously need more support than extensive designs.
  • Next, a protection board is usually laid down that prevents root penetration into the supporting structure.
  • Then, a waterproofing membrane is laid down to keep everything dry. If an existing roof is being retrofitted, this will usually be whatever is in place currently, such as asphalt shingles. The protection board would be placed down on top of this.
  • A layer of insulation may be placed down, but not always.
  • A drainage layer is then laid that captures, collects and distributes water across the roof evenly.
  • A filter layer separates the drainage system from the soil, to prevent clogging.
  • Finally, there is the growing medium (soil) and plants.

The plants can be individually planted, but a more economical approach being used is to grow the plants in a layer, much like sod, which can then be rolled or placed onto the roof in mats. The type of plants selected will depend on local climate conditions, the thickness of the growing medium and the objectives of the roof. For example, a grass roof may require more water and provide less diverse habitat than a sedum or native plant roof.

OK, so let’s talk money

The cost of green roofs in the US is estimated at $15-20 per square foot, with the highest cost being the organic materials (i.e. the soil and plants). This can be compared with an asphalt shingle roof that averages between $2.75 and $4.50 per square foot. However, green roof costs in Germany are substantially less ($8-$15 USD/square foot) than in the US as the high demand has created a large service industry associated with green roof systems. So as demand increases in the US, costs will drop.

Though there may be a high initial cost to green roofs, they can have a much longer life than other conventional roof types. The construction materials of green roofs are protected beneath their vegetative layer from ultraviolet rays, wind and other elements that are known to degrade these materials. As well, costs associated with heating and cooling the home are usually reduced with a green roof design.

Some greener minded cities have started to set standards for green roof construction that not only meet building code requirements, but also help to fulfill sustainability objectives for the city. There may even be incentive programs to help offset construction costs. If you think a green roof might be for you, ask around your city offices to see what funds might be available to you.

Watch this video to see an example of green roof construction in action.

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