Ask the Green Architect:

The Skinny on Green Roofs

Eric Corey Freed
Green architect Eric Corey Freed answers your questions on sustainable building performance, materials, and design.


  • The Skinny on Green Roofs
  • Retiring Your Refrigerator
  • Earthen Floors

    How does a green roof work? What are the benefits and drawbacks?

    Not only do green roofs protect your roof membrane, they also add insulation and beauty to your roof.

    Green roofs are not recommended for a roof area that is not seen, as there are easier ways to get an efficient roof. But for a roof with visibility, such below an apartment building or office, a green roof will enhance and beautify the surroundings, all while purifying the water, cleaning the air and insulating the roof.

    A green roof system is an extension of the existing roof, and not potted plants as many assume. The system consists of a special water proof and root repellant membrane, a drainage system, filter cloth, a lightweight growing medium and plants on top.

    Green roof systems may also be modular, with the drainage layers, filter cloth, growing media and plants already prepared in movable, interlocking grids or trays. These are good for areas where access could be an issue.

    In North America, the benefits of green roof technologies are poorly understood, despite the efforts of several industry leaders. In Europe however, these technologies have become very well established. Cities like Chicago have launched a
    Green Roof Initiative, serving as a model for other cities.

    Before you initiate a green roof installation, you will need to know the slope, the structural loading capacity, and existing materials of the roof, as well as the nature of any drainage systems, waterproofing, and electrical and water supply in place. You should also consider who would have access to it, who will do maintenance, and what kind of sun and wind exposure the roof gets.

    Plant selection depends on a variety of factors, including climate, type and depth of growing medium, loading capacity, height and slope of the roof, maintenance expectations, and the presence or absence of an irrigation system. A landscape architect would be able to advise you on suitable plants and design of the plantings. (See the landscape contacts below.) Here in California, Rana Creek is the leader in green roof systems and design.

    The cost of a green roof varies considerably depending on the type and design factors. As a budgetary figure, a green roof may be installed for $12.00-$24.00 per square foot. While this might seem expensive, it also reduces the need for additional insulation, and protects the real roof below it.

    The biggest obstacle in adding green roofs to smaller projects is the added weight they create. As a rule of thumb, a green roof weighs the same as water, 300 pounds per cubic foot. The additional weight might require additional roof structure, and therefore, additional cost.

    A final issue to consider in looking at a green roof system is how other functions like photovoltaic solar panels could make better use of a particular roof space.

    When installed, green roofs provide a beautiful and environmental alternative to traditional roofing. By knowing the considerations, you will be able to choose an appropriate solution.

    Further Reading:

    Green Roofs: Ecological Design and Construction

    California Department of General Services Quick-Response Studies Guide (PDF)

    Architectural Group to Install Green Roof at D.C. Headquarters
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