Recently planted green roof at Evergreen College, Olympia WA (Photo courtesy of O'Brien & Co.)

A green or vegetated roof provides the function of a conventional roof while allowing plants to grow on the surface. A vegetated roof includes water proofing, a drainage system, filter layer, a lightweight growing medium, and plants. The technique is well established in Europe and is beginning to be applied in the U.S for both commercial and residential applications.

An ‘extensive’ green roof has a relatively shallow soil profile (1 to 5 inches) and is planted with ground cover plants that are adapted to the harsh conditions of the rooftop microclimate. ‘Intensive’ green roofs refer to more substantial roof gardens with deeper soil (6 inches or more) and are often planted with shrubs, and trees as well as ground cover.

Green Roof at Chicago City Hall (Photo courtesy of City of Chicago)

A vegetated roof can be a major asset to a building’s developer and owner operator. By adding to the long term energy efficiency of a building and, as a result, reducing initial HVAC size requirements, first cost and operational costs for conditioning the building can be significantly decreased. Modeling by Environment Canada estimated that with grass in less than 4 inches of soil a one storey building would reduce summer cooling needs by 25%. Field studies in Ottawa Canada, found that a 6 inch extensive green roof reduced heat gains by 95% and heat losses by 26% compared to a reference roof.

In addition, jurisdictions in the U.S. are beginning to recognize the value of green roofs in stormwater management and are considering providing flow credits to those who install them. As this becomes more common, vegetated roofs are likely to become a preferred alternative to complex and/or land-hogging stormwater management techniques. These significant economic advantages, as well as multiple social and environmental, have proven vegetated roofs to add to a building’s overall value.

What does this have to do with concrete? Concrete is the structural system of choice for vegetated roofs because it provides a continuous load-bearing surface for the potentially moist growing medium and plants. In addition, cast-in-place concrete provides a uniform surface.

Projects constructed using waterproof concrete made with proprietary admixtures allow for the elimination of membranes, and therefore simplify design, construction, and maintenance processes.

Lightweight concrete topping can be used to create the sloping deck of a vegetated roof system. In addition, structural lightweight aggregate can be used as a lightweight, absorptive portion of the growing medium. Lightweight applications reduce the deadload on the roof structure.

 

 

 

Durability. The vegetated cover protects the roof membrane resulting in a longer service life before replacement is required

 

Acoustical Comfort. Increased sound insulation results from higher sound absorption by the growing medium and plants

 

Energy Efficiency. Greater energy efficiency typically results in reduced size requirements for the building’s cooling equipment. May contribute to LEED Credit EA 1.

Heat Island Reduction. The vegetated roof minimizes the heat island effect of the building. May contribute to LEED Credit SS 7.

Stormwater Management. Because rainwater percolates through the growing medium peak runoff rates are reduced. May contribute to LEED Credit SS 6.

Human Well being. Increased aesthetics for those with access or site line view to the vegetated roof, enhancing psychological well-being of building users.

Outdoor Air Quality. The plants absorb and filter outdoor pollutants, helping improve outdoor air quality.

Habitat. The roof provides habitat for birds and other small creatures. May contribute to LEED Credit SS 5.

 

Water Efficiency. Appropriate vegetation can eliminate the need for landscaping irrigation in green roofs after establishment. This can contribute to LEED Credit WE 1. Using roof runoff from a green roof can contribute to WE Credit 2.