GREEN ROOFING
INSIDE THE FLOODPLAIN | OUTSIDE THE FLOODPLAIN

What Is It?

Cost: Extensive: $5-25 per sq ft
Intensive: $25-40 per sq ft
Annual maintenance costs: 0.75-$1.50 per sq ft
Applications: Can be installed in retrofits or in new construction. Intensive green roofs can only be installed on flat rooftops that can support the additional weight. Extensive green roofs can be used on sloped rooftops.
Service Life: Extensive (>50 years with maintenance). Can extend life of conventional roofs by 40-60 years.

Green roofs consist of a vegetative layer grown on part or most of a rooftop, shading the roof surface and helping to remove heat in the air through evapotranspiration. Whereas conventional rooftops absorb heat and can be up to 90 degrees hotter than ambient air temperature, green rooftops can often be cooler than the surrounding air and can reduce building cooling costs. Widespread use of green roofs in urban environments can reduce urban heat island effect: a study of Toronto suggested that use of green roofs on 50% of rooftop surfaces in downtown could lead to cooling the entire city by up to 1.4⁰F. In winter, green roofing provides an insulating effect, reducing building heat loss and heating costs.

Green roofs can also enhance stormwater management, capturing over 50% of rainfall and reducing runoff to storm drains. Careful selection of vegetation can allow green roofs to also capture pollutants from stormwater, including phosphorous, nitrogen, and heavy metals. Green roofs can reduce temperature variations and ultra-violet ray exposure on roofing materials, potentially extending the roof’s life by 40-60 years.

There are two types of green roofs. Intensive green roofs are over 6” deep and allow for a wide range of vegetation to be grown on the roof, including shrubs and potentially trees. Intensive green roofs can provide social and recreational uses, but are typically more costly to install and require more regular maintenance and potentially, irrigation. They can only be installed on flat rooftops that can support the additional weight, potentially limiting their application in retrofits. Extensive green roofs are shallower, lighter weight systems that typically have lower upfront and maintenance costs than intensive green roofs. Extensive green roofs have been successfully grown on roofs with slopes of greater than 30⁰, and tend to be more cost-effective in retrofits.

Benefits

  • Reduces building heating and cooling costs. In summer, green roofs reduce building and ambient air temperatures. In winter, green roofs reduce building heat loss.
  • Enhanced urban stormwater and pollutant management.
  • Reduced sound transmission and reflection.
  • Prevents UV and ozone damage to waterproofing systems.
  • Can be used for gardening to save on food costs.
  • Ecological benefits include safe habitats for birds and insects, potentially including endangered species.
  • Improved air quality, carbon sequestration, and building aesthetics.

Drawbacks

  • Green roofs (particularly intensive green roofs) add additional weight to the building roof. Additional structural reinforcement may be necessary for installation.
  • Green roofs require extensive rooftop waterproofing, as water becomes retained on the roof during storms. Roots may penetrate waterproof membranes, leading to structural damage.
  • Higher upfront costs than traditional roofing.
  • Most systems cannot be placed on sloped or curved roofs.

Regulatory Impacts and Requirements

A summary of potential regulatory touchpoints follows below.
Green-Roofs

Financing Options, Incentives, And Rebates

Additional Resources

Project Examples

SAMPLE OF SUPPLIERS

SOURCES

Photo credit: User TonyTheTiger [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0), Wikimedia Commons