Green Roofs Are Multi-Functional Roofing Systems

PURPOSE & DESCRIPTION

Green roofs are multi-functional roofing systems that provide significant economic and environmental benefits. Green roofs mitigate the impact of urban development by managing stormwater, reducing building heat absorption, and creating valued green space.

APPLICATION

Green roof systems can have a variety of objective applications including: storm water runoff reduction, storm water pollutant reduction, thermal buffering of the building envelope to save energy in cooling and heating, creation of urban wildlife habitat, creation of human recreational green space, air pollution mitigation, and/or mitigation of heat island effects in urban areas. Green roof systems have been used in Europe for centuries, the first systems appeared as grass or sod roofs that provided thermal insulation in Northern Europe. The engineered green roof systems we use today began to appear over thirty years ago in Germany. Today they are widespread throughout Europe and are becoming common throughout the United States.

All green roof systems utilize the following components:

  • High-quality impermeable waterproofing membrane covering the roof structure.
  • Drainage layer.
  • Lightweight growing medium layer.
  • Vegetation.

There are many commercially available green roof systems that vary only slightly. Some systems utilize a drainage layer that unrolls into one continuous cover over the waterproofing while other systems utilize interlocking trays or grids that are combined to create a continuous drainage layer. Additional components, such as root barriers, are also used in some systems.  

Green roofs may be divided into two broad categories, intensive and extensive. Intensive green roof systems generally consist of a deep (6 in+, 150mm+) soil layer and vegetation that may include groundcovers, turf grass, shrubs and trees. Extensive green roof systems have a shallow (1-6 in, 25-150mm) soil layer and only hardy, drought-tolerant groundcover vegetation. Extensive systems are less expensive, lighter weight, lower maintenance and more common than intensive green roof systems. Green roof systems may be installed during new building construction or retrofitted onto an existing building. Extensive green roof systems are feasible on roofs with up to a 30 degree slope. The key consideration for extensive systems is whether the roof structure is adequate to hold the weight of the green roof.

Extensive green roofs can weigh 12-50 lbs/square ft. (59-244 kg/square m), and intensive roofs often way over 50 lbs/square ft (244 kg/square m). Consult a structural engineer to verify the roof and building structure is adequate before installing any green roof system. Green roof soils are typically engineered to be very lightweight. They are primarily made of inorganic components such as expanded slate and shale, and extruded clay and perlite, which provide volume and rooting structure with little additional weight.

Extensive green roof systems are specifically designed to be low-maintenance. Typical maintenance procedures include: irrigation to establish vegetation, semiannual inspection and removal of invasive weeds.

RAINFALL ABSORPTION/RUNOFF REDUCTION

While green roof systems can absorb between 50 and 90% of total rainfall, increasing the organic matter content of the soil media will increase the rainwater absorption and initial abstraction properties of the green roof soil media. It is important to understand that many green roof plants, including sedums, do not thrive in organic rich soil media. If rainfall absorption/runoff reduction is one of the key design objectives, plant material selection may need to account for organic matter tolerance.

For every ? in (15mm) of GrowingMedia™ added to the green roof soil media, it will hold approximately 8 cubic ft (0.2 cubic m) of water per 1000 sq. ft (93 sq m), or 360 cubic ft/ac (26 cubic m/ha). Alternatively, GrowingMedia™ typically holds approximately 1.6 oz (45 g) of water per 3.6 oz (100 g) of GrowingMedia™ (dry weight); 1 gal (0.004 cubic m) of water per 20 lbs (9 kg) of GrowingMediaTM (dry wt) or per 30 lbs (14 kg) of GrowingMedia™ (wet wt). This equates to approximately 40 gal (0.15 cubic m) of water per cubic yard (0.76 cubic m) of GrowingMedia™ and 5,400 gal (722 cubic ft, 20 cubic m) of water per acre inch (0.01 ha meter,103 cubic m) of GrowingMedia™, and 10,800 gal (1444 cubic ft, 41 cubic m) of water for a 2 in (50mm) GrowingMedia™. An acre inch (0.01 ha meter) of GrowingMedia™ requires approximately 135 cubic yards (103 cubic meters) of material.

INSTALLATION

Installation should follow the manufacturer’s specifications for the specific system being installed. Green roof soil media and GrowingMedia™ may be installed using a blower truck in most situations. Green roof vegetation should be installed soon after soil installation.

INSPECTION & MAINTENANCE

Inspection and maintenance should follow the manufacturer’s specifications for the specific system being installed. In general, it is recommended that inspections be performed at least semi-annually to check to make sure the vegetation is healthy and monitor for invasive weeds.