Living Green Roofs

Living Green Roofs

Living Green Roofs

A green, vegetated, or living roof is an innovative stormwater management solution for absorbing water and reducing runoff during light rain events. It also helps reduce the temperature in hot cities on hot days by reducing the "heat island effect," The heat island effect refers to the occurrence of increasing ambient temperatures on hot days in urban areas due to the concentration of reflective or light colored surfaces (or albedo from roofs, walls, roads, parking lots, etcetera) that reflect the sun’s heat back into the air.

It is an issue in cities like Phoenix, Houston, and Tucson where temperatures increase by 10 to 15 degrees. A living roof helps mitigate this phenomenon and has the potential to save energy. There are 2 types of green roofs: extensive and intensive. Extensive green roofs consist of drought-tolerant, self-seeding vegetated roof covers. Native plants, selected from locations with dry or semi-dry grass conditions, that require little or no irrigation–such as succulents, grasses, and meadow flowers–are often chosen for the plant cover. Intensive green roofs, like roof gardens and parking garage roofs, are more costly and elaborate in design and use different growth media types. Flowering shrubs and trees can bring out the intensity in the design. Intensive green roofs allow for more variety in plant selection compared to extensive green roofs. Both roof types offer extra insulation which results in energy savings and may reduce or eliminate the need for air conditioning.

Living Green Roof Realities

Green roofs provide green space from which urban and suburban areas can benefit. They reduce stormwater runoff by harnessing the water retention abilities of the vegetation.

A traditional impermeable roofing system does not capture, retain, or filter water at the source. Green roofs also improve air quality by collecting dust and pollutants. The air in urban environments like those in California is hot and dry; in this situation the limited number of trees cannot transmit enough water into the air to cool it. Green roofs can improve air quality when installed in this type of climate.

Drawbacks to green roofs include costs and architectural constraints. Contrary to common belief, flat roofs are not an ideal green roof setting since they require an extra layer to drain extra water away from the root zone. A roof with a slope between 5 degrees (1:12) and 20 degrees (4:12) is ideal because gravity will cause the water to drain naturally. Costs are another factor, they range from $15 to $20 per square foot, which includes everything from waterproofing to plant selection. But consider the reputation and expertise of the company providing the waterproofing for the green roof. A bad waterproofing installation can end in roof leakage, resulting in water damage and expensive repairs.

Living Green Roofs

Construction Materials

Installers can use 60- to 80-mil reinforced, heat-seamed PVC or Polyethylene, single-ply roof systems for efficient, cost-effective construction waterproofing that provides protection against root penetration. However, PVC is not sustainable, is highly toxic, and is not readily recycled. Other possible materials for waterproofing and root protection include rubber membrane (EPDM) or hypolan (CSPE). One potential problem with EPDM is the high risk of leakage leak because seams are simply glued with adhesive or tape. Another more environmentally acceptable option is thermoplastic polyolifins (TPOs), which is a green roof base waterproofing material.

Additional Information

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