Overview
Green roofs (also known as vegetated roof covers, eco-roofs or nature roofs) are constructed of a lightweight soil media, underlain by a drainage layer, and a high quality impermeable membrane that protects the building structure. The soil is planted with a specialized mix of plants that can thrive in the harsh, dry, high temperature conditions of the roof and tolerate short periods of inundation from storms. Green roofs help to reduce stormwater runoff and improve water quality by absorbing, detaining, and filtering rain water. 1
Two typical types of green roofs are:
--Extensive systems, with 2 to 4 inches of soil, 12 to 40 pounds per square foot of roof area, short plants with shallow root systems, and easy maintenance. Extensive systems absorb stormwater and provide insulation.
--Intensive systems are similar to gardens on the ground. They have a minimum of 6 to 12 inches of soil, 80 to 150 pounds per square foot, host deeper-rooted plants, including shrubs and trees, and require more maintenance. This type of roof provides more stormwater benefits, including insulation, water filtration, storage, and increased habitat opportunities. 2
Benefits
- Significantly reduces runoff volumes and rates (50 to 90% reduction in annual runoff).
- Can reduce on-site stormwater detention needs.
- Can reduce energy requirements associated with heating and cooling the building interior.
- Creates outdoor space such as rooftop gardens.
- Prolongs the life of the roof (see cost considerations below).
- Reduces urban heat island effect. 3
Cost Considerations
In the United States, green roof costs - including everything from waterproofing to plants - tend to range from $18 to $25 per square foot, depending on how intensive the system is. The initial capital and ongoing maintenance costs of a green roof are offset by some long-term
cost savings - most notably roof maintenance and replacement and utility costs. A vegetated roof, on average, can be expected to prolong the life of a conventional roof by at least 20 years because the vegetation prevents the roof from being exposed to ultraviolet radiation and
cold winds. 4
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