Eco-roof/ Green Roof
An ecoroof is a lightweight layer of vegetation and soil installed on building rooftops. The shallow growing medium of ecoroofs intercepts rainwater and evapotranspirates a portion of it, particularly in the summer months. Rainwater not evapotranspirated is slowed, filtered, and cooled through its movement through the soil layer. In addition to reducing the amount of stormwater generated from a roof, ecoroofs also reduce urban heat island, provide habitat and amenity value, improve roof longevity and reduced heating and cooling costs. Simple ecoroofs consist of a waterproof membrane, drainage material, a lightweight layer of soil and a cover of plants. They can be two to six inches deep. A variety of plants are available for ecoroof plantings, such as sedums, succulents and hardy perennials. The rooftop growing environment requires plant species to be drought-tolerant and able to grow in shallow soils. Ecoroofs are not intended to be accessed except formaintenance, unless walkways or plazas are incorporated into the design.
Applicability
Ecoroofs are appropriate for all building types - residential, commercial, industrial and institutional - and are applicable both to new and existing buildings depending of roof type and load-bearing capacity of the building. Some considerations for implementing ecoroofs include the growing medium, plant selection, waterproofing and drainage systems.
Benefits
Ecoroofs produce both stormwater quantity and quality benefits by reducing stormwater flows and filtering pollutants.An ecoroof can capture and retain 60% of
the annual precipitation that falls on it.Nearly all of the rainfall is absorbed duringwarm weather storm events and stormwaterdetention and peak attenuation is alsonotable during saturated winter months. Thisreduces the runoff flow rate, volume andtemperature of roof runoff.They also filter air pollutants and reduce outdoor air temperatures that contribute to the urban heat island effect. They provide habitat and biodiversity and provide an attractive amenity for the building and for buildings with views to the ecoroof.The insulation provided by ecoroofs also lowers building heating and cooling costs. Ecoroofs are more durable than conventional roofs delaying the need to costly roof replacement by twenty years or longer.
Vegetation
There are many options for ecoroof plantings. Choose drought tolerant species such as
sedums, succulents, or hardy perennials. Plants should be low maintenance, able to
tolerate shallow soil, and be self-sustainingwithout fertilizers or pesticides and minimum
summer irrigation.
Maintenance Considerations
Once an ecoroof is well established, maintenance requirements are usually minimal. This may include some summer watering, weeding and mulching. During the first few years when the vegetation systems are establishing, some additional watering may be required. Inspection of the roof membrane and drainage flow paths should be performed on a regular basis. If grasses or other annual plants are included, then cutting and removing dry vegetation is recommended to avoid the accumulation of combustible material.
Cost Considerations
An ecoroof initially costs more than a conventional roof but typically lasts twice as long (about 40 years). The construction costs for a new ecoroof tend to range from $10 to $15 per square foot. Re-roofing costs are higher, ranging from $15- $25 per square foot. 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. The building owner can also realize cost savings from lower heating and cooling costs. Since the ecoroof industry is relatively new to the United States, ecoroof construction costs may decrease as the industry develops.
Safety and Siting Requirements
- Consult a professional for designing and building an ecoroof
- Ecoroofs can be on flat roofs or pitched structures up to a slope of 25%
- Roof strength must be adequate to hold the additional weight of the ecoroof
- Overflow structures – such as drains or downspouts - must also be incorporated into an ecoroof