Difference between revisions of "Green roofs"
Green roofs

Revision as of 15:51, 2 July 2013

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The information on green roofs will be updated in early summer of 2013

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The anticipated review period for this page is June through September 2013

Acknowledgements for green roofs

Green roofs consist of a series of layers that create an environment suitable for plant growth without damaging the underlying roof system. Green roofs create green space for public benefit, energy efficiency, and stormwater retention/ detention.

Design criteria

Photo of a green roof.

Structural load capacity, how much weight the roof can hold, is a major factor in determining whether the green roof is “extensive” or “intensive”. Vegetation selection is based on numerous factors including, growth medium depth, microclimate, irrigation availability and maintenance. A leak detection system is recommended to quickly detect and locate leaks.

Benefits

Reduce, delay, and cool stormwater runoff
Can significantly lower building energy costs
Provide habitat for birds and insects
Increase longevity of traditional roofing systems by protecting from ultraviolet rays
Reduce carbon dioxide levels and heat island effect
Local food production
Decreased stormwater utility fee
Aesthetics
Create gardens even where open space is not available at grade
Can be used for stormwater treatment in many areas where other LID techniques might not work (e.g. redevelopment projects with limited open space, areas with high bedrock, karst topography, contamination)
Beneficial in hospitals (see Ulrich, 2002)

Limitations

Initial capital cost is higher (Note: considering lifecycle costs may make green roof a more attractive investment due to increased roof lifespan, increased energy efficiency, and stormwater benefits).
Leaks can cause significant damage and can be hard to locate and repair without an electronic leak detection system. ANSI/SPRI RP-14 Wind Design Standard for Vegetated Roofing Systems, available at http://www.spri.org, gives guidelines on how to design to minimize wind damage in green roofs, including, for example, how to evaluate what roofs are too windy for a green roof, and where a green roof may need extra wind protection.
Conditions can be harsh for vegetation establishment.
Maintenance needs can be higher than traditional roofing system.
Building codes and insurance

Management suitability

Water Quality (V_wq) - High
Channel Protection (V_cp) - Med.
Overbank Flood Protection (Vp₁₀) - Low
Extreme Flood Protection (Vp₁₀₀) - Low
Recharge Volume (V_re) - Low

See Unified sizing criteria for explanation of these terms.

Mechanisms

Screening/ Filtration
Temperature Control
Evaporation
Transpiration (if vegetated)
Soil Adsorption
Biological/ Micro. Uptake

Pollution removal

Total Suspended Solids - 90%
Nutrients - Total Phosphorus/Total Nitrogen - 100%/ 20%
Metals - Cadmium, Copper, Lead, and Zinc - 80%
Pathogens - Coliform, Streptococci, E. Coli - 65%

NA Toxins - Hydrocarbons, Pesticides

Site factors

Drainage Area - Rooftop
Max. Slope - NA.
Min. Depth to Bedrock - NA.
Min. Depth to Water Table - NA.
SCS Soil Type (can be used in C&D soil types with modifications (e.g. underdrains)) - NA
Freeze/ Thaw Suitability - Good
Potential Hotspot Runoff (requires impermeable liner) - Suitable

Note: Pollution removal percentages apply to volume of runoff treated, and not to volume by-passed

Description

Schematic of green roof

There are two systems of green roofs, extensive and intensive, composed of the same system of layers. Extensive systems are lighter, typically have 6 inches or less of growing medium, use drought tolerant vegetation, and can structurally support limited uses (such as maintenance personnel). Intensive systems are heavier, have a greater soil depth, can support a wider range of plants, and can support increased pedestrian traffic.

Rainfall is initially intercepted by vegetation, held on foliage, or soaked up by plant roots. Any remaining runoff filters through the growing medium and is drained away from the roof’s surface by the drainage layer. This water can be captured in cisterns to irrigate plants or for other re-use purposes. Some drainage systems use small depressions to store excess water for uptake during drier conditions (RCWD), while others provide an overflow for larger rainfall events.

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+<!--Several [[Fact sheets for green roofs|fact sheets]] for green roofs provide overview information and information on design, construction and maintenance, and volume and pollutant removal.-->
-Several [[Fact sheets for green roofs|fact sheets]] for green roofs provide overview information and information on design, construction and maintenance, and volume and pollutant removal.-->
 Green roofs consist of a series of layers that create an environment suitable for plant growth without damaging the underlying roof system. Green roofs create green space for public benefit, energy efficiency, and stormwater retention/ detention.

Difference between revisions of "Green roofs" Green roofs