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. Modular products can increase installation and repair efficiency.

Benefits

• Reduce, delay, and cool stormwater runoff.
• Insulate buildings and lower energy consumption and 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.

Limitations

• Cost is higher than traditional roofing systems – can be significant for retrofits.
• Leaks can cause significant damage and can be hard to locate and repair without an electronic leak detection system.
• Conditions can be harsh for vegetation establishment.
• Maintenance needs can be higher than traditional roofing system.

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 4 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. 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.