There are many reasons to consider having a green roof. This eco-conscious choice is recognized by LEED (Leadership in Energy and Environmental Design) and can earn credits towards an environmental building certification. Green roofs reclaim green space, provide wildlife habitat and promote biodiversity. Additionally, they lessen storm water discharge and filter/detoxify water by removing pollutants.
Green roofs consist of vegetation and growing media placed in layers, installed over an existing roof structure. These are not typical point-loaded roof gardens, as their live load is evenly distributed partially or fully over the roof surface. A structural engineer must determine load-bearing impacts of the green roof prior to design and installation.
The two major systems of green roofs are:
Extensive (shallow), less than 4" deep, covering a wide area, mainly planted with succulents growing in modular trays
Intensive, 4 to 6" deep, usually planted with perennials and ground-cover plants
The Intensive green roof offers diversity and visual appeal, but also requires more maintenance. The Extensive system, once established, requires very little maintenance.
A commonly used growing media for green roof systems is a mixture of 75% inorganic, expanded shale to 25% organic material.
Environmental, Health and Economic Benefits of Green Roofs
Protect waterproof roof membrane from solar and temperature exposure, extending the life of roof membrane 2x longer than a conventional black roof
Mitigate the urban heat island effect. According to the American Society of Landscape Architects, on a hot summer day green roofs are as much as 40 to 50 degrees cooler than a conventional black roof.
Lower energy costs. Canadian studies have shown that reduction in cooling demands of up to 25% can be achieved with green roofs.
Filter particulate air pollutants and store carbon
Reduce CO2 gas through photosynthesis
Manage storm water, thus reducing the amount to flow to storm water facilities
Diminish noise from traffic, airplanes and machinery
Increase neighborhood biodiversity and wildlife habitat
Earn credits toward environmental building certification status (LEED)
Offset initial costs by increased jobs/industry and increased energy savings
Qualify for New York State Tax Credit
In order to determine if a green roof is a suitable choice for the site, create a feasibility plan to balance costs and benefits at the start of the project. A structural engineer is required to:
Calculate structural dead loads and expected live loads
Determine if core samples of roof are necessary
Decide to replace existing roof or install over the top of it
Prepare preliminary budget estimates including costs for roof repair or replacement
Establish design goals, colors, elements
Consider aesthetics, storm water management, heat island mitigation, habitat restoration, etc.
Develop strong contract documents
Develop specifications using FLL guidelines (The German Research, Development and Construction Society) and ASTM standards (American Society for Testing and Materials)
Include performance and maintenance requirements
Choose plants based on hardiness zones, exposure (sun, wind, shade), water and nutrient requirements, overall toughness and wildlife value
Choose hardy succulents for use with shallow, extensive roof systems as they photosynthesize via CAM (Crassulacean Acid Metabolism)
For intensive roof systems with deeper growing media, hardy perennials and evergreen ground covers work best
Plant Installation Methods
Cuttings are lowest in cost but take approximately 12 to 18 months to become established
Plug-rooted cuttings are spaced 12" on center and the costs are reasonable
Vegetated mats have actively growing plants but are higher in price and difficult to transport
Planted tray modules are the most expensive option but do allow for specific design goals to be met easily
Green Roof Components
The roof top, insulation layer and waterproofing layer
An additional protection layer
A layer for drainage, aeration, water storage
A root barrier layer
Provide sufficient water for newly planted installations
Plant early in the growing season to promote root growth
Specify detailed maintenance plan for establishment phase
Plan for monthly maintenance, then bi-monthly, then seasonal