Stormwater Quality Design Manual (May 2007) - City of Sacramento ...

Technical RequirementsGreen RoofVegetationEngineered Growth MediaGeotextile Filter FabricDrainage/Water Storage/AerationWater Retention MatProtection Layer/Root BarrierWaterproofing MembraneRoof DeckFigure GR-1. Typical Green RoofConfigurationImage source: American Hydrotech, Inc.®,http://www.hydrotechusa.com/. Text detail added.Step 1 – Choose Vendor and Specialized ConsultantsGreen roofs are typically designed and installed by an established vendor. At a minimum, also consult:• Structural engineer – to ensure the roof loading capacity is adequate• Architect—to integrate green roof design with the building design, including planning forpossible use by future building occupants• Landscape architect – to design the planting areas, select vegetation and design the irrigationsystem• Mechanical engineer – to calculate the heating and cooling implications of the green roof and todiscuss how to integrate the green roof with rooftop mechanical equipment and drainage needsStep 2 – Calculate Water Quality Volume (WQV)The growth media volume is a key criterion governing the sizing of the green roof. Provide sufficientvolume within the pore space of the growth medium to contain the water quality design volume (WQV),which is determined using Appendix E information, based on a 12-hour drawdown period. Take intoaccount the presence of established vegetation when determining the needed pore space. To calculate thevolume of growth medium required to contain the WQV in the available pore space, use the followingequation:Volume of growth medium = WQV/Porosity of growth media (with vegetation)Step 3 – Select Green Roof TypeDecide whether to install an extensive or intensive green roof. Extensive green roofs, which have simplevegetation and shallow growth medium, are characterized by their low weight, low capital cost, and lowmaintenance, and can be retrofitted onto existing structures with little or no additional structural support.Intensive green roofs, which have complex vegetation and deeper growth medium, are characterized bytheir higher weight, capital cost, and higher maintenance requirements. They are more elaborate in design(sometimes even incorporating fountains and ponds), are typically intended for human use andStormwater Quality Design Manual for the Sacramento and South Placer RegionsGR-4 May 2007