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

Runoff Reduction Control Measure Fact SheetGreen Roofinteraction, and need to be engineered to conform to the load requirements. Characteristics of each greenroof type are summarized in Table GR-1.Make the decision whether or not to provide public access in this step, since this affects green roof typeand load design (Step 4). If access will be provided then at a minimum foot traffic must be accommodatedwith walkways or turf grass. (Turf grass must be irrigated and requires the deeper growth media of anintensive green roof). Typically, green roofs with public access have a complex array of vegetation (i.e.are intensive green roofs) and may even have other garden features such as a fountain, ornamental pond,or patio/deck.Step 4 – Determine Required Structural Support and Green Roof DesignDesign the structure to support the green roof, considering the saturated weight of the mature green roofsystem and the expected live load from human activity on the roof (e.g., maintenance staff, tenants,visitors). If a green roof is planned for a new building, the architects and structural engineers need tofactor the green roof into the architectural and building design process. To retrofit an existing building,confer with an architect, structural engineer, and/or green roof consultant to ensure the proposed greenroof can be supported—either as is or with additional support such as additional decking, roof trusses,joists, columns, and/or foundations, as indicated in Table GR-1.If the roof will be accessible to the public, design the traffic flow paths and integrate decks, patios, orpavers into the design. Turf grass will stand up to regular foot traffic but requires an intensive green roofsystem with deeper soil and mandatory irrigation. Alternatively, less intrusive, lightweight steppingstones or decomposed gravel walkways can be used to provide access and interpretation with extensivegreen roofs. Design roof entrance and exit routes to design standards to be safe and efficient formaintenance staff or the public.Step 5 – Select Component LayersGreen roof systems typically contain the following specialized component layers (see Figure GR-1):Waterproofing Membrane — Choose an adequate waterproof membrane that resists penetrationby roots. The waterproofing component is essential to the long-term success of a green roof.Generally, a composite of several layers of protective materials is used . Materials used includemodified asphalts (bitumens), synthetic rubber (EPDM), hypolan (CPSE), and reinforced PVC.Protection Layer/Root Barrier — The need for a separate root barrier (dense materials thatinhibit root penetration) depends on the selected waterproof membrane . Modified asphaltsusually require a root barrier, while synthetic rubber (EPDM) and reinforced PVC generally donot. Check with the manufacturer to determine if a root barrier is required for a particular product.Insulation/Air Barrier (optional) — If the thermal requirements of a building necessitateadditional insulation, a layer of moisture-resistant insulation may be added.Water Retention Mat — If additional water retention is necessary to sustain the selectedvegetation, a moisture retention mat may be used.Drainage/Water Storage/Aeration Layers — A green roof must safely drain runoff from theroof to an approved stormwater destination. Provide a drainage layer over the entire roof area toconvey excess water to the building’s drainage system. Drainage layers usually consist of moldeddrainage channels and retention cups.Geotextile Filter Fabric — Include a geotextile filter fabric layer to keep the growth media outof the drainage layer.Engineered Growth Media — Green roof growth media differ from soil in that they generallycomprise lightweight mineral material containing a minimum of organic matter. Use a growthmedium uthat meets established FLL or ASTM guidelines for both water retention and drainage.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsMay 2007GR-5