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DesignR Typically a green roof needs to beengineered to hold the extra loadsimposed by soil, snow, and water.R The roof needs to have a slope fo rwater drainage. Keep in mind thattoo much slope may cause problemswith erosion.R An EPDM (synthetic rubber)membrane on plywood provides thebe base surface of the roof. TheEPDM system is typically used forflat roofs and industrial buildings.R The edges of the roof need a layer ofgravel and drainage pipe, much likethe treatment around the foundationof the house. The gravel should coverthe drainage pipe around the edgeswhere drainage occurs.R On top of the EPDM, gravel anddrainage pipe, a Delta (or similar) water membrane should be laid down andsecured with soil (the next layer). This membrane has dimples for trapping waterfor the plants roots. This layer should be laid starting at the bottom, with eachlayer moving up the roof laid on top, overlap ping by about a foot. This layering isthe same process as shingling.R A drainage cloth (the type used for landscaping), should be laid on top of theDelta water membrane. This prevents soil from filling the cups in the Delta watermembrane.R On top of the E PDM put a six inch layer of soil.R A layer of straw mats or other material may be required to reduce the effects oferosion where greater slopes exists.163