The drainage board is installed above the protection layer. This product manages excess rain water and prevents roots from becoming saturated. In the event of a heavy rainstorm, the drainage board is engineered to store water below the green roof media layer while simultaneously creating a lateral flow of excess rainwater that is directed to the roof drains, scuppers or gutters. This layer creates a proper balance of water in an extensive green roof system by providing natural subterranean irrigation. We offer drainage/retention boards in .4”, .5”, 1.0”, 1.5” and 2.5” depending upon the type of vegetation installed on the roof and rainwater retention requirements.