Parking lot drainage design Parking lot drainage design mar2805 (Structural) (OP) 20 Mar 16 10:41 Hi folks! Any books or guide lines on designing drainage system for outside parking lots...minimum grades, "drainage sheets" layout... Any help would be nice. Thanks RE: Parking lot drainage design Ron (Structural) 20 Mar 16 12:33 This is usually dictated by local ordinance and practice. Design storm events and allowances for onsite storage and offsite impact are often unique to the area. If your area is covered by a regional water management district, the will also have a say as to design criteria and "downstream" affects. RE: Parking lot drainage design nkrigPE (Civil/Environmental) 30 Mar 16 02:51 As far as the parking lot drainage itself. I work in a very flat area and for asphalt parking I try to have 2% slope for sheet drainage, 1% is the absolute flattest I will go. Concrete parking I try to have a minimum slope of 0.50% for sheet flow and 0.25% for longitudinal gutter/channel flow. Once again, I am in an area where natural prevailing slope is less than 0.1%. We don't have any detention or mitigation requirements for downstream impacts, but that is something you should consider if required by the jurisdiction.