Gravity plays a big part in carrying stormwater from your catch basin drain to your drainage system, and the outlet plug can help. Use the 6 in. Outlet Plug to close off any unused catch basin opening so water is directed toward an unplugged opening connected to drain pipe for release downstream. Seals off opening in 9 in. and 12 in. catch basins; seals off opening in 18 in. catch basin with additional Reducer Ring (1890); seals off opening in 24 in. catch basin with additional Outlet Adapter with Reducer Ring (2410). Made of high-impact polystyrene (HIPS) treated with UV inhibitors.