The Polylok 3017-C is 12" in diameter and is designed to work with the 3017-R risers or 3017-12C septic distribution boxes. The covers are made of HDPE which is highly resistant to harsh septic environments, providing years of trouble free service. This Polylok cover will ship with the stainless steel screws required for installation. WARNING: This product can expose you to chemicals which is [are] known to the State of California to cause cancer and birth defects or other reproductive harm. For more information go to www.P65Warnings.ca.gov.