Post Collar For 3.5 Inch x 3.5 Inch Wooden Fence Post Protection From Rot and Yard Equipment Steel with adhesive fungus resistant mastic rubber sealant to prevent rot from occurring where it starts: at the base of the post. Bury half below ground for maximum protection. Installation is easy: Peel off the adhesive covering, attach around post so half will be underground. Align the holes and secure with two screws or nails. Fits 3.5 inch posts (which are advertised by Home Depot and Menards as 4" nominal)