Detailed Product Description A post spike is a heavy metal spike about 24 or 30 inches long with a metal boxA post spike is a heavy metal spike about 24 or 30 inches long with a metal box (usually 4" x 4") attached to the top, designed to hold the base of a wooden fence post. You simply drive the post spike into the ground where you want to put your fence post and then insert the wooden post into the box.