Chase covers are the metal pans that sit on top of the chimney chase commonly on prefabricated chimneys. Their purpose is the shed water away from the chimney protecting the chimney from water penetration. Unfortunately, many chase covers are poorly designed or made of materials that develop leaks after years exposed to the elements.Standing water on the chase top causes the cover to rust prematurely, eventually causing deterioration. Make sure that when you replace your chase cover that you install a stainless steel chase cover which is resistant to rust. It may cost a little more,but it is money well spent.