Pre-fab fireplaces have a metal covering (chase top) to prevent water from entering the interior of the chimney structure. The original chase top is usually made of a cheap galvanized sheet metal, some 1-piece and some several pieces (never acceptable). When you get leaves and/or pine needles blowing around some are bound to get stuck on the water that has collected during rain, snow, and ice storms. Most tops have a low spot that holds water as well. Pine needles and many types of leaves are very acidic when they are decomposing, especially when sitting in water.The acids break down the galvanized coating and you soon get bare metal. After a while the metal starts to rust, and when water runs off the top during a rain, it carries the rust with it down the side of your beautiful siding, causing a stain that gets bigger over time. You may even hear water dripping on the inside of the chase after a rain if the rust has penetrated the metal, allowing water to seep through. Then you know you have a problem. The rust stains on the outside of a chimney are the first clue, as it can take several months for the metal to pit enough for water to get in. The time to act is when you first notice rust stains, not when you hear water dripping.The way to fix the problem is to replace the old rusted chase top with a new stainless steel chase cover. Our 304 stainless steel 24 gauge chase cover includes creases on the top to prevent standing water and increase strength, choice of collar height and side skirts, and drip edges that flick the water away from the chase siding. We use and recommend stainless steel for its strength and ability to resist rust. Galvanized metal is not long lasting, and the cost difference is minimal compared to stainless steel. Also, our stainless steel chase cover has a lifetime warranty against rust, corrosion and manufacturer's defects. The galvanized cap only comes with a 12 month warranty.