Most modern houses have prefabricated metal fireplaces and flues rather than traditional brick and mortar fireplaces with clay tile flues. The frame built around a prefabricated chimney is called a chase, usually constructed from wood framing and finished with siding, masonry or stucco. In well-built homes the space inside the chase, around the flue, is insulated. The chase cover is the fitting that goes over the top of the chase, with a hole in the middle that fits around the flue.
Why Cheap Chase Covers Fail
Many chase covers are made from galvanized steel, which resists rust and corrosion only as long as the paint holds and the temperatures stay moderate. A lot of them are poorly designed with flat tops that collect rain and snow rather than shedding it. Between heat from the combustion gases, intense summer sun and corrosive standing water, cheap galvanized covers peel and rust.
When to Replace
Once a chase cover shows rust — or you notice rusty streaks running down the side of the chase — it's time to replace it. Rust turns into holes quickly, and those holes let water into the chase and down the flue. Water infiltration causes mold inside the chimney and the structure nearby, damages sheetrock and ceilings, compromises insulation, and corrodes the flue itself. It's a small part whose failure gets expensive fast.
What to Replace It With
A quality replacement pays for itself in avoided repairs. Chase covers can be fabricated from corrosion-resistant materials that last decades even in the harsh conditions at the top of a chimney. Stainless steel and copper are the popular choices, and both are available with lifetime warranties.
You may find an off-the-shelf cover that fits, either online or at a hardware store. If not, custom fabrication is reasonably priced. Whichever route you take, look for a cover that's slightly domed or arched on top so water runs off rather than pooling, and specify stainless steel, copper, bronze or another corrosion-resistant metal.
Cover Only, or Cover and Cap?
You can buy a chase cover with a chimney cap built into it as a single piece, or buy a plain cover to seal the chase and choose a separate cap to suit the look of your home. A chase cover doesn't have to be a plain lid with a hole in it — some custom chase covers are genuinely decorative, while still doing the structural job of keeping your chimney and home dry.