Chase Cover Materials: Galvanized, Stainless, Aluminum or Copper

The Factory Pan

A chase cover is the metal pan that closes the top of a framed chimney chase, with a collar where the metal chimney passes through. On many factory-built fireplaces the original is a simple galvanized sheet. The Superior wood fireplace instructions, for example, tell the installer to fabricate the chase-top flashing from 28 gauge galvanized steel, with a hole and a 2 inch collar for the pipe, set on noncombustible spacers, and not to seal its ventilation openings.

That pan does its job when new. The trouble is what happens to its coating over years of sun, rain and standing water, and that is where the choice of metal for a replacement matters.

The Four Metals Compared

Each line below is about the material itself. The fit, the slope of the top and the fasteners matter as much, and they are covered further down.

• Galvanized steel: steel with a zinc coating; protected while the zinc lasts, then it rusts, first at cut edges and scratches; can be primed and painted.
• Stainless steel: the steel itself resists rust without a zinc coat; keep fasteners and flashing compatible with it.
• Aluminum: light and rust-free as a metal; must not sit in contact with galvanized steel or copper where water collects.
• Copper: does not rust and weathers to a patina; must be kept apart from galvanized and aluminum parts.

Why Galvanized Steel Is the One That Rusts

Galvanized steel is steel wearing a zinc coat. The zinc corrodes slowly and protects the steel underneath, but it is used up over time, and once it is gone, ordinary rust begins. The Majestic MPL11 fireplace manual is frank about the weak spot: its chimney cap metal has a rust-protective coating, but the cut edges of the parts are not protected. To prevent rusting and rust staining, it tells owners to wash exposed parts with detergent and paint them with a galvanized primer.

On a flat or sagging pan, water sits in puddles, and every puddle is a small area where the zinc works harder. The stains you see running down the chase siding below a cover are usually that rust on its way out of the metal.

Weather That Speeds It Up

The American Galvanizers Association lists five factors that set how fast a zinc coating wears outdoors: temperature, humidity, rainfall, sulfur dioxide pollution and air salinity. It notes that the zinc's protective film stays stable unless it meets aggressive chlorides or sulfides. Where a home sits in warm, wet weather, or close enough to salt water that sea air reaches the roof, more of those factors are working on a galvanized pan at once.

That is the honest answer for a home near salt water: no metal is promised a number of years, but a galvanized cover has more working against it where the air carries salt, and a stainless, aluminum or copper cover does not depend on a coating to survive it.

Gauge and Shape

Sheet metal thickness is sold by gauge, and the numbers run backwards: a smaller gauge number is a thicker sheet. The Florida Building Code, Residential, for instance, equates No. 26 gage steel with 0.0187 inch in its rule for chimney insulation shields. The 28 gauge named in the Superior instructions is thinner than that. A thicker sheet resists denting and sagging, and a pan that does not sag does not hold water.

Shape matters just as much. A cross break, two shallow creases running corner to corner, or a sloped or hipped top makes water run off instead of sitting in the middle. A drip edge on the skirt carries water past the siding. A collar tall enough to sit well above the pan keeps water from running down the pipe.

Mixing Metals on the Chase Top

A new cover is never alone up there: there are screws, the storm collar, the termination and sometimes old flashing. The American Galvanizers Association explains that when two different metals touch in a corrosive environment, one corrodes faster while the other is protected, and that a small part touching a large one of the other metal is the worst case. So a stainless or copper cover gets fasteners and collars that suit it, not leftover galvanized screws.

Choosing and Fitting a Replacement

We measure the chase, the pipe and the termination, and fabricate or order a cover in the metal you choose, with a slope or cross break, a drip edge and a collar sized to the listed termination. The ventilation openings the fireplace maker calls for stay open. Our chimney chase cover service covers the measuring, the fitting and the sealing, and our chimney repair work covers any rot the old pan let into the chase.

A cover is only as good as its fit, so our experienced crew measures every chase itself. We are a local, family-owned company and Proudly American. Book a free inspection, and the Proudly American family that runs the company will show you what the old pan has done and price each metal in a written estimate.