Chase Cover Materials: Aluminum, Galvanized, Stainless, Copper

What a chase cover has to survive

With a factory-built fireplace, the metal chimney pipe usually climbs through a boxed-in, wood-framed shaft known as a chase. The chase cover closes the top of that enclosure, with a collar where the chimney pipe comes through. Above it sit a storm collar on the pipe and the chimney's listed termination cap.

That one piece of metal deals with standing water if it sags, constant sun, daily temperature swings, snow load, wind-blown debris and the fasteners and sealant at the collar. When it fails, water goes straight into the chase, onto the framing, the chimney pipe and the fireplace below. Our chase cover page describes what that damage looks like.

Manufacturers treat the chase top as part of a working water-shedding detail. Hearth & Home Technologies' instructions for one of its chimney cap adapters, as an example, state that a chase top must be used in a chase installation, call for a turn-down and drip edge to keep water from seeping into the chase, call for sloping the chase top downward from the penetration, and say the storm collar should cover the opening in the chase top.

Shape does half the work

A flat panel holds water. A puddle that sits for days after every rain gives corrosion the one thing it needs: a wet surface. A cover that slopes from the collar to the edges, or has cross-breaks or a raised center that drains it, stays wet for minutes instead. The raised collar keeps water from running into the pipe opening, and the storm collar above it sheds water over the collar. The skirt laps down over the siding, and the drip edge throws water clear of the wall so it does not run behind the trim.

This is why a well-shaped cover in a modest metal can outlast a flat cover in a better one. When you compare materials, compare shapes at the same time.

Galvanized steel: a coating that wears away

Galvanized steel is ordinary steel coated with zinc. The zinc protects the steel in two ways: as a barrier, and by corroding in its place. The American Galvanizers Association says zinc generally corrodes at about 1/30 the rate of bare steel in the same environment, and that the corrosion resistance of a zinc coating is determined primarily by the thickness of the coating, varying with the severity of the environment.

Two things follow for a chase cover. First, its life depends on how much zinc is on it and how long water sits on it. Thin coatings on sheet steel and flat, ponding panels are the worst combination. Second, failure has a recognizable look: once the zinc is used up in a spot, the steel underneath rusts, and orange or brown streaks run down the siding below the chase. See rust and brown stains on a chimney.

Galvanized steel is strong, stiff and easy to form, and it is widely available. Painting adds a layer of protection that has to be maintained.

Aluminum: a film that repairs itself

Aluminum protects itself differently. ASM International's handbook chapter on aluminum corrosion describes aluminum and its alloys as highly corrosion resistant, protected by a self-healing oxide film that passivates the surface. When the surface is scratched, the film reforms. Aluminum does not rust in the way steel does, so a failing aluminum cover does not leave rust streaks.

Its practical traits for a chase cover:

• Light weight, which makes a large cover easier to handle and install.
• Softer than steel at the same thickness, so stiffness comes from the metal's thickness and the shape: bends, cross-breaks and a formed collar.
• Sensitive to some neighbors. The Copper Development Association names aluminum and zinc as the principal metals of concern in direct contact with copper, and says wash from copper surfaces should be prevented from draining onto exposed aluminum. Fasteners and any metal it touches should be compatible.

Chimney 911's chase covers are aluminum, custom-made to measure for each chase, including the collar position.

Stainless steel: chromium does the protecting

Stainless steel is steel alloyed with chromium. The Specialty Steel Industry of North America explains that chromium gives these steels their corrosion resistance and that an alloy must contain at least 10.5 percent chromium to be called stainless. It needs no applied coating.

Grade matters. SSINA's roofing and flashing handbook describes Type 304 as the alloy most often specified for roofing and flashing, and Type 316, which contains 2 to 3 percent molybdenum, as more corrosion resistant and usually preferred for long-term service in aggressive industrial, chemical and seacoast atmospheres, at a higher initial cost. For a house close to the New Jersey shore, that distinction is worth asking about.

SSINA also warns that fasteners made from other materials, even with an anti-corrosion coating, will eventually corrode and cause rust staining. A stainless cover held down with plain steel screws is only as good as the screws. Stainless is stiff and strong for its thickness, which keeps a large cover flat where it should be flat and crisp where it is bent.

Copper: long-lived, with a few rules

Copper is chosen for appearance as often as performance. It starts bright and darkens, and with exposure it develops the familiar green patina. The Copper Development Association explains that patina forms through oxidation in the atmosphere, and its weathering chart shows the color changing over many years depending on climate.

The CDA's design guidance raises three points for anyone considering copper:

• Staining. Water running off copper picks up small amounts of copper salts, which can stain porous, light-colored materials such as limestone or marble. The CDA recommends drip edges and directing runoff away from such surfaces.
• Neighbors. Copper is not harmed by contact with the more active metals, but aluminum and zinc are, so copper runoff should not drain onto aluminum or galvanized parts.
• Concentrated runoff. When water collected over a large surface is directed onto a small copper piece, the copper may deteriorate before it develops a protective patina.

Soldered copper seams, the CDA notes, should last the life of the copper. Copper is soft and easy to form, so thickness and stiffening matter here as they do with aluminum.

Where metals meet: the galvanic problem

A chase top is rarely one metal. There is the cover, the storm collar, the termination cap, the fasteners, and often flashing or gutters nearby. When two different metals touch in the presence of water, one of them can corrode faster than it would alone. This is galvanic corrosion.

The American Galvanizers Association explains that zinc is more anodic than copper, stainless steel and aluminum, so in contact with them the zinc corrodes while the other metal is protected, and that a large area of the more noble metal touching a small area of the less noble one makes things worse. Its recommended defenses are electrical isolation with non-absorbent spacers or gaskets, durable paint on the joint, and choosing metal pairs that sit closer together on the galvanic series. The CDA likewise recommends taping or gasketing with nonabsorptive materials to separate copper from other metals.

In practice that means matching fasteners to the cover, looking at what the cover's water will run onto, and paying attention to old parts left in place when a cover is replaced.

The four side by side

None of the four is the right answer for every chase. All of them can perform when the cover is shaped to shed water, fastened with compatible hardware and kept apart from metals it reacts with. Price is driven by the size and shape of the chase, the number of flue openings, the metal and its thickness, custom fabrication, and whatever repairs are found under the old cover, such as wet framing or a damaged cap.

Chimney 911 installs aluminum chase covers, custom-made to measure. If you prefer stainless or copper, the points above are what to compare when you talk to a fabricator who works in that metal. Either way, the old cover's removal is the moment to check the framing, the chimney pipe and the cap. See factory-built fireplaces: care and limits.

Questions homeowners ask

Sources

1. Hearth & Home Technologies: CT-14A cap adapter installation instructions (PDF)
2. American Galvanizers Association: How long does hot-dip galvanizing last?
3. American Galvanizers Association: Dissimilar metal corrosion with zinc
4. ASM International Handbooks: Corrosion of Aluminum and Aluminum Alloys (chapter abstract)
5. Specialty Steel Industry of North America: Standard Practices for Stainless Steel Roofing, Flashing and Copings (PDF)
6. Copper Development Association, Copper in Architecture Design Handbook: Architectural Considerations

This guide is general information. It can't diagnose a specific chimney; an inspection can.