Copper vs Stainless Steel Chimney Caps

What the cap has to do

Whatever it is made of, a chimney cap does the same job. The lid keeps rain and snow out of the flue, where water would rust the damper, soak the smoke chamber and crack clay tile as it freezes. The mesh sides keep out birds, squirrels and raccoons, and on a wood-burning flue they catch larger embers. A well-shaped hood also takes the edge off wind blowing across the top of the flue.

The metal decides how long the cap lasts and how it looks over the years. It does not change how well a properly sized cap keeps water and animals out.

The metals caps are made from

Most caps on houses in this region are one of three metals, plus painted versions of them. Each has a place.

• Galvanized steel: the lowest-cost option and the first to rust, usually at the legs, base and screws
• Stainless steel: stays bright, resists rust and handles the exhaust from most fuels
• Copper: weathers to brown and green, mostly chosen for appearance
• Black or painted caps: usually steel underneath, so ask what the base metal is

How stainless steel weathers

A stainless cap looks much the same years after it goes up, apart from soot on the mesh and a slow dulling of the shine. It stands up to rain, snow, salty coastal air on Long Island and the Jersey Shore, and the acidic exhaust from oil and gas appliances far better than plain or galvanized steel.

Stainless is not one product, though. Grades and thicknesses vary, and a thin sheet with flimsy mesh and poor welds can still pit and rust at the seams. When comparing caps, look at how heavy the metal feels, how the mesh is attached, and whether the fasteners are stainless too. A good lid held on by plain steel screws will fail at the screws.

The mesh deserves the same attention as the lid. It is the part that sees the most heat and soot, and on a cheap cap it is often the first piece to tear, sag or pull away from the frame, which leaves an opening that a squirrel finds quickly.

How copper weathers

New copper is bright orange-pink. Within months outdoors it darkens to brown, and over a longer stretch it can develop the blue-green patina people picture on old church roofs. How fast that happens depends on rain, air and exposure. Some owners love the change and some do not, so it is worth deciding before the cap goes on rather than after.

The part people miss is the runoff. Rain that washes over copper carries a little of the metal with it and leaves green streaks on what it drips onto. On a dark red brick chimney it is rarely noticed. On a brownstone or limestone facade, a pale brick chimney, white trim or a light-colored roof, the streaks can be obvious and hard to clean off.

Mixing metals

When two different metals touch in the presence of water, the less noble one corrodes faster at the contact point. Copper in direct contact with steel, galvanized parts or aluminum flashing is a classic example: the steel or aluminum near the joint wears away. A copper cap should be fastened with copper or compatible fasteners and straps, and kept from touching other metals on the crown or the flashing.

The reverse also applies. If a house already has copper flashing or gutters, a galvanized cap mounted where water from the copper runs over it can rust out sooner than expected.

Matching the cap to the fuel

The fuel matters more than the metal. Wood-burning fireplaces need mesh fine enough to stop larger embers but open enough that it does not clog quickly with creosote and choke the draft. Oil flues produce soot that can build up on fine mesh, so the mesh size and the cap style are chosen with that in mind. Gas appliances, especially water heaters and boilers, may have specific requirements in their installation instructions for how the flue ends.

In many older homes one chimney holds a fireplace flue and a boiler flue side by side. They may need different caps, or a multi-flue cap that covers the whole crown and suits both. We check what each flue serves before choosing anything.

Fit, fastening and the crown underneath

A single-flue cap clamps onto the top of one flue tile or liner. A multi-flue cap sits on the crown and covers several flues under one lid, which is common on brownstones and rowhouses where flues end close together. Either type has to be measured to the flue or the crown, not guessed, and fastened so a nor'easter cannot lift it.

The cap is only as good as what it sits on. A multi-flue cap bolted to a crumbling crown loosens as the crown breaks up, and a single-flue cap clamped to a cracked tile can tip. When the crown is cracked, it gets sealed or replaced before a new cap goes on, whatever the metal.

Checking your current cap from the ground

Binoculars from the sidewalk or the yard tell you a surprising amount. Orange streaks running down the crown and the brick below usually mean a galvanized cap is rusting. A lid sitting crooked, a cap leaning to one side or a gap between the cap and the flue mean it has come loose. Mesh that looks solid black from below may be clogged with soot or creosote.

Green streaks under an existing copper cap are normal wear rather than a fault, unless they bother you on a light facade. Inside, rust flakes on the damper or water in the firebox after rain suggest the cap is missing, damaged or too small for the flue.

Which one fits your chimney

For a working fireplace or a boiler flue on a typical brick rowhouse, colonial or twin, stainless steel is the practical pick. It lasts, it fits almost any fuel and flue, and it does not change color or stain the masonry. Copper earns its place when the chimney is a visible part of the house from the street, the house has other copper details like gutters, bays or flashing, and the owner wants the cap to age with the building.

On a landmarked house or in a historic district, check whether visible rooftop changes need review before choosing a cap that stands out. Once we have measured the flues and seen the crown, both options go into a written estimate so you can compare them side by side.