Main Factors: It is hard to find the best balance between style, longevity, strength, and cost. These factors do not naturally blend together. I work to reach the best balance possible on all the projects I build.

(a) Chimney caps need to handle a great deal of heat, unless the exhaust is just from a natural gas source. I have personally stoked up a wood stove so hot the steel exhaust pipe was glowing orange and lit up the living room. It was over 20 years ago, so I was not trying to test this theory. I was just burning some paper and cardboard waste.  You can see these statistics at a web site called Online Metals.

Most metals handle high heat well, but steel commonly has a thin zinc coating for rust protection that melts at a lower temperature and will result in rust stains or worse.

(b) A ticker heavier sheet metal is most always best, so that this structure is strong and less prone to wind damage. But that of coarse increases the cost of the metal used as well as the shipping charges a bit. Although, consider how projects like this the labor is far more than the material cost, so a thicker copper does not increase the cost as much as you might imagine. Compared to the flimsy 16oz copper most sheet metal shops would use 20oz copper feels 50% stronger, even though it is just 25% thicker. The 24oz copper is just 50% thicker, yet feels at least 2X stronger. 32oz copper is just 2X thicker, but feels 4X strongerr than 16oz copper sheet stock. The 48oz copper is sturdy enough to stand on supported by boards spred 2' apart, so it is stronger than the plywood on your roof. I often impliment more internal bracing than other shops as well. Most of our projects are priced by the weight of the copper, so when you can get 4X the strength for just 2X the cost that is a huge advantage for the $ spent.

(c) The cost of the metal type is a small consideration from your end for any custom work like this. Most of the cost is the design, custom fabrication, a shipping crate, truck freight costs, and the final installation. A 500% increase in the metal cost would only have about 20% increase in the total cost for this sort of project, so a better metal is by far your best value any way you slice it. Anything less is a waste of money. My custom copper chimney caps will normally increase the resale value of houses more than the cost difference, so you can make a profit from using better materials that will last. Even short term.

(d) A long life metal that will stay looking good decades later and need little to no maintenance is a much better value, but even more so in just the cost of labor to replace cheaper units more often. Rust stains from common steel units can be very unpleasant if the unit is not replaced every 10 to 15 years. A well made copper unit should last well over a century. That saves you the hassle of 7 or more replacement over that timeframe, beingmade heirloom quality.

The galvanized steel cap shown here was less than 15 years old.  It was so rusty, pitted, and worn thin that I could crush it with my bare hands like an aluminum pop can. You can see here dozens of daylight pin-holes through the metal (photo above left).  Two of the three bracing that held this hood up had disintegrated.  The strap clamp had disintegrated so badly that we were able to lift it off the stainless steel chimney pipe it was attached to without loosening the clamp.  The worst part was how it left terrible rust streaks down the sides of the stainless steel chimney pipe and on the roofing that are now pretty well permanent (photo above right). I tried to use a wire brush in it, but it did not seem to help.

The simple flat roof chimney cap most commonly seen like in these photos below is not the type of chimney caps I make.  Those are small generic size caps that are mass-produced and sold in many hardware stores.  It is normally designed to bolt onto a flue liner, but most of the chimneys I've seen do not have a flue liner tile protruding up over the bricks to attach that specific type of cap to.

That type of cap may work on your chimney, but it's not very decorative. It is fine for an inexpensive temporary solution. Steel can handle a good deal of heat, with a melting temperature of 2,500 degrees F. Although rain and heat will dissolve the zinc galvanizing away. The zinc melts off at only 787 degrees.  Even though the high temperature paints can handle more heat than a zinc coating, it is not much better, since it is prone to suffer UV damage from the sun and oxidization.  You can see these statistics at a web site called Online Metals.

If you let it go too long steel chimney caps will rust and make unsightly permanent rust stains down the side of the chimney and on the roofing shingles around it, that are near impossible to remove. Eventually it will deteriorate so badly that it will literally fall apart. If you factor in the labor cost of replacement each 10 to 15 years, spending $3,000 on 1 copper chimney cap is cheaper than getting an inexpensive $1,500 steel cap built the same way.  A steel chimney caps is not a good value for your dollar.  Not when a copper chimney cap should last well over 100 years, if built well. 

Steel is good for repeat sales; due to it's planned obsolescence.  That's the same reason we do not see more car bodies made out of aluminum, like with the Acura NSX sports car pictured below.

Aluminum (good for low temp natural gas exhaust only):

Aluminum would also need to be very well built and attached, since it is a lot softer metal and a lot lighter, and therefore is more prone to wind damage. Aluminum is great for aircraft construction where weight is very important, but it is not an asset for a chimney cap.

Stainless Steel (better):
This is a good strong high temperature metal, but it is just as expensive as copper; costing 6 to 10 times more than just galvanized steel.  It may look good outside a diner car, but one of the biggest drawbacks to stainless steel is how it will stay bright and shinny, will get dirty, and mildew on the North side (just like in this photo taken on the Pacific Coast, over a church near Cannon Beach, Oregon). It is easy enough to clean once you are able to safely access it, but that can be enough of a hassle that it just doesn't happen.

Stainless steel is preferred by most sheet metal shops because is quick and easy to just spot-weld together, so most shops will opt for stainless steel when forced to work with a rust free metal. As with copper other shops will usually use very thin sheets of stainless steel to to work with in order to help reduce the cost and make it a lot easier to cut, bend, and drill. The thinness of the metal will tend to show a lot of irregularities and buckles on a flat surface. Hence the big  'X' bend commonly seen in these flat panels. Most people think it is a design feature, but that actually has nothing to do with why they bend that 'X' through the larger metal panel.

One of the cautions with stainless steel products is how many clients have been swindled by getting just steel, which will begin to rust within a few years. Well past any chance to resolve a dispute over that fraud. Even if it is made with a high grade stainless steel that will not attract a magnet the fasteners may not be SS. Any product is only as good as it's weakest link.

Here is a example of a very nicely built 30" wide pizza oven, but at over $6k it cost more than one of my copper chimney caps that are 4X larger w/truck freight included. I'm flattered if their design was inspired by my work shown just below, but they deny it of course.

Copper (best): Other than cost and tricky handling copper is the best value for your money, since copper is one of the few metal meant to be seen in it's natural state or after oxidization. Being the same cost as stainless steel it seems the clear choice when given this choice.  The fabrication may cost a little more, since it is not as easy to work with. Copper will tarnish to a nice satin brown within a few months of exposure to the elements. The striated chalky green patina actually takes several decades to form.

We fabricate most of our custom copper work in 20oz copper sheets or thicker, which is 1/4th thicker copper than the standard 16oz copper normally used in the roofing industry.  Copper is a heavy and sturdy metal that handles around 2,000 degrees F. It needs no coatings over it for protection from the elements.  It has a melting point about 700 degrees higher than aluminum.

Unlike most other shops, we go to the trouble to rivet the overlapping seams together, since the melting point of the lead type solder is far less than even aluminum and less than the zinc coating on steel.  It would be terrible if the cap were to fall apart from the solder welds melting apart. We have seen plenty of evidence of this happening.  We have to design our chimney caps to be strong enough for high winds, and not to leak, without the use of solder or even caulk, and yet be easily removable for future cleaning and servicing.

Fabricators who are more about mass production will not work with copper, because it cannot just be quickly spot welded together like steel and stainless steel.  Handling is also tricky, since finger prints will cause the copper to tarnish sooner than the rest of the copper surface, making it spotty looking until it evens out.  So will wearing protective gloves while handling bare copper, but most of it will have a clear plastic film over the outside when you get it, that you simply peal off after installation.

New copper looks nice, but we have yet to find a good method to keep it from tarnishing.  Raw copper is actually more pink, but most people see it as an orange'ish color, since it has already has begun the tarnishing process by the time they see it just from the humidity in the air, beginning it's process to turn brown.  Any clear coat applied over the copper surface can be a problem, since it will not handle the heat well.  Even without the heat issue, the sun's UV rays and the elements will break the clear coat down before long, causing unsightly peeling and a spotty tarnishing where it has cracked and peeled off the copper surface.

Fortunately most people we have talked with seem to love the natural aged striated tarnish look that real copper gets, and some ask if we can pre-tarnish the copper here in our shop.  I explain how within a few months it will naturally tarnish, so it is not worth the added cost, but it can be done before shipping by applying an acid wash to pit the metal surface and cause this premature reaction. Not well advised if I may say.

Titanium (excellent, if you can afford it): Other than the fact I have not seen this sheet metal available because of such small demand given it's extremely high cost, it would be very hard to work with. As an example: I jokingly say how I would be willing to pay more for a titanium extension ladder to have a stronger lighter ladder, but the $5k it would cost replacing it when it gets stolen would be a real drag! I do have a titanium hammer and cats-paw that were about 4 times the cost of a normal steel tool of it's kind. They are great tools and I feel well worth what I paid. Real Titanium is truly an amazing metal, but they would have to be mass produced to be any where near feasible. The head on the hammer is still nearly as smooth as when I first got it. I am eager to see more products and tools made from this metal, but not many contactors are willing to pay 3 to 4 times as much for a tool for their employees, so few manufacturers are willing to make and market them. You also will need to be careful not to get swindled with an inferior grade or a complete substitute. Think of how many products are advertised using the word platinum or turbo charged (LOL). Real platinum is more expensive than gold for goodness sake!