Copper is a semi-precious metal that has earned its place as a beautiful choice for exterior design accents and even entire roofing systems. You can find copper roofing systems on prominent buildings throughout the United States and Europe. I'm a native of Asheville, NC of which the downtown skyline is known for the inclusion of a prodigious domed roof on the First Baptist Church of Asheville that features an intricate copper cupola and copper tile roofing system.
Why copper?
Why use copper as a metal roof instead of a lower cost material like steel or aluminum? There are numerous reasons why copper may be a more suitable fit for any given metal roofing situation.
1. Displaying Grandiosity
As stated earlier, copper is a semi-precious metal. It's character is unmatched by other metal materials and it is rarer than steel or aluminum and is thereby much more expensive. You often find copper roofing systems on elegant homes or prominent architectural buildings for this reason.
2. Longevity and Efficiency
Copper is a durable material that when properly installed and maintained can last 50-100 years or more as a roofing system. Copper is also very friendly to the environment. It's prominent sheen makes it is an excellent choice for maximizing energy efficiency inside a structure. It is also 100% recyclable at the conclusion of it's life.
3. Malleability & Ease of Installation
Copper is a soft metal which makes it an excellent choice for intricate designs featuring domed, curved or otherwise convoluted roofing systems. While material costs for copper are high, installation is quick and easy for an experienced roofer. Copper is very lightweight and easy to handle and its ductility make it easy to install over irregular roof surfaces. New tools and installation methods developed in recent years have streamlined old processes to aid in a quick, proper, and economical installation.
Noble in more ways than one
All metals have a property called nobility. This is a measure of the metal's resistance to corrosion when in contact with another metal. A greater relative difference in nobility between the two metals in contact represents a greater corrosion potential. The table below ranks the most common metals used in construction in order of increasing nobility, called the galvanic number. Metals closer to the anodic side of the scale are less noble and metals closer to the cathodic side of the scale are more noble.
When dissimilar metals are in contact with one another in the presence of an electrolyte (like rain water or humidity), an event known as galvanic action occurs (a conductive path for electrons and ions to move from one metal to the other). Galvanic action results in the deterioration of the metal with the lowest galvanic number (less noble). Since copper has a higher galvanic number than other metals it won't be affected by them. However, less noble metals can be affected by contact with copper. The solution is to prevent direct contact between the dissimilar metals by using specific paints, gaskets, or physical distance. The principal metals of concern are aluminum and zinc (and galvanized steel).
Copper can be affected by another type of corrosion called "erosion corrosion". This type of corrosion is caused by the flow of acidic water concentrated on a small area of copper. Such an event may occur when rain falls on a non-copper roof such as tile, slate, wood, or asphalt. The acidic water is not neutralized as it flows over the inert material. When water, collected over a large surface, is diverted to a relatively small copper flashing like a drip edge or gutter, the copper is at risk of deterioration before it can develop a protective patina.
Selecting a substrate
This is another particularly important part of using copper as roofing material. When copper is used as a vertical metal panel like standing seam panels it requires a nail-able deck like plywood (the most common substrate for copper is 1/2" or 3/4" plywood). Lumber should be kiln-dried and all joints true to provide a smooth surface for installation.
There have been many recent developments in fire retardant treated (FRT) plywood and lumber. Most of these products use wood or plywood that is pressure-impregnated with chemical salts in water solution to inhibit combustion. Many of these salts are corrosive to copper, as well as other metals and materials. If leaching of theses salts brings them into contact with the copper, corrosion will occur. In regard to such corrosion events, areas prone to condensation, high humidity or where water is introduced during construction or at a later time are particularly auspicious. Any areas where salt laden moisture can collect then evaporate, thereby increasing the concentrations of salts, will accelerate the corrosion process.
In conclusion, Care should be taken when installing copper around dissimilar metals but is nevertheless a dependable roofing material that has proven itself as such for centuries. Copper comes at a premium cost but will return the investment with a splendid display of opulence and a lifetime of durability and efficiency.