The gray-colored zinc patina develops over time as the material weathers naturally; it is a process similar to the way copper turns brown then green. With zinc, the patina is actually a layer of zinc hydroxy-carbonate that develops - typically over 2 to 5 years - as the material is exposed to carbon dioxide in the air. The time it takes for the patina to emerge depends upon environmental factors such as air quality. In areas with a higher concentration of pollution, the patina appears more quickly. Likewise, zinc used in interior applications will rarely, if ever, develop a patina. Once formed, however, the layer is compact and insoluble to rainwater which thereafter controls the rate of corrosion. This patina is said to be a self-healing protection for the zinc. If it becomes scratched or is removed, it will naturally redevelop from continued exposure to rainwater and carbon dioxide. The scratch would appear as a shiny area while healing. Once the patina develops, the zinc has a uniform and consistent color of gray for its lifespan.

Zinc manufacturers today offer pre-patina materials (also called pre-weathered) with a dark gray look that is very true to the dull gray that all zincs eventually acquire after natural weathering. They also offer factory-applied transparent coatings that include subtle blues, red and greens. Over time these materials will all develop the natural gray zinc patina. The length of time for this to happen, however, still varies with air quality. Typically, these factory-applied colors will last up to 30 years before gradually changing to the natural gray patina.

Architect Bruce Norelius of Elliott Elliott Norelius Architecture in Maine said the firm began researching zinc after becoming interested in materials being used in Europe. They've used zinc now in several projects where it compared favorably to other metals. "Zinc has very little reflectance. Here in a coastal area by the water, people are conscious of that. Zinc has that low reflectivity and beautiful appearance that gets better with time." 

Located at number thirty on the Periodic Table, zinc is an essential element. The zinc metal used in building products is an alloy that meets the European standard EN 988. While the U.S. doesn't have a similar standard, most of the zinc building materials sold here adhere to EN 988. The standard dictates the dimensional tolerances of the metal - such as thickness, width, length, saber and flatness - and also dictates the level of trace elements that are contained in the mix. These elements are copper (0.08 − 1.00 percent), titanium (0.06 − 0.20 percent) and aluminum (less than 0.015 percent), all of which contribute to the unique characteristics that distinguish zinc EN 988. The copper slightly increases the mechanical resistance of the alloy, where as the titanium provides increased "creep resistance."  Creep is an occurrence in which the zinc will actually become thicker at the bottom of a roof than at the top as it ages. It actually "creeps" down the roof over time. Titanium helps protect the zinc against creep. Design professionals can confirm with a supplier that the material specified conforms
to EN 988.

 Beyond the patina, zinc manufacturers are educating building professionals about other misunderstandings related to the product. First, while it is true that the initial cost for zinc building products is higher compared to other choices, overall costs for maintaining the product, when factored by its years of longevity, actually yield a more favorable return on investment for owners when compared to other materials. Second, building professionals experienced at using copper sometimes assume zinc is a similar metal and fail to take into consideration its specific installation needs, therefore some avoidable problems have been a deterrent to the use of zinc. A poor installation can result in corrosion and improper adjustments for the metal's expansion and contraction. Finally, a perception that zinc is not environmentally friendly is being reevaluated. Some of the ways zinc contributes to sustainable building include recycling and recycled content, low or harmless levels of leaching into adjacent soil and the low embodied energy requirements needed for manufacturing. These advantages will be explained further in this article.

FLEXIBLE ZINC

Architectural metals such as stainless steel, copper, aluminum and zinc bend to accommodate curves, textures and other design elements, thereby providing architects with more options for creating unique building forms. Zinc contributes to the architectural styles possible today, a trend seen in such projects as Will Bruder's Nevada Museum of Art in Reno and the entrance to The Centers for Disease Control in Atlanta.

According to Dean Marchetto, "[Zinc] has qualities that lend itself to a modern aesthetic. It can be shaped into a variety of patterns, tiles, sheets and panels which offer architects almost unlimited opportunities in terms of the way it is applied to a building façade or a roof."

A perception that zinc is hard to work with and install is being overcome through manufacturers' efforts to explain the product's specific needs. Many manufacturers work with building professionals during construction to advise on installation, and many strongly recommend or even require training prior to installation.

The University Town Center II in Hyattsville, MD features flatlock and interlocking zinc panels. Architect: WDG Architecture, PLLC. Installer: Roofers Inc, LLC. Photo courtesy of Umicore Building Products

 

One aspect of working with zinc is the metal's expansion and contraction. Zinc will move approximately one inch in thirty feet and so the design must be engineered to meet this thermal movement. To accommodate, a combination of sliding and fixed clips must be installed in zinc roofing and wall systems. Fixed clips alone will not allow sufficient expansion and contraction. The sliding clips, however, consist of two-parts in which one part can slide to accommodate thermal movement.

Temperature is another misunderstood aspect of working with zinc and is particularly important when bending the material on site.  The metal is malleable but can become cold and brittle at lower temperatures. Therefore, zinc should not be folded when the metal is at temperatures of less than 45 degrees. Crazing, which is seen as long wrinkles in the material, can occur if this happens. When working in temperatures below 45 degrees, installers must use a heat gun to warm the material to the right temperature for bending and installation. Therefore, it is important to factor temperature into construction scheduling when planning to bend or fold the material on site.  Once the material is installed correctly, however, low temperatures do not cause problems because accommodations have been made for thermal expansion and contraction of the metal. Pre-manufactured wall panels and flashing limit the need for bending zinc on the job site. When bending zinc is necessary, installers typically use a heated enclosed area inside the building. This is a standard technique used with all metals in regions with very low temperatures.