Think Zinc For Metal Roofs And Walls
By Tom Diamond, E.I., Garland Industries
What is the first metal that you think of when considering design options for a new standing seam roof or wall panel system? Are steel and aluminum your primary choices, or do you lean more towards exotic metals such as copper or stainless steel? When was the last time you considered zinc? Its manufacturability and resistance to weathering are only a few of the benefits of this beautiful metal that are driving the increasing popularity of zinc for building cladding.
Beauty And Longevity
Zinc’s striking aesthetic makes it an attractive alternative for metal roofs and walls. Photo courtesy of IMETCO. Click image to enlarge.
The physical properties of zinc make it a very unique and useful material to be used as building cladding. Many of us are familiar with the oxidation process of steel, which forms rust. Steel will continue to corrode when rust is present on it surface, eventually rendering it unusable. Zinc, on the other hand, has the ability to resist continued corrosion due to a very unique reaction. When zinc is exposed to the moisture and carbon dioxide that is present in our atmosphere, a protective layer of zinc carbonate forms on its surface, prohibiting the corrosion process that steel experiences. The resulting zinc-carbonate layer creates a beautiful bluish patina for a very desirable appearance.
In addition to its beauty, this protective barrier provides longevity that will allow zinc to protect a building’s exterior for years to come. It is not uncommon for a properly designed zinc roof or wall panel system to last over 60 years in most climatic conditions. Another unique characteristic of zinc is that it can “heal” itself overtime. As it continues to patina throughout its life, scratches and imperfections that were once present will virtually disappear.
There are however certain conditions that will prohibit the protective patina from forming. If zinc is exposed to moisture, from humidity, condensation or even submersion in water without the presence of carbon dioxide, white rust will form. Just as with rust on the steel surface, this white rust will continue to deteriorate zinc. This condition can occur on the backside or underside of a zinc panel that is not exposed to the exterior environment where carbon dioxide is prevalent. To prevent the white rust from forming, the backside or underside of a standing seam roof panel or wall panel, or the interior side of a gutter, should be always be coated to prevent deterioration of the zinc. Ventilation should also be provided behind panels to promote air flow that will move away any moisture that may accumulate due to condensation.
Clean And Green
Zinc is also a very environmentally friendly metal for a number of reasons. It is a 100 percent recyclable metal that can be reused over and over again. Zinc is also a fungistat, which is an agent that prohibits the reproduction of mold, mildew and fungus. This can greatly reduce the risk of moss or even black mold from forming on the exterior of a building in areas that are in contact with the metal.
Interestingly, the rainwater run-off from zinc is clean, unlike the rainwater run-off from other metals such as copper, which is a fungicide. However, in addition to killing fungus, fungicides can also kill vegetation. So zinc, unlike copper and other metals, can be used in areas where vegetation is present, as it will not damage or compromise plant growth.
Zinc is a very soft and malleable metal that can be formed into desirable shapes with ease. This characteristic allows zinc to be used for a wide range of unique architectural applications. However, it should be noted that zinc becomes much more brittle when exposed to temperatures below 50 degrees Fahrenheit. Forming zinc in temperatures this low will cause it to rupture very easily. Therefore, it is not recommended to break, form or install zinc panels or trim components in cold weather. Even mechanically seaming a standing seam roof in cold temperatures can cause the seam cap or seam legs to crack, rupture or split. If the installer is unsure of the temperature, it is recommended that the surface temperature of the zinc material be measured with an infrared thermometer to assure it is above 50 degrees Fahrenheit.
Thermal Expansion And Contraction
Preventing direct contact with dissimilar metals that have a high electrical potential in relation to zinc is critical to successful zinc applications. Chart courtesy of IMETCO. Click chart to enlarge.
With all standing seam metal roofs, regardless of the material used, it is important to allow for unlimited thermal movement so that the panel can expand and contract without restriction. It is especially critical to allow for sufficient thermal movement for zinc panels, as it can expand over twice the distance of a steel panel. Zinc will actually expand almost an entire inch for every 30 feet of panel when exposed to a temperature differential of 160 degree Fahrenheit, which is very common throughout much of the United States. If the panel is restricted by the clip or fasteners when the expansion or contraction of the metal occurs, the panel will buckle and billow. This will increase the oil canning effect that is more prevalent in standing seam panels that have been improperly installed. Restricting the movement of the panels will also cause the sealants that help make the system watertight to break down over time. Trim components should also be given approximately ¼ inch at end laps to allow for thermal expansion.
Galvanic corrosion also needs to be considered when designing zinc panel or trim applications. Galvanic corrosion occurs when two dissimilar metals with a high electric potential between the two are in contact with each other in the presence of an aqueous solution. The higher the electric potential between the metals, the more severe the galvanic reaction will be. Referencing a galvanic corrosion chart will give you a good idea of which metals will react with each other. The further apart these metals are on the chart, the larger the electric potential will be. The effects of galvanic corrosion are much less severe when metals that are close to each other on the chart are in contact with each other.
When designing with zinc, care must be taken to enable adequate expansion and contraction due to this metal’s comparatively large propensity for thermal movement. Chart courtesy of IMETCO. Click chart to enlarge.
Viewing the accompanying chart (on the left), it is clear that bare zinc should not be placed in contact with metals such as copper or non-galvanized or unprotected iron. However, zinc can come into contact with aluminum and galvanized steel with minimal galvanic reaction.
It is also critical that zinc not be installed in locations that will receive rainwater runoff from dissimilar metals. The runoff will carry electrically charged ions from the metal that will cause galvanic corrosion of the zinc. If zinc will be in contact with a dissimilar metal after installation, it is imperative that the two metals be separated. Protecting the surface of the zinc with a plastisol coating will help to eliminate the risk of galvanic reaction from occurring.
Other considerations that need to be taken into account when designing zinc applications include the risk of deterioration due to abrasive or corrosive chemicals. Zinc should not be installed near salt water coasts that receive little or no rainfall, as the salt spray in the atmosphere can have a damaging effect on the unprotected zinc material if it is not frequently washed away by rain.
Unprotected zinc should not be in direct contact with acidic woods such as oak, birch or cedar. Other woods such as pine, spruce and poplar have a lower acidity and are less abrasive. If zinc is in contact with an acidic wood, it should be protected with a plastisol coating.
Concrete and felts hold moisture that can damage bare zinc in enclosed areas where carbon dioxide is not present. If zinc will be in contact with bituminous felts or concrete, it is recommended that the zinc be protected with a coating. It is also recommended that the area between the zinc and concrete be ventilated to promote good air flow.
Protective gloves and long sleeves should be worn when installing zinc to prevent bare skin from contacting the panels. Oils in the skin that contact bare zinc can create undesirable marks. While these marks will disappear over time, it will create an unsightly blemish on a newly installed panel. A strippable clear film should be applied to zinc to help protect it during shipping, handling and installation, and should not be removed until the panels are installed and all finish work is complete.
When installed properly and appropriate considerations are taken to assure its surrounding are suitable, zinc is an aesthetically pleasing, long-lasting and beneficial alternative for building cladding. Very few metals, practical for use in building design, offer such an attractive array of characteristics, making zinc an easy choice for many designers. Its architectural appeal is continually growing as more designers are realizing the benefits of this beautiful metal. So whether you are a building owner, designer or contractor, the next time you are faced with an opportunity to specify metal, think zinc!
The author, Tom Diamond, E.I., is a product engineer for Garland Industries, a Cleveland, Ohio-based manufacturer and distributor of high performance solutions for the commercial building envelope. In that capacity, he designs optimum performance metal roof and wall panel systems and oversees metal system building code compliance at the national, state and local levels. As a member of Garland’s Speakers Bureau, Diamond presents educational seminars on how to implement edge metal designs for code compliance.
The Garland Company Inc. is one of the worldwide leaders of quality, high-performance roofing and building envelope solutions for the commercial, industrial and institutional markets. For over 120 years, Garland has continually developed unique product and service offerings that have raised the bar of performance while exceeding the individual needs of customers throughout the world. Today, Garland's network of over 200 local building envelope professionals is strategically positioned throughout the United States, Canada and the United Kingdom to provide quality building envelope solutions for single and multi-property facilities. The Garland Company Inc., headquartered in Cleveland, Ohio, is an ISO 9001:2015 certified company. For more information about Garland, visit www.garlandco.com or call 800-321-9336 to be connected with a local Garland representative.
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