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It is very nice to use a roofing material that develops a patina with time. The patina often helps to protect the metal from corrosion. It is a living finish that changes and evolves with conditions, as opposed to paint, that is a flat, unchanging surface that only evolves in the sense that it gets scratched and dirty over time. Patinated metals can be naturally healing and are generally very long lived. Some examples of such metals are copper, zinc, and cor-ten steel.
As a non-ferrous metal, Zinc is naturally self-protecting. With time (usually 2-5 years), it forms a light grey matte patina. The patina is created when the zinc metal reacts with water and oxygen to form hydroxyde, then with carbon dioxide to form a layer of hydroxycarbonate. This natural patina protects zinc and naturally reforms when accidentally scratched.
Zinc roofing usually starts off shiny, but can be installed with a patina already created. Some sources suggest that this can enhance the longevity of the material by preventing early accelerated corrosion.
While the basis of the patina is alkaline zinc carbonate, additional substances
are incorporated from the local environment. This is why the color of the patina can vary slightly from one place to another.
The patination speed can vary between six months and five years, depending on climactic conditions & roof slope. The more exposure to wetting and drying cycles, the quicker the patina will develop.
Summary of recommendations for Zinc roofing
Recommended thickness of material:
.030" (0.8mm) thick (22 GA) or .027" (0.7mm) thick (24 GA)
EN988, ALLOY 710 material
According to the metal roof guide by the NRCA: "Compared to previous zinc alloys, current zinc alloys have a lower coefficient of expansion, improved tensile strength, and improved cold-weather formability"
Zinc Sheet typically comes 1-meter (39.4") wide x up to 3.048m (10') long
- According to many sources, It is important to ventilate or protect the underside of the zinc roofing
- According to the National Roofing Contractors Association, It is important to avoid contact with copper and copper runoff at all times. Also avoid contact with red rosin paper, wet mortars, cement, & acidic sealers. Red Cedar. Larch, and Chestnut can also cause damage to the material.
- According to the NRCA, the field temperature during fabrication should be greater than 48 degrees F to prevent fracturing.
- Properly installed, your zinc roofing should last at least 60 years. Some Zinc roofs are well over 100 and going strong. Unfortunately your roofer will likely be dead at that point and will not be able to honor your warranty.
Zinc has many environmental benefits when used as a building material:
1. The amount of energy used to produce zinc from ore is the lowest of all non-ferrous metals. Energy consumption is even lower when zinc is produced from recycled material.
The primary energy required to manufacture zinc sheets and coil is substantially lower than most metals for two reasons. First, the concentration in zinc ore is higher than other metals in their respective ores. Second, zinc has a relatively low melting point, approximately 785 F, and thus a relatively low level of fuel is required to extract, smelt, alloy, roll, and fabricate the metal. By comparison, the primary energy content of zinc is significantly less than that of aluminum, and also less than that of copper or stainless steel. Less energy is used for refining and recycling zinc because of its low melting point, whereas copper and stainless steel need twice as much energy, and aluminum even four times more.
2. Zinc is 100% recyclable. Recycled zinc conserves 95% of its initial energy
content. When a zinc installation reaches the end of its service life,
the material is 100% recyclable, without degradation or loss of
3. Zinc’s long life span means
that it seldom needs replacement. Zinc roofs have been know to last hundreds of years.
4. Because of its light weight, shipping costs are much less than with stone or brick.
History & manufacture of zinc as a building material
The website of the International Zinc Association has a lot of information on the history, manufacture, properties, and uses of zinc.
5. There are different opinions on the toxicity of zinc. According to industry sources, runoff from a zinc roof is "non toxic," "It is also a non-corrosive, environmentally friendly product with a 100% clear water runoff." unlike lead and to some degree copper
The AIA Continuing Education Center published an article: Essential Zinc: Building For The Future (Sponsored by Umicore Building Products) that details the effect of runoff from zinc roofs, claiming that if not actually beneficial to life, it is at least not harmful.
This appears only to be true when concentrations stay below a certain threshold. It seems that in most cases large concentrations of zinc in the environment are caused by things other than architectural metals. It is not far fetched to assume that the other benefits of architectural zinc outweigh potential hazards.
I found many articles and studies that showed that concentrations of zinc in San Francisco Bay are actually above the safe, background threshold.
Here are a few research papers that examine the effects of high concentrations of zinc in waterways. These high concentrations seem mostly to result for mining and industrial activities, not runoff from zinc roofs:
There are several different alloys of zinc that are commonly used, but Alloy 710 is preferred for architectural applications.
Alloy 190 - copper hardened zinc - good strength & ductility (0.7-0.9% copper) Typical uses: Plumbing hardware, bright automotive trim, cable wrap, EMI-RFI shielding, coinage, expanded metal, terrazzo stripping
Alloy 500- commercially pure, softest of all zinc alloys (0.001 to 0.004% titanium, less than 0.003% copper) Typical uses: Lens finning pads; galvanic protection where the specification dictates high purity; die striking of ornamental parts where die filling is difficult.
Alloy 710 - copper hardened and titanium refined zinc- good strength and ductility with better creep resistance than most zinc alloys (0.10-0.25% copper, 0.06 to 0.10% titanium) Typical uses: Architectural applications, painted or powder coated hardware parts, low amperage electrical conductor.
Alloy 750 - copper hardened and titanium refined zinc- good strength and moderate ductility; best creep resistance of zinc alloys (0.50 to 0.70% copper, 0.12 to 0.18% titanium) Typical uses: Automotive fuses, leaf spring interliners, low voltage electrical terminals, bussbar.
According to Janet Zaso in her AIA Continuing Education article, "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, whereas 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"
Workability & Architectural Benefits
Compared to other architectural metals, zinc alloy is soft. Titanium improves zinc’s tensile strength and hardness, copper imparts color and malleability, and aluminum hides the spangles commonly seen on galvanized hot-dipped surfaces. Zinc alloy still has the same malleability and patina of natural zinc.
Because of its softness, zinc can be worked in the field with hand tools. It can be formed into curves and unusual shapes more easily than other metals.
Another advantage to zinc roofing is that runoff does not stain other building materials.
Sources of more information on zinc building materials
- Aia continuing ed article sponsored by RHEINZINK America, Inc.
- Contrarian Metal Resources: Metallic Corrosion
- The Metal Initiative - Zinc
The maintenance-free aspects of zinc are making it an increasingly popular commercial construction element. Learn more about how this durable metal can benefit your next project.
- Metal Roof Company - Products and Services
- AIA Continuing Education Article
Essential Zinc: Building For The Future Low-maintenance zinc gains popularity for buildings that last Sponsored by Umicore Building Products By Janet Zaso April 2009
- Atlas Metal Sales
- International Zinc Association
Titanium zinc for roofing and facade cladding - RHEINZINK is the trademark for our zinc copper titanium alloy, specifically developed for the building industry.
source for zinc sheets
- UMICORE | VM ZINC WORLD | VMZINC.COM | Standing Seam Roofing - VM ZINC Custom-built Standing Seam Ro
As for galvanic corrosion, zinc is not compatible with copper or iron, so zinc is usually installed with a decent slope in order to drain water that might contain trace copper or iron residue. Obviously it is important not to allow contact between zinc and any metals that are at the opposite end of the galvanic chart.
According to VM ZINC, Zinc is highly corrosion resistant in marine environments as well as inland. Regardless of location, erosion rates of the metal are relatively similar. In marine environments, zinc combines with salt in the air mainly to form zinc oxychloride (Zn2 O Cl2). Water soluble and not very adhesive, the white zinc chloride washes off the surface easily, and then the zinc patina develops and maintains the blue-gray color. However, unless washed away by rainfall, direct saltwater or sea spray contact ultimately reduces the lifespan of the material in dry, low rainfall maritime environments when the white chloride (essentially dried salt) stays on the surface.
Depending on atmospheric conditions, corrosion rates are low and vary between 0.5μm and 1.0μm per year. About 60% of the worn material continually washes off the surface while the remainder stays to form the patina layer. The highest wear rates are found in heavily polluted industrial areas, where sulfur dioxide concentration is high. In general, the service life of zinc as a roofing material in a marine atmosphere also ranges between 80 and 100 years, depending on exposure, roof geometry and installation quality.
Corrosion due to condensation trapped on the underside:
Several sources indicate that there are problems with corrosion from the underside of zinc roofing if moisture is allowed to condense there.
If the roofing system is designed such that warm moist air from the inside can travel up through the framing, insulation, and sheathing, as it gets colder, water vapor is likely to fall out of the air and be trapped. If the conditions are not right for patina formation (no air) or if the water sits on the metal for a long time, the zinc is likely to corrode. The manufacturers of zinc roofing all recommend either venting the roof, using zinc panels with a protective coating on the underside, or using a drainage layer directly under the roofing to allow moisture to escape.
According to an article by Janet Zaso in the AIA Continuing Education Center,
"Zinc roofing systems can be successfully installed on warm and cold roof applications. The term “cold roof” used here refers to one with a ventilated substrate rather than a reflecting “cool roof,” although some zinc roofing may also have this characteristic. Cold roofs are typically seen in residential homes with gable roofs and involve a roof ventilation system in which air is introduced at the eaves and ventilated at the ridge. Typically the layers of a warm roof include a metal deck, a vapor barrier and a layer of rigid insulation that’s on top the rafters rather than between or below them. Because a warm zinc roof doesn’t have the same air ventilation system as a cold one, and because zinc metal will corrode if moisture isn’t wicked from it, proper detailing will avoid roofing failures. Correct installation of a zinc roofing system is essential for protecting the metal against corrosion. Corrosion can be seen visually as a white chalkiness on the metal surface. Typically, the causes of zinc corrosion are water from condensation adhering to the back of the panel or standing water on the surface due to a lack of slope."
"Generally, one of three approaches is used to prevent zinc corrosion from moisture. Skip sheathing is the oldest method and involves placing pine boards beneath the zinc roofing. The boards are spaced apart so that when condensation on the back side of the metal drips off, it will drip behind the pine boards where it can dry from air flow behind it — essentially a cold roof. This approach is still followed in Europe in some places and contributes to the longevity of some European buildings. Another method for protecting against condensation is installing a drainage mat beneath the zinc roofing. The newest protection against corrosion is a special backside polymer coating. The coating is often between 30 microns and 60 microns thick and protects the zinc metal from coming into contact with moisture from condensation."
According to Contrarian Metal Resources, Zinc panel systems must be back ventilated in order to prevent corrosion from the interior surfaces caused by trapped moisture. More about the ventilation requirements of ZINC can be found in the INFORMATION TOPICS section. For additional information regarding the properties of zinc, Alltrista Zinc Products is an excellent resource."
A competitor to zinc in the premier roofing material market, states that their product is more durable because: Ice dams in valleys and along eaves are a common winter occurrence. Water trapped behind these dams can (and does) penetrate locks and seams. With zinc roofs, this can lead to “underside corrosion.” Moisture trapped on the underside of copper, on the other hand, has no negative effects, "making FreedomGray the better choice for long-term durability."
According to an Old House Web article on flashing:
"The underside of zinc flashing requires ventilation to allow protective patination to develop. If the underside is allowed to stay damp, white rust and corrosion can reduce service life severely." The underside must be protected by "bituminous sheet material" against alkaline materials such as fresh concrete or mortar; acid reacting antifreeze agents; wood preservative, or contact with copper.
My recent experience with zinc roofing
I recently built a small studio in my back yard. I intended to install a zinc roof because it forms a nice deep grey patina, It lasts virtually forever, and can be easily recycled. I did a lot of research and hired a local (San Francisco area) metal roofer/fabricator. He had never installed a zinc roof before and intended to use the same system and details that he used with other metal roofing materials that he had worked with (copper and galvalume.) From my research (summarized above) I concluded that the zinc was more susceptible to corrosion from moisture trapped underneath the roofing material. I sent him this email expressing my concerns.
Everything I have read, including the National Roofing Contractors Association Metal Roofing Manual, indicates that zinc is particularly susceptible to corrosion from moisture on the underside. Excerpt from the NRCA 2006 Manual: "For roof applications, ventilation of the zinc underside is standard practice and necessary for durability. Standing seam roof applications require zinc sheet to be elevated above the underlayment with a capillary break, such as tangled mesh ventilation mat. This flat-panel installation practice ensures the bottom side of the zinc panel will not be in contact with trapped moisture. Flat-lock zinc shingles create an airspace inherently and may not require additional separation. As an alternative to ventilation, bottomside coated zinc is available for roof panels, shingles, or edge metal details that do not provide adequate ventilation or a capillary break." I found this product: http://www.benjaminobdyke.com/visitor/product/key/cedarBreather. I was wondering if you had used something like this before. I think that in our climate, the likelihood for condensation under the roofing is minimal. It is also possible that using pre-patinated sheet might keep corrosion from happening. BUT everything I have read mentions this as a very important consideration. Please give me a call when you get a chance so we can discuss this issue.
After that Jim raised his price significantly and indicated that he wasn't very excited about the idea of using zinc roofing. He mentioned the sacrificial zinc anodes that he uses on his outboard motor to divert the galvanic corrosion away from his propeller. I decided to go with a red painted galvalume material instead. I still don't totally understand whether zinc is really more susceptible to corrosion or if you need to be equally careful with copper or other metal roofing. My basic conclusion is that zinc can last for a few hundred years if installed properly, but if installed incorrectly, it can fail. It is also a lot more expensive than galvalume or similar products. Interestingly, for the painted 26 gauge panels that I purchased from Metal Sales, the warranty on theMS Colorfast45 paint is 45 years. I can't find the warranty information for the metal itself, so hopefully as long as the paint lasts, the metal is protected.