Zinc roofing
Natural metals
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.
Zinc Patina
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.
Environmental benefits
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 properties.
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.
Toxicity
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.
Technical considerations
Galvanic corrosion:
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.
Marine environments:
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 and 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 propagated by trapped moisture. Our InvariTique With Back CoatTM has a coil coating on the back side that eliminates the need for traditional zinc ventilation requirements. Warranties for our zinc products are available on a specific project basis, upon our assessment of the environment and design review. 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. Requires proper detailing – the underside must be protected by bituminous sheet material against: alkaline influence (e.g., fresh concrete or mortar); acid reacting antifreeze agents; harmful influence of wood preservative. Contact with copper should be avoided.
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