Meeting sustainability goals: Designing and specifying architectural zinc roofing

by arslan_ahmed | October 6, 2022 3:02 pm

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By Charles (Chip) McGowan

With a potential lifespan of 80 years or more, architectural zinc roofing delivers longevity, sustainability, and resiliency to meet or exceed specified requirements on commercial, municipal, residential, educational, health care, and other thoughtfully designed projects.

The history of zinc as a construction material reaches antiquity. Modern architectural zinc is specified for roofing, rain management, wall cladding, and other architectural details.

It can be fabricated into a variety of panel styles, sizes, and shapes. Today’s zinc material options also offer different colors and coatings, complemented by numerous installation techniques and systems.

This proven, natural construction material is extremely long-lasting and requires minimal maintenance. Throughout its long lifetime, its dynamic aesthetic evolves as the zinc material’s patina is influenced by the project location’s unique climate and protects this ecologically friendly material for many generations.

Elemental, natural, and sustainable

While zinc could be considered a precious metal for its usefulness, it is not limited in its supply. Zinc is the 24^th most abundant element in the earth’s crust. The largest working reserves are in the United States, Asia, and Australia. Research[2] indicates zinc resources should be available for at least the next 700 years, without taking recycled zinc scrap into account.

In design and construction, zinc is known to be a dense material which is easy to form, cut, and fabricate. Pure zinc is somewhat brittle and works best as a building material when alloyed with small amounts of titanium and copper.

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Supporting environmental and human wellness, the metal alloy that industry-leading architectural zinc manufacturers use does not contain lead, iron, cadmium, selenium, manganese, magnesium, or chromium-6.

Energy-efficient, low-emission production is part of what makes zinc roofing ecologically friendly. As little as one-quarter to one-third of the energy is necessary to produce architectural zinc alloys in comparison to other metals such as stainless steel, copper, and aluminum. This is because of zinc’s low melting point and its malleability. Manufacturers can minimize emissions during smelting and processing through state-of-the-art production equipment. In the highest quality manufacturing companies, pre-consumer scraps are 100 percent recycled.

High-quality zinc building products are infinitely recyclable without loss of its chemical or physical properties. Recycled content in architectural zinc products approaches 40 percent for pre-consumer content and 10 percent for post-consumer content, depending on the source.

In Europe, the recycling rate for zinc is more than 90 percent in the scrap industry. One reason for this is the high residual material value of up to 60 percent of the original material price, which makes it a strong incentive to collect scrap and waste materials. Architectural zinc’s material price and local labor costs may be higher than short-lived roofing materials; however, its multigenerational longevity[4] make it a cost-effective choice over a building’s lifespan.

Further, zinc’s natural abundance, resiliency, recyclability, and longevity are recognized attributes in meeting the criteria of green building programs such as the Leadership in Energy and Environmental Design (LEED) rating systems by the U.S. Green Building Council (USGBC). Product and material testing and documentation are necessary to support sustainability claims. When assessing an architectural zinc product, one should request an environmental product declaration (EPD). EPD documents are third-party verified, internationally recognized, and standardized tools for assessing a building product’s sustainability.

Considering the entire lifecycle of a product or material, the Cradle to Cradle Products Innovation Institute’s (C2C) evaluation differs from other certifications in its focus on safe and sustainable products for a circular economy. To receive C2C certification, products are assessed across five categories: material health, product circularity, clean air and climate protection, water and soil stewardship, and social fairness.

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Zinc roofing products

European building construction has relied on zinc roofing since the 1800s and continues to do so today. For example, 80 percent of the roofs in Paris are zinc, and their patina contributes to the city’s sophisticated architectural allure. There is even a movement to designate the Parisian zinc roofs[6] as a United Nations Educational, Scientific and Cultural Organization (UNESCO) World Heritage Site.³

Drawing from its European design heritage, titanium zinc was introduced to North America as an architectural material in 1992. In the U.S., successful projects now can be seen from coast to coast, in all climate zones, and in a variety of building applications (Refer to Figure 1 and 2).

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Regardless of the project’s location in the continental U.S., manufacturers commonly produce architectural zinc in continuous 1000 mm (39.3 in.) wide coils. From these large coils, they ship material as either flat sheets or smaller coils in 24, 22, 20, 18, 16, and 14-gauge thicknesses. One can order custom width to minimize waste and to facilitate in manufacturing various profiles. Manufacturers can fabricate tiles and panels as roll-formed or brake-formed engineered roofing systems. Any scrap generated through this process
is 100 percent recyclable.

Manufacturers can further define and customize dimensions, system orientation, shapes, sizes, to meet a project’s individual specifications. Examples of typical zinc roofing products include:

• Double-lock standing seam (DLSS) pre-profiled panels. These are ideal for roof pitches greater than 0.625:12, for creating curves and custom shapes, and for integral snow-retention or photovoltaic systems.
• Angle-lock standing seam panels. These offer distinctive lines and shadows with a wider profile seam than double-lock panels and can integrate snow retention or photovoltaic systems. Along with a conventional vertical orientation, the panels can be installed horizontally or diagonally, on rounded parapets or mansard roofs, and on roof pitches greater than 6:12.
• Batten seams in traditional wide-seamed applications. These are used as an alternative to double-lock standing seam or in combination with standing seam panels. Available in up to 18.3 m (60 ft) lengths, the batten seam panels form boxlike joints by lengthwise connections, with a wood batten or a galvanized steel clip (Figure 3).
• Interlocking tiles for curvilinear surfaces and with roof pitches greater than 3:12. These tiles are individually sized in diamond, rectangular, and square shapes to create a unique aesthetic for roofs with complex geometry (Figure 4).

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Architectural zinc can be shaped into a multitude of forms—geometric, organic, or curves—to provide innumerable design options. For more budget-conscious projects, specifying standard panel sizes and shapes helps to reduce manufacturing and fabrication costs, as well as time and expenses associated with installation labor. Some architectural zinc material manufacturers also offer prefabricated half-round gutters, round downspouts, and other rainwater management products to complement the roofing system.

Specifying for performance

The current industry standard, ASTM B69-20 Standard Specification for Architectural Rolled Zinc is applicable to roofing, wall cladding, and other exterior building products. It details Type 1 and Type 2 architectural rolled zinc, where Type 2 has a higher copper composition and a graphite-gray patina.

Material compatibility and cautions

Architectural rolled zinc is affected by its surrounding conditions. This means one must protect roofing products loosely stacked on pallets or platforms with a weatherproof and ventilated covering. They cannot be stored in contact with other materials that could cause staining, denting, or other surface damage. Acidic materials are of key concern.

Avoid placing zinc in contact with or downstream from acidic woods including oak, red cedar, white cedar, larch, hemlock, or chestnut. Red rosin paper produces an acidic vapor and should never be used as a slip sheet. Further, use only pH-neutral sealants. Also, follow the manufacturer’s recommended storage and handling guidelines.

Glass, aluminum, galvanized steel, stainless steel, painted steel, and most solid, dry building materials present no known compatibility issues with zinc. Electrochemical reactions can cause corrosion when copper is installed above zinc. Chlorides as an additive in mortar or concrete as a bonding agent, or to inhibit freezing are also problematic. The construction team and facility management should protect architectural zinc from acidic cleaning agents, avoid contact with human perspiration, and wear clean gloves when handling and installing the zinc material.

Joints and sealants

The designer should accommodate zinc’s high coefficient of thermal expansion in the design to allow for movement. Caulking is often unnecessary at these critical junctures, which helps lower initial and lifetime costs, and reduces recurring maintenance.

The first consideration at any joint should be how to minimize the use of solder or sealant. Sealants can limit airflow or trap moisture behind the zinc panels and decrease the panel’s lifespan. If soldering is essential, choose one that does not contain lead, cadmium, or copper to maintain zinc’s environmental material attributes, and choose one which LEED and other green building programs recognize.

Resilience and longevity

Properly installed zinc roofing systems will resist air and water infiltration. For regions with high winds and hurricane conditions, zinc roofs satisfied tests to withstand high winds, as per code and local jurisdictional requirements. In areas which are susceptible to fires, zinc also provides a non-combustible solution.

Zinc’s inherent metallic properties allow the material to deliver low-maintenance and long-lasting performance in roofing applications. Runoff from zinc roofs is non-staining and does not adversely affect the environment as zinc’s natural patina forms and minimizes corrosive elements.

Patination process and appearance

Mill finished architectural zinc starts with a bright surface. However, most architects and specification professionals select zinc for its future patina appearance. Like copper, zinc develops its distinctive patina, or surface crust, based on the alloy composition and local environmental conditions.

The patination of architectural zinc occurs through a two-step chemical reaction. First, the zinc combines with water and oxygen to form zinc hydroxide. Then, with carbon dioxide (CO2) in free-flowing air, the zinc generates a dense outer layer insoluble to water and a packed inner layer of alkaline zinc carbonate.

The patina starts forming as zinc carbonate “freckles” start growing together; the rate of its formation is a function of the slope of the surface. Patination will be slower on a vertical roof surface than on a slightly sloped one. Its speed can vary between six months and five years or more, depending on climatic conditions. The more exposure to wetting and drying cycles, the quicker the patina will develop.

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While the basis of the patina is alkaline zinc carbonate, additional substances are incorporated from the local humidity, rainfall, snowfall, and air pollution levels. Therefore, the color of the patina can vary. Natural patina will appear lighter when used in marine locations because the air contains chlorides. In environments where sulfur levels are higher, the patina may appear somewhat darker.

White deposits from salt in the atmosphere can develop on all metal roofing materials. In marine environments, lighter colors of architectural zinc will not show salt deposits as much as darker surfaces. For aesthetic reasons, lighter surfaces are often chosen for coastal applications.

After the true patina has fully formed, the process will slow down but never stop. Natural patina will form to a soft blue-gray or graphite-gray color, depending on the alloy type. To achieve a vintage look at the time of installation, some architectural zinc manufacturers can accelerate the patination process under factory-controlled conditions before they fabricate the material into roofing.

Additional color and coating options for zinc roofing include:

• Pickling—a pre-treatment process which chemically etches the metal and derives the color from the alloy. The resulting appearance brings out the metal’s natural gray color and subtle grain texture.
• Phosphating—a treatment where phosphate crystals are deposited on the surface to create a darker color. The resulting color is produced by the durable phosphate coating, which looks like the zinc structure of natural patina.
• Color coating—a finish applied to the zinc, which seals the metal with a selected color during its manufacturing process. The resulting color is imparted by the coating.

While coating the zinc surface will delay the visible patination process longer, all these coloring techniques will eventually fade and give way as the material continues to naturally weather and form its patina.

Selection criteria and considerations

The architect can evaluate and qualify zinc manufacturers based on material quality, flatness, finish color, texture, selected panel profile, application system, availability, customer service, and technical support. The specifications should stipulate a zinc manufacturer who certifies its material to ASTM B69-20 and has demonstrated its experience in similar climates and applications to help ensure successful projects. As appropriate, designating one zinc manufacturer as the basis of design will further minimize opportunities for variations in material performance.

At the onset of construction, the contractor’s submittals should include the manufacturer’s product data, details, instructions, material samples for submittal, as well as engineering calculations and shop drawings. A professional structural engineer licensed and registered in the same location as the project must prepare and stamp the calculations.

To ensure a zinc roofing system meets a project’s aesthetic and performance requirements, the architect may specify a mock-up. The contractor should construct the mock-up using specified materials, methods, and quality control standards. If the contract requires it, the contractor should install a full-size mock-up on location for final approval.

A high-quality architectural zinc manufacturer can provide in-depth information on efficient and cost-effective fabrication and installation. For instance:

• Roll-forming zinc panels are almost always less expensive than brake-forming, and there are numerous profiles that can be roll-formed for roofing applications.
• Since labor often represents 66 percent or more of the contract, expensive time-consuming details should be reviewed to ensure they are necessary to fulfill the architect’s vision.
• Fewer pieces can save time and labor. However, heavier gauge zinc or smaller panels may be necessary to reduce perceived waviness (caused by oil-canning).
• Panel sizes should be chosen to optimize the width of the zinc sheet or coil and to minimize scrap material. Similarly, panel lengths should be limited for easier handling, fabricating, and installing—as well as to reduce waste in the shop and the field.

Zinc manufacturing partners also should provide care and cleaning guidelines. Some manufacturers may require washing for its warranty. For aesthetic reasons, it is recommended to clean the surface of the material in marine environments with clean water (not seawater) at least twice a year or more, depending on local conditions. Generally, zinc roofing takes minimal maintenance to retain its distinctive aesthetic and long-lasting, sustainable performance.

Author

[11]Charles (Chip) McGowan is the president of RHEINZINK America Inc. He has more than three decades of experience working with architectural, specifications, and installation professionals on projects featuring metal building products. McGowan is a member of ASTM B02 Nonferrous Metals and Alloys Committee and represents RHEINZINK’s membership in the Metal Construction Association (MCA), U.S. Green Building Council (USGBC), and the American Institute of Architects (AIA). He can be reached at charles.mcgowan@rheinzink.com.

Source URL: https://www.constructionspecifier.com/designing-and-specifying-architectural-zinc-roofing/

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