by Steve Shull, CSI, CDT, LEED AP 2.0
Architectural zinc (titanium-zinc alloy, rolled zinc, zinc strip) is a soft, natural-weathering metal that has been used for centuries in various roof applications. Its value reaches far beyond beauty and durability. Following ‘zinc-appropriate’ design, a quality metal roof specification will not follow a standard painted metal specification, but instead cover issues unique to this material. In some cases, prescriptive requirements for the material quality, technical support, panel profile, installer qualifications, and detail quality will be necessary.
Zinc can perform for 80 to 100 years when proper design and installation techniques are followed. While the use of rolled zinc for architectural applications can be traced back more than 150 years in the United States, it is often apparent many in the construction industry do not fully understand this unique metal. Awareness, education, and communication are critical in realizing the full potential of a zinc roof.
Communication between the project team and zinc manufacturer is an important first step. While these roofs will be similar to other metal installations, there are a few design, assembly, specification, and execution details that make them unique. A well-crafted zinc roof specification helps establish the quality thresholds to minimize contractor error and eliminate the possibility for engineering the ‘value’ out of the roof. While the metal quality may remain high, lower-quality details and workmanship can sometimes undermine the roof assembly’s durability.
Understanding the metal
Rolled zinc ‘strip’ (sheet or coil) is a soft metal produced by alloying special high-grade (SHG) zinc (with 99.995 percent purity) with very small quantities of copper, titanium, and aluminum (<1.0 percent total) as outlined in EN 988 and ASTM B69-13, Standard Specification for Rolled Zinc. This titanium-zinc alloy provides improved mechanical properties required specifically for architectural applications, including:
- roof-edge flashing and trim;
- gutter systems;
- façades; and
- building ornamentation.
Sheet thicknesses common to roof applications include:
- 0.7 mm (27 mils);
- 0.8 mm (31 mils); and
- 1 mm (39 mils) for wide-pan engineered systems or requirements that include long lengths or durability beyond 100 years.
Standard coil width is 0.5 m (19.7 in.). However, most mills produce 1-m (39.4-in.) wide coil that can be custom-slit to any smaller dimension yielding zero waste when ordered directly from the mill. While zinc is similar to copper and aluminum, there are fundamental differences required for design
Color of zinc
Natural or mill-finish zinc alloy has a bright surface similar to stainless steel. Designers, however, prefer an aged zinc aesthetic. A manufactured ‘preweathered’ finish provides an initial gray color. This is achieved by either:
- ‘pickling’ with a sulfuric/nitric acid bath, which etches the mill finish and produces a blue-gray color; or
- ‘phosphating,’ which deposits a phosphate-crystal coating on the shiny zinc surface.
Either method can show some tonal variation in the initial color. Dark gray can be achieved when a modified titanium-zinc alloy (Type 2 per ASTM B69) is pickled. A heavier phosphate coating gives the zinc a temporary matte black aspect. As the sun’s angle, viewer’s perspective, seasons, cloud cover, and sky clarity change, so too does the perceived color. Only after the natural zinc-carbonate patina has developed, does the final natural warm gray of zinc appear.
With several variations of preweathered color where there is no true ‘equal,’ the project team may want to specify one acceptable preweathered color from each zinc manufacturer named. This will help eliminate confusion and boost competition during the bid process. Designating one zinc manufacturer as the ‘basis of design’ (BOD) will indicate what zinc rolling mill is preferred by the architect. However, if there are other products that would be acceptable, then including those selections in the specification further clarifies the project to the bidders.