Specifying architectural zinc

by Katie Daniel | February 2, 2016 11:33 am

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:

• roofing;
• 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
and installation.

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.

Zinc roof design considerations
Long before the specification is written, the groundwork for a 100-year zinc roof should be laid. Good zinc design may be facilitated by a specification writer who understands the metal and is involved by the design team early in the process.

Slope
Steep slope is the best roof design strategy an architect can provide. While three degrees (5/8 in 12 in.) can be an acceptable roof pitch, it does not make any zinc joinery or profile acceptable. Double-lock standing seam (DLSS) is the most popular zinc roof panel application and is the required joinery for the low-slope zinc applications (often with in-seam sealant). There can be exceptions, as a limited section of a barrel roof will demonstrate.

A coping is theoretically a hybrid low-slope roof and façade panel in one application. Seaming and expansion details for coping applications can be easy for those who understand zinc. As the roof pitch becomes lower, the durability can be reduced as the zinc ion loss is elevated. Large roof areas without a lot of slope (e.g. < 3/12 pitch) may require an engineered standing-seam system. Designers should consult a zinc manufacturer for recommendations. (Zinc is a reactive metal and those atoms at the surface of the zinc sheet are more vulnerable to atmospheric influences. Zinc ions are formed as atoms at the surface lose electrons through the interaction with oxygen, water, etc. These charged ions have a weaker bond to the sheet surface and can sometimes be removed by rainwater(some remain to complete the patina process).

Expansion and contraction
Zinc alloy has a high coefficient of expansion. Indirect attachment with concealed clips is the preferred attachment method for any panel. For those longer than 3048 mm (120 in.), this becomes more important. All zinc profiles should be ‘fixed,’ but only for a short distance—914 to 1219 mm (36 to 48 in.) is a good rule of thumb.

Locating the fixing zone will depend on the roof slope. Steep slopes require fixing the panels closer to the ridge allowing more downslope movement. While fixing a 4000-mm (157-in.) panel in one climate may work, the same application executed in a different location, exposure, or season will produce different aesthetic results regarding panel waviness. Attachment of all zinc panels should accommodate panel movement of approximately 6.3 mm (1/4 in.) per 3048 mm (120 in.) per 55-C (100-F) temperature change. The total design temperature fluctuation should be 67 C (220 F).

Above-sheathing ventilation
In some respects, zinc roof applications may look familiar to rainscreen or ‘drain-screen’ façade designs. Zinc roofs should not be the primary waterproof ‘barrier.’ Instead, zinc roof profiles should be applied as a ventilated dry-joint cladding or a ‘rainscreen roof’ strategy. This design alternative allows for pressure equalization, backside drying, and moisture escape, rather than damaging the zinc (or creating a roof leak).

While the airflow may be limited and the ‘ventilation’ label overstated, an 8- to 10-mm (3/10- to 4/10-in.) structured underlay comprising entangled nylon wire that elevates the zinc—keeping the profile underside dry— should be used. Substitution of this air space and capillary break with a backside paint coating or other ‘barrier’ strategy should never be accepted. Above-sheathing ventilation mats must be a requirement of every zinc roof assembly.

Roof underlayments
Synthetic felts applied with cap nails can provide fewer seams and better weather protection than conventional #30 felt. Self-adhered high-temperature (HT) roof underlayments are often specified at vulnerable roof conditions (e.g. perimeters, penetrations, and valleys) where openings in the deck are found. Other underlayment properties to specify include:

• good slip resistance;
• unreinforced membranes for better self-sealing capabilities;
• good adhesion; and
• extended exposure time (i.e. three to six months).

Red rosin paper and any other moisture-holding materials should be prohibited in every zinc application. Moreover, forbidding use of red rosin paper within any zinc application should be cited in every zinc (and zinc-coated metal) specification.

Assembly components
All attachment components should be fabricated from stainless steel (Type 304) or exterior-grade polymer-coated steel.

Fasteners, clips, and cleats
Fasteners for panel clips will typically be #10 screws, at least 25 mm (1 in.) long with a drill point appropriate for the substrate (e.g. plywood, 1x lumber, or steel decking). With so many fastener penetrations, specifying an added waterproof membrane gasket under each clip base can be
an added option for leak prevention. Generally, exposed rivets or screws connecting two zinc profiles should be avoided unless specifically approved by the architect in advance.

Concealed clips should be fabricated from light-gage (26 to 28 ga.) type 304/316 stainless steel. The thin metal section is important to reduce resistance during the final seam-closing operation. This lighter design may also increase clip length or decrease clip spacing to achieve uplift requirements. Two-piece expansion clips designs vary (butterfly and hooked designs are common) and offer different levels of wind resistance. Clips should allow the roof panel to move freely during periods of thermal stress. A 110 C (200 F) temperature change for the zinc should be considered, which in extreme conditions translates to 3.2 mm/m (50 mils/ft). Based on metal temperature at the time of installation, the contractor should calculate the maximum movement and ensure panel ends do not disengage or ‘bottom out’ in extreme conditions. Clips can be sized to equal the sum of the vertical seam plus ventilation mat, or match the seam dimension (compressing the mat).

Concealed offset cleats used to retain the leading edge of the panel should be corrosion-resistant metal (stainless steel, coated steel, or zinc) sized appropriately, often 22-ga. metal and attached with the appropriate fastening method (often screws). To facilitate moisture drainage from the vented space, the roof panel should have a soft bend past the cleat that produces an acute angle and finishes parallel to the ground (rather than a closed panel return). This open hook promotes water drainage from the end pocket formed by the panel hook. Zinc profile end folds should also be ‘soft’ with the raw zinc edge parallel to the ground and not closed tight. Final instruction for clip and cleat material thickness, sizing, spacing, fastener pattern, and spacing requirements should be confirmed by a licensed engineer.

Sealants and solder
While a zinc profile will not allow the passage of water, the joinery can be compromised if the seam is not properly designed or executed. On occasion, the forces that move water ‘uphill’ can also cause a leak to appear. It is possible for a properly lapped water-shedding zinc connection to leak. The solution may or may not be to waterproof the joint with sealant or solder. Concealed sealants often create more problems for zinc than they prevent. The zinc manufacturer should be consulted for any proposed use of tube or tape sealants within laps or other concealed applications.

Excessive use of sealants by an unsuspecting contractor can plug weep holes, limit airflow, trap moisture, create adverse reactions, or restrict the metal’s movement. In-seam sealant at the top of the vertical seam is rarely a problem; it is recommended when the roof slope drops below 2/12. One should specify a non-curing butyl sealant for any moving joints. For other high-point termination conditions where weeping cannot occur, a zinc-compatible pH-neutral sealant can be used.

Due to the high coefficient of expansion of zinc, limited details should be soldered. When a small roof area requires a permanent waterproof connection, soldering may be a solution after consulting the zinc manufacturer. The keys to successfully soldering zinc include:

• surface prep and zinc-specific flux;
• 50/50 solder (or silver solder for a lead-free solution); and
• a ‘cool’ iron (i.e. no open flame torch).

Strength in the solder joint is achieved by the solder concealed in the tight space between the two layers of zinc. The joint overlap should measure only 13 mm (½ in.) so solder flow can be seen on the back side. Plumbing vent flashings, chimney crickets, and gutter joints are zinc roofing details that are soldered.

Rigid insulation
Insulated metal deck roof assemblies are also possible for zinc. Nailbase insulation composite
or structural insulated panels (SIPs) can provide a solid substrate (e.g. oriented strandboard (OSB) or plywood) to attach the panel clips. In the absence of a nailable substrate, rigid insulation (at least 172-kPa [25-psi] polyisocyanurate [polyiso]) with metal bearing plates matching the seam clip pattern can be fixed to the metal deck. The steel bearing plates will compress the ventilation mat when fastened to the structural deck and help distribute the compressive load of the seam clips. The size, metal thickness, and spacing of the bearing plates should be confirmed by the system fabricator as recommended by the zinc manufacturer.

Flat seam profiles
Low-profile zinc shingles and interlocking or overlapping tiles applied parallel to the eave offer a more familiar aesthetic and a technically easier installation method. These flat-seam profiles will always be applied as a ‘dry-joint’ roof system without solder or sealant.

Flat-seam zinc profiles rely on gravity and at least a 4/12 slope to typically maintain weathertightness (greater pitch is better). Attachment of flat-seam units can be direct with concealed fasteners or with clips. Tiles can be small and all offer good wind resistance. Usually oriented parallel to the eave, flat-seam profiles can offer custom sizes, shapes, and staggered patterns. Although most interlocking, overlapping, or other low-profile seams cannot provide the same level of weather protection as a vertical seam, that disadvantage is reduced as the roof pitch increases. Variations of the flat-seam joinery are available in many combinations of top, bottom, and side laps (Figure 1).

If future roof panel salvage and reuse is a goal, preference should be given to any indirect attachment profile that allows panels to be more easily removed without penetrating the metal and damaging the panel.

Vertical seam profiles
Standing-seam profiles with mechanical-lock connections are the most common zinc roofs. Long panel lengths make this design strategy more vulnerable to oil-canning, panel disengagement, and wind uplift. Roof slope, local weather conditions, warranty requirements, and roof scale are all driving factors in selecting a specific seam type. Although there are many standing seam designs, most may be unsuitable without limiting length and taking other precautions.

Vertical joints are attached on one vertical side joint, overlapped, and closed on the opposite side. Standing seam profiles are typically more weather-resistant by design than low-profile flat-seam roofs. The soft nature of zinc makes hand-seaming or power-seaming an easy task. The primary zinc standing seam roof panel options can be found in Figure 2.

Taller and geometrically interesting seams with added capillary breaks typically look more ‘industrial,’ but offer better water and wind resistance and longer panel lengths. Shorter seams provide a more crafted metal aesthetic and higher material use (for same length panels). Although seam geometry and panel dimensions should be provided on the drawings, the specification can eliminate any questions by outlining seam style, height, and panel width requirements.

Laps and panel lengths
As long roof panel lengths will limit the seam and roof system fabricator options, defining the longest allowable panel length by the architect is important. Zinc has a high coefficient of expansion, so longer panels require lots of movement accommodations. The challenge with long zinc panel lengths is thermal movement and wind uplift where concealed panel clips bear the stresses.

Roof systems with taller seam heights generally can accommodate longer panel lengths and lower-slopes. While most zinc roofs will have panels shorter than 7.6 m (25 ft), panels greater than 15 m (50 ft) are possible when there has been a close collaboration between the project team, roof system fabricator, and zinc manufacturer.

By contrast, shorter length panels provide more visual ‘character’ and greater wind resistance
while reducing the thermal movement. Traverse-seam detailing requires added time. Therefore, locating the approximate horizontal seams locations on the roof plan and specifying a maximum panel length will help clarify installation requirements.

Where the slope is steep, horizontal laps using simple flat-lock (hook seam) transitions from one panel to another is customary. As the roof slope is reduced, increasing the overlap distance (head-lap) and adding a soldered cleat several inches below a water-check end will improve the detail’s weathertightness. This overlap distance can increase up to 254 mm (10 in.) as the pitch is reduced. At slopes below 10 degrees (2/12 pitch), a substrate ‘step’ of at least 75 mm (3 in.) should be provided. A general specification recommendation for water-check folds at the top of the lap and staggering of traverse seams (609 mm [24 in.] or other visually appropriate offset) should also be prescribed in the specification reference.

Panel surface textures
A vertical seam roof panel oriented perpendicular to the eave will reveal oil-canning (i.e. panel waviness). This aesthetic phenomenon should be expected and is not a cause for rejection. In normal cases, zinc is a metal that will never achieve a truly flat surface. Although a smooth zinc panel surface without intermediate bends is the norm and will provide a more traditional look, any of the following measures can help minimize oil-canning:

• increase zinc thickness from 0.7 mm (27 mils
  [24 ga.]) to 0.8 or 1 mm (31 or 39 mils);
• reduce the panel width (304 mm [12 in.] is often the narrowest seen);
• add additional bends within the pan (e.g. striations, stiffening ribs, pencil ribs);
• select a pre-weathered treatment providing a gray aesthetic (rather than mill-finish zinc);
• tension-level zinc sheet and coil prior to fabrication; and
• add an embossed texture/pattern (e.g. stucco, pebble, woodgrain, or any raised design).

Specifying crafted-metal details
A big advantage of soft, natural-weathering metals is malleability. With the ability to fold the metal rather than ‘cut and caulk,’ zinc allows knowledgeable craftsmen (and architects) to execute project details that will last the lifetime of the roof. Providing water-checks (soft hems) is an easy way for every architect and contractor to make it more difficult for water to get into the building. Cut metal edges should typically not be allowed. Premium metal-working tools help make the execution of many crafted-zinc details not only possible, but also easier. One should consult a zinc mill for recommendations.

Vertical-seam panel closures: Bread-pan ends
Folding the zinc rather than relying on J-channels and sealant to close off the roof panel end is a fail-safe detail. ‘Bread-panning’ is a crafted metal technique used to create a folded ‘dog-ear’ or ‘boxed’ end.

The best way to eliminate the possibility of roof leaks at the up-slope end is to fold a vertical seam across the pan’s width. Requiring ‘folded’ zinc detailing eliminates the need for cuts, rivets, sealant, or solder at the ridge termination or at the head flashing details.

Changes in plane: The ‘pinched’ seam
Pitch-break transitions (or vertical roof-to-wall transitions) can expose the weaknesses of many rigid roofing materials. Zinc’s ability to be folded allows it to retain the weatherproof integrity of the vertical seams without cutting the profile. This dry-joint technique provides a designed solution that does not rely on sealant. Execution involves:

• straightening of the vertical seams and ‘twisting’ the flattened metal legs with needle-nose pliers;
• a pan ‘crease’ along the pitch break line to keep the standing seam panels in alignment; and
• folding and ‘tucking’ the overlapping metal to shed water to effectively eliminate the need for cutting the roof panel—which can expose the roof to more termination points and the potential for leaks.

Roof edges: Valley pans, ridge caps, and eave terminations
Intersecting roof planes with opposing slopes are vulnerable details. Improvements to the traditional valley profile design can help make this condition a lifetime solution. Valley pans collect and direct water toward the roof edge. An improved zinc valley will include:

• ventilation mat underlays (as required for
  roof panels);
• a W profile at valley centerline (for lateral expansion and better water management);
• a wide-open valley channel (at least 152 mm [6 in.] each side of centerline);
• clip attachments (allowing movement and eliminating any pan penetration);
• soft bend water-check ends;
• integral S-fold cleats or non-penetrating soldered cleats;
• an extended distance between cleat and watercheck; and
• a profile step 50 mm (2 in.) below the cleat.

The top of the roof is where the water, wind, and blowing snow can create havoc for zinc or any roof. Attention to function and aesthetics is especially important for a zinc roof expected to last for generations. Consulting with a zinc manufacturer may expose the design team to new ways to solve common metal roofing issues with more style.

Soldered pipe boot
Long-term and watertight plumbing vent penetrations are possible with natural metals like zinc. Specifying a prefabricated or field-fabricated zinc flashing boot soldered to the roof pan will provide a durable and watertight solution. Eliminating rubber boots, fastener penetrations, and sealants is an important aesthetic and durability consideration, while soldered zinc flashings are another specification requirement.

Other panel preparations should include communicating with the plumbing contractor so penetrations occur in the panel center without interfering with standing seams. Peening the panel cut prior to soldering the boot also provides a secondary water check for added protection.

Quality assurance
Specifying architectural zinc can be difficult. Two European producers have dominated the architectural zinc market in Europe for more than 40 years and in the United States for about half of that. With a limited knowledge of zinc in the American market, dedicated technical services and support for architects during the design and construction phases is critical. Zinc-specific training for contractors should be a specification requirement. Prior to the bid, specifiers should carefully qualify each zinc rolling mill that offers a unique combination of:

• material quality;
• flatness;
• strip thickness and width;
• customer service;
• technical support;
• finish color;
• availability;
• texture;
• panel profile; and
• application system.

Specifying manufacturers can offer a high level of service and can be especially beneficial to the architect after the installation begins.

Panel fabricator/system suppliers
With only a limited number of distinguishing features between zinc rolling mills (i.e. zinc manufacturers) and resellers (i.e. those who may purchase zinc strip from secondary mills), ‘rebrand zinc’ may make tracking the quality more difficult. Any metal roof system fabricators who offer zinc strip as a metal option for their profiles or systems should be considered ‘system suppliers’ and not zinc manufacturers (i.e. rolling mills that produce semi-finished sheet and coil).

The architectural metal panel market has numerous metal distributors and fabrication companies. Some offer zinc profiles and should have fabrication equipment fitted with zinc-appropriate tooling. Use of rounded-edge (bullnose) bending equipment with a radius (at least 1.5 x the material thickness) that will not fracture or micro-crack the zinc should be noted in the specification.

Roofing contractors
Installers with verifiable zinc roofing experience and craftspeople that have received zinc training should be a specification priority. Quality-minded contractors typically understand they need the proper tools and equipment to execute ‘zinc-friendly’ profiles and details, yet they still may be missing some of zinc’s finer points. Requiring a minimum two-day attendance to a zinc manufacturer’s roofing workshop for installation crews (ideally the foreman and two or three journeymen) and the project managers can help ensure installation quality.

Zinc is easier to apply in warm temperatures. Therefore, any extended cold-weather installations below 0 C (32 F) may require provisions for temporary heat. Zinc becomes less malleable in colder temperatures, making field bending more difficult when the metal temperature falls below 10 C (48 F). Requiring and enforcing contractor quality submittals—including zinc training, previously completed project references, detailed shop drawings (an indicator of architectural zinc knowledge), and crafted zinc detail mockups—require specification consideration.

Engineering
Large-scale testing is generally not available for wind, water, and fire resistance in most zinc roof applications. Architectural zinc alloy is an inorganic metal and therefore non-combustible (alloy melting point is 420 C [786 F]). Steep-slope zinc roof systems are typically dry-joint and are not intended for waterproof applications—therefore, water penetration testing may not
be appropriate.

High wind resistance is likely the most common hazard a roof covering must resist. While minimal zinc roof wind uplift testing has been completed, specifying engineering calculation requirements that meet code and additional project load conditions can be easy and effective. Prepared by a registered engineer of the project state (with registration stamp), this specification requirement can be an economical alternative to costly full-scale testing. As an added piece of the shop drawing submittal, engineering calculations should provide specific direction regarding fastening patterns, clip spacing, and added density at the roof perimeter and corners.

Material testing, environmental standards, approvals, and certifications
Zinc roofing does not follow many of the modern metal roofing references typically cited. ASTM B69-13 is the material standard for the United States. UL has limited testing from zinc roof system providers. Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA) guideline details are useful, but they do not have extensive information on zinc. The National Roofing Contractors Association (NRCA) has upgraded its metal roofing technical reference, but mostly details target commodity painted steel rather than custom-crafted metals. As the construction industry moves toward more transparency and concern for environmental, social, and health issues, specification writers may want to evaluate their zinc manufacturers according to Environmental Product Declarations (EPD) complying to ASTM International Organization for Standardization (ISO)14025, Technical Report–Environmental Labels and Declarations–Type III Environmental Declarations and EN 15804, Sustainability of Construction Works–Environmental Product Declarations: Core Rules for the Product Category of Construction Products. Cradle to Cradle certifications are also available from a few zinc mills. One should consult a preferred zinc rolling mill for additional quality references.

Quality assurance: Warranty and guarantee
The best quality assurance for long-term performance is great design properly executed by the contractor. Design and specification review by an experienced zinc representative is critical. The project manual can require one or more limited warranty/guaranty coverages for the building owner (Figure 3).

Typical ‘finish’ warranties are not applicable for natural weathering metals like zinc since the preweathered aesthetic will change as the patina develops. The contractor guarantee is often the most valuable because poor workmanship will have greater consequences. Architects and owners are conditioned to rely on weathertightness warranty coverage, which can limit competition, raise the initial cost, and overshadow quality fabricators that cannot offer ‘big company’ warranties. Simply substituting zinc in standard ‘painted metal’ detailing may be problematic. One should consult a zinc manufacturer and regional fabricator for recommendations.

Conclusion
Building a 100-year zinc roof is a process that does not have to be difficult or time-consuming. Communication is the first and most important step. Zinc can be a long-lasting solution when the architect utilizes experienced and knowledgeable resources to design and specify a roof that can endure a century.

Steve Shull, CSI, CDT, LEED AP 2.0, is a technical sales manager at RHEINZINK America, Inc. and is responsible for promoting the proper use of titanium-zinc alloy for exterior building applications in 14 Midwestern states. For the past 12 years, Steve has been working with architects, spec writers, contractors, and metal fabricators/distributors open to learning more about the technical, economic, and aesthetic merits of architectural-grade zinc. Steve’s perspective is enhanced by his years of roofing industry experience as a manufacturer’s rep and project manager for a commercial roofing, waterproofing, and sheet metal contractor. A long-time advocate for the environment and efficient design solutions, Steve is a graduate of Purdue University with a degree in industrial management. He can be contacted via e-mail at steve.shull@rheinzink.com[1].

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