Specifying architectural zinc

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Figure 1: Flat seam roof panel profile edge-seam combinations. NS=Nail-Strip where concealed fasteners are located.

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.

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Figure 2: Primary zinc standing seam roof panel options with corresponding seam descriptions, seam heights, pan widths, and minimum slope.

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.

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