BASICS OF ZINC ROOF SYSTEMS

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. Warm roofs are more typical on commercial buildings, although many residential projects today are turning to warm roofs for their energy efficiency and other benefits such as cost and aesthetics. 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. Zinc corrosion can also be accelerated by salt spray, acid rain and construction dust, such as masonry dust. Even with these atmospheric conditions, however, zinc roofs and walls have been successfully installed in most climates across the United States. Often success is determined by good communications between the builder and manufacturer to assess and accommodate any environmental factors.

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. The first generation coatings were white colored, but today gray coatings can be found to aesthetically match the natural patina color of zinc.

What about protecting zinc from corrosion due to sodium in salt spray? Sodium can alter the chemical patina process and result in a lighter or streaked color. Therefore, in seaside locations, zinc products may need regular rinsing with fresh water. According to a spokesperson from Umicore, a producer of zinc worldwide, "Zinc is a natural material and may vary in color from panel to panel. It may also vary in color uniformity within the same panel. Although Umicore (uses) its best efforts to provide a uniform color for each order, it cannot warrant, either expressly or impliedly, that all panels will be free from variation in color." In dry coastal climates, Umicore expects that pre-patina material will not retain its original color and hue. 

Besides warm versus cold roof considerations, zinc is suitable for all types of roofing applications. Depending upon the length and slope, a zinc roof with a length exceeding the maximum length for one panel will require transverse seams, which are those joints that run perpendicular to the roof slope between the top and bottom of one metal panel.

Standing seam and batten seam roofs are also suitable for zinc applications. In a standing seam roof, the long seams on adjacent zinc panels are bent up, overlapped or even folded to prevent moisture from getting in and to "seam" the panels together. When working with zinc in this manner, the height of the seam is typically one inch, however the height can go to a one-and-a-half inch seam for snowy areas or low slope (1:12) roofing applications.

Batten seam roofs can be adapted to complex domes, and experienced installers need only simple tools for the work. The installer fastens a strip or bar to the structural deck and the zinc roofing panels are attached to it. When working with zinc, if the batten cap (the top of the seam) is to be bent downward, the legs of the cap must be bent at an angle less than 90 degrees. The angle is important to accommodate for thermal expansion and contraction of the metal. A straight 90 degrees could not allow enough play for this movement. The proper leg angle is a common mistake made by inexperienced installers. In some cases the leg is not bent down at all, but is simply capped off.

ZINC CLADDING AND RAINSCREEN

Zinc rainscreen applications are increasingly popular with design professionals. As with a masonry rainscreen, flashing and weep holes need to be provided at the base, as well as the head details, of the exterior wall. An advantage with zinc wall construction is that it doesn't require caulking in the joints or adjacent to the connecting panels. Successful rainscreens keep water from penetrating the wall system, by allowing it to flow down and off an outside wall surface and by wicking away any moisture that does penetrate the wall system before it causes decay and corrosion.  This system utilizes an air cavity between the wall and cladding to produce a type of chimney effect that provides air flow and improves energy efficiency. The wall behind the rainscreen is water resistant. Some water will get past the zinc surface and is planned for in the rainscreen design. The flow of air in the cavity between the rainscreen and the wall, allows for the evaporation of water that penetrates the zinc surface wall panels. Additionally, the cavity between the rainscreen and the building wall is drained at the bottom, thus allowing any moisture that gets between the rainscreen and the wall system to both evaporate and drain at the base and head details of the wall.

Bloorview Kids Rehab hospital in Toronto, Canada includes interior walls of zinc flatlock shingles. The hospital also has a zinc roof. Architect: Montgomery Sisam. Installer: Flynn Canada Photo courtesy of Umicore Building Products

 

Flatlock and interlocking panels are two common styles of zinc cladding. For rainscreen applications, flatlock panels are attached to a substrate, and their corners are actually open in order to allow condensation to drain or dry. Flatlock panels are often designed in a running bond pattern as well as stacked bond pattern, a fact that's also true for interior applications. Diagonal diamond shaped patterns are also being used. The size of the patterns will depend on which panels are selected. Interlocking panels are mounted on hat channels placed either vertically or horizontally. These provide the required air flow as well as ability for water and condensation to weep out either at the base of the walls or at fenestration head details.