Metal roof panels are fabricated from a variety of metals. The most common metals used are aluminum, copper, galvalume, galvanized steel, pre-finished steel, stainless steel, and zinc. Some are naturally weathering and therefore do not require paint or coating for protection; these include aluminum, copper, stainless steel, and zinc. Steel, on the other hand, requires coatings of paint or other metals, such as zinc (galvanization) or aluminium (aluminized steel), for corrosion protection.
Galvanic Series
The following chart shows part of the galvanic series for metals. The principle of the galvanic series is that a more “active” metal will corrode more rapidly when in the presence of moisture and a more “noble” metal, which will experience a reduced corrosion rate. The term “active” refers to electrode potential, which is greater in active metals than noble ones. For example, two metals that could be selected from this chart are plain steel and zinc, which are used for galvanized steel.
When zinc, with its high electrode potential, is used to coat plain carbon steel, the zinc coating corrodes sacrificially, thereby protecting the base steel. The more anodic zinc coating, if penetrated, corrodes to prevent the more cathodic steel substrate from rusting. Once the zinc is nicked or scratched, the steel continues to be protected by galvanic action. Because zinc has good self-healing properties, it will actually “heal” a nick in the coating and provide protection at cut edges and drilled holes. As the zinc oxides wash away, and more zinc oxide is formed. This process continues until all the zinc in a localized area is consumed. Only then does the base steel begin to rust in that location. While the zinc remains, it also acts as a barrier between the atmosphere and the base steel. All factors being equal, the thicker the zinc coating, the longer it will protect the base steel.
The galvanic chart is also useful for judging the interactions of different metals that may come in contact with one another in the presence of moisture. As the chart shows, incompatible metals should not be used in contact in roofing systems.
Metallic-Coated Steel
There are a wide variety of metals used in the manufacture of metal roof panels, but metallic-coated steel is the most popular. The most common types of metallic-coated sheet steel are galvanized steel (steel with a coating of zinc), Aluminized steel (steel coated with aluminum). The characteristics most commonly associated with steel are its strength and susceptibility to rust. Rusting results from the steel reacting with oxygen. The primary purpose of metallic coatings is to protect the steel from the environment and reduce the chance of corrosion.
Galvanized Steel
Galvanized steel is one of the oldest and most common metallic-coated metals. For metal roofing applications, it is almost always sold with a factory-applied paint finish. Factory applied paint finishes are generally longer lasting than field-applied paints. The most common steel thickness used for architectural metal roofing is 24 gauge used in applications over solid substrate decking.
Aluminized Steel
Aluminized steel was introduced more recently than galvanized steel as a metal roofing product since 1950’s. Unlike galvanized steel, aluminized steel rely almost solely on the aluminum’s ability to act as a barrier for protecting the base steel. The durability of aluminized sheet steel varies with the harshness of the environment.
Galvalume
Galvalume is the third type of metallic coating commonly used for steel roof panels. It was introduced commercially in 1970’s. This proprietary alloy coating is reported to be 55 percent aluminum and 45 percent zinc by weight. Because of its composition, it protects steel both sacrificially and as a barrier.
Natural Weathering Metals (Uncoated)
Stainless Steel
Stainless steel is a durable, corrosion-resistant material. It is often used in harsh environments that would harm other metals or paint finishes. It is also used when a non-weathering finish is desired. Stainless steel is an alloy, unlike copper and some other metals. Chromium, nickel, manganese, and sometimes, molybdenum are added to the base steel for corrosion resistance. Stainless steel is available in a number of different mill finishes, from dull matte to highly polished.
Aluminum
The minimum nominal thickness used in aluminum panel roofing is 0.8 mm (0.032 in), although 1 mm (0.04 in) is used when greater span is required or when increased control of oil canning is desired. Aluminum has a substantially higher coefficient of expansion than other roofing metals (except zinc). Aluminum provides a lightweight, easily formed panel.
Stainless steel or aluminum fasteners are recommended for use with aluminum panel roofing. Steel and copper fasteners cannot be used with aluminum. While aluminum does not require protective coatings for most exposures, it is readily available with pre-painted finishes and with anodic coatings.
Copper
Copper does not require a protective finish. Once exposed to the weather, it quickly loses its shiny surface and turns an earth-tone brown. Over a period of years, it develops a blue- green colour (referred to as patina) resulting from the formation of a protective copper sulfate. The amount of time required for the formation of patina depends on a variety of factors, including the slope of the roof, weather, and contaminants in the air. Vertical surfaces and soffits sometimes never turn green. For roofing and flashing, both 4.9 kg/m2 and 6.1 kg/m2 (16 oz and 20 oz) copper are common.