The thin aluminum flashing widely used today is inexpensive, but is a poor choice in many applications. Always choose metal flashings that are compatible with the adjoining building materials to prevent galvanic corrosion. Also choose flashings that will last at least as long as the siding or roofing materials where they are to be placed.
When incompatible metals are in direct contact, and the materials remain wet, the more active metal corrodes and the passive metal is protected (see The Galvanic Scale, below).
Aluminum. Most residential wall flashing today is made from light-gauge aluminum “coil” stock because it is low-cost, easy to bend, and holds paint well. However, the aluminum alloy commonly used in flashings tends to pit and oxidize and pit in salty or polluted air. Unpainted aluminum flashing will also corrode in contact with pressure-treated wood, concrete, mortar, or other alkaline masonry materials. Also aluminum cannot be soldered, limiting it to simple profiles.
If using aluminum, use at least .019 in. coil stock, preferably .022 to .024-in. Some home center coil stock is .017 or thinner. Choose prefinished stock for difficult environments such as urban areas with air pollution, as these are much more resistant to corrosion. However, cut edges are still vulnerable. Also be aware that the actual thickness is less than the nominal thickness on pre-painted stock. In coastal areas, .024 to .027 in. coil stock is preferable and still thin enough to be formed with a portable sheet-metal brake. However it’s best to avoid aluminum altogether if you are close to salt water. Heavier .027 and .032 in. material is called “gutter stock” and used to form seamless gutters.
Copper. Copper flashings are a good choice for many applications, but expensive. Two types are available: soft and cold-rolled. Soft copper is very malleable and useful for molding into irregular shapes. The harder cold-rolled material is a better choice for most applications, because it is stronger and more durable. Copper flashing thickness is rated by oz. per sq. ft. and is typically 16 or 24 oz.
Copper flashings are easy to solder and corrosion-resistant, even in polluted air and in contact with masonry, but should not be in contact with galvanized steel. Nails in contact with the copper should be copper or stainless steel. Over time, all unpainted copper will oxidize and develop a green patina that protects the underlying copper. This a gradual process that occurs over 5 to 10 years. While most people find the patina attractive, the runoff of the oxidation can leave streaks of blue-green stains on the siding or trim.
Some experts caution against using copper or lead-coated copper in contact with red cedar or redwood or rainwater that drains over the wood. Over time, the copper surface will be etched by the acidic wood runoff. Although failures of copper flashings are rare, they have been reported in areas of the Northeast after 10 to 20 years of service. The combination of acidic rain and exposure to runoff from red cedar is suspected as the cause.
Lead-Coated Copper. Copper flashing coated with lead on both sides can work well where staining of building components from runoff may be a problem. Lead-coated copper has a less noticeable gray runoff. Also, copper flashing will react in contact with galvanized steel unless the copper is lead-coated.
Lead. For special flashing applications where a high degree of malleability is required, such as chimney step flashing, lead is a good option. Lead is easy to bend and mold, and is very resistant to corrosion. Lead is relatively soft, however; so it should not be used where it will be bumped or walked on. Also, it is best to leave lead flashings unfastened on one side to allow movement. If pinned on all sides, the flashing can fatigue and tear due to thermal movement.
Galvanized Steel. This is the least expensive and least durable metal flashing material. It is not recommended in harsh climates or in contact with masonry materials or pressure treated wood. Like other galvanized products, the galvanic coating will eventually wear away exposing the underlying steel to corrosion. A proprietary version called Galvalume has a much longer service life, but is still not recommended for contact with masonry or treated wood. Steel flashing is typically 26 or 28 gauge, or about 1/64 in. thick.
With metal flashing, roofing, or any metal building components, the safest strategy is not to mix metals that come in direct contact with one another. Use aluminum flashing and fasteners in contact with aluminum, copper flashing and copper nails with copper roofing, gutters, etc. When this is not possible, choose a second metal that is not likely to lead to galvanic corrosion or use a physical barrier to separate the two metals.
The Galvanic Scale. The galvanic scale (see Table above) ranks a metal’s tendency to react in contact with another metal in the presence of an electrolyte, such as water or even moisture from the air. Metals at the top of the chart are called anodic, or active, and are prone to corrode; metals at the bottom are cathodic, or passive, and rarely corrode. The farther apart two metals are on the chart, the greater their tendency to react and cause corrosion in the more active metal. Metals close to each other on the scale are usually safe to use together.
The Area Effect. The rate of corrosion is controlled by the area of the more passive metal. For example, a galvanized steel nail (active) will corrode quickly if surrounded by a large area of copper flashing (passive). For that reason, you should never use galvanized nails or screws with copper flashing.
If a copper nail is used in galvanized steel flashing, however, the corrosion of the steel will be slow and spread over a large area, so it may not be noticeable. In each case, the active metal corrodes, and the passive metal is protected.
Where incompatible metals must be used in close proximity, use the following precautions:
• Separate the two dissimilar metals with building paper, bituminous membrane, durable tapes, or sealants so they are not in direct contact.
• Coat the cathodic (less active) metal with a nonconductive paint or bituminous coating.
• Avoid runoff from a cathodic metal (e.g., copper gutters) onto an anodic metals (such as galvanized steel).
In addition to galvanic corrosion, a number of other common building materials can harm the finishes on metal flashing or lead to etching or corrosion of the material itself:
Wet Mortar. Aluminum flashing materials can be damaged by alkali solutions such as wet mortar. Where contact with wet mortar cannot be avoided, one option is to spray the metal with lacquer or a clear acrylic coating to protect it until the mortar is dry.
Pressure-Treated Wood. Aluminum and galvanized flashing should not come into direct contact with today’s pressure-treated (PT) wood, which contains much higher levels of copper than older CCA-treated lumber. Contact with the wood can cause corrosion in both aluminum and galvanized steel. One approach is to separate the flashing from the wood with peel-and-stick flashing tape, such as Vycor’s Deck Protector.
Another good option is membrane-type flashing, used alone or as a layer to separate the pressure-treated wood from the metal flashing or steel framing connectors. Other options, although more expensive, include stainless-steel (the best choice for coastal projects), copper, and lead. Plastic flashings made from PVC and other plastics are safe to use, but may not be as durable as metal in most applications.
Salt Spray. Saltwater spray is very hard on steel and uncoated aluminum products and may lead to corrosion within 5 to 7 years. In these areas, the best choices are copper, stainless steel, or prefinished aluminum with a durable, heavy coating.