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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 (see The Galvanic Scale, below).

CLICK TO ENLARGE Avoid placing different metals in direct contact unless they are close together on the galvanic scale. *Note: Most stainless steel used in residential construction is passive, usually Type 304. The more durable Type 316 is recommended for coastal areas exposed to salt water and salt spray.

CLICK TO ENLARGE   Avoid placing different metals in direct contact unless they are close on the galvanic scale.
*Note: Most stainless steel used in residential construction is passive, usually Type 304. The more durable Type 316 is recommended for coastal areas exposed to salt water and  salt spray.

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. 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 lest 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 propriatary 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.

Galvanic Corrosion

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). 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).

Incompatible Materials

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.


  1. Will Copper Flashing Corrode Aluminum?

    I’ve got a reroofing project where we have a stone chimney that has copper cap flashing that someone has installed a membrane lap and apron flashing below. When we repair the roof, I want to replace the membrane flashing with copper. I’m wondering if we can use a prefinished aluminum drip edge without it suffering damage from the copper chimney flashing, which is about 6′ away up the roof. This is a budget consideration to not use a copper drip edge if we don’t need to.

    • buildingadvisor says:

      Contact between copper and aluminum, in the presence of water, will definitely lead to the premature pitting and corrosion of the aluminum. How fast this occurs depends on a number of variables – particularly the aluminum alloy used, frequency of wetting, and the surface area of the copper (the passive material) relative to the aluminum (active material). A large area of copper can rapidly corrode a small area of aluminum, such as a fastener, due to the “area effect”.

      The fact that the materials are separated by six feet is in your favor, but of course water runs downhill and will continually leach a little bit of the copper onto the aluminum. For example, algae-resistant shingles made copper granules can destroy aluminum gutters in several years in a rainy climate. In this case the copper granules are designed to carry dissolved copper down the roof surface.

      In your case, the corrosive effect will be much less, but you may notice some pitting in a few years. If you go with aluminum drip edge, look for a heavy-gauge, pre-painted stock. How much protection you’ll get with the typical light-gauge, pre-painted drip edge from your local home center is hard to say. To be on the safe side, you might consider using copper drip edge just in area below the chimney flashing.

  2. Best Flashing for Coastal Home?

    Two years ago, I had powder coated aluminum flashing installed on two large exterior balconies. The flashing is in contact with vulcrum (not sure of spelling) and is cover on the superior surface with porcelain tile. Now the flashing has completely disintegrated pushing up surface tiles and requiring tile removal and replacement. I live at the beach 3 houses back from the actual ocean. What product should be used on the reinstall and what happened?

    • buildingadvisor says:

      I think you might be referring to Vulkem, which is a polyurethane sealant and is not corrosive to metal. However, salt air and salt spray are highly corrosive. It’s also possible that the flashing was in contact with pressure-treated wood, which can be very corrosive as well.

      If you are three houses in from the ocean, you need to use a non-corrosive metal. Your choices are copper, lead-coated copper, and stainless steel. All three are expensive, but should last 50 years or more.

      Copper would be fine as long as you don’t have redwood or cedar nearby which can stain or be stained by the copper flashing. Also, the copper with develop a greenish patina over time. If you use copper, only use copper or stainless steel nails to avoid galvanic corrosion.

      There are some less expensive laminated copper flashings such a Cop-R-Shield, made for pressure-treated decks, but I don’t think they would be a good choice here.

      Lead-coated copper is a variation that eliminates the staining issue as well as the patina.

      Stainless steel is the most expensive and least reactive metal you can use. It is the fix-it-and-forget it choice and the best choice for most exposed metal hardware, flashings, nails, and fasteners in a coastal home. While the material is expensive, as a percentage of the total cost of a home, it’s not that significant.

      Type 304 Stainless steel is fine for most uses. The more expensive Type 316 might be used for critical details with direct exposure to salt spray.

      Another product widely used in coastal climates is PVC flashing. It is corrosion-proof, but does not have the strength or long-term durability you are looking for.

      Best of luck with your project!

  3. Looking For Box-Shaped Gutter

    I am looking for a box shaped rain gutter, with straight sides, similar to this image:
    It could be a rain gutter or some type of roof flashing. I am not using it as a rain gutter but for an alternative use- I use them for growing microgreens.

    Any suggestions on a piece of metal this shape, that I can find at a normal hardware store or order easily online?

    • buildingadvisor says:

      Here in the U.S., this is not a standard gutter or flashing profile, so I doubt you would find it premade. You would need to find someone with a sheet-metal brake to form it out of “gutter coil” which is thick aluminum coil stock — .027 in. and up — often used to fabricate seamless gutters on the jobsite.

      Anyone who installs vinyl siding has a portable sheet-metal brake and this is an easy shape to form, so it shouldn’t cost an arm and a leg.

      You could also contact a sheet-metal shop, metal roofing company, or heating/ac contractor who works with metal ductwork. All of these folks could easily fabricate what you are looking for. Not sure what type of metal you want for growing, but aluminum and galvanized steel would be your cheapest options. Best of luck!

  4. PVC-Coated Aluminum For Treated Lumber?

    Will PVC-coated aluminum coil stock work well as a flashing for a pressure-treated deck?

    • buildingadvisor says:

      In general, aluminum flashing is not recommended for direct contact with pressure-treated lumber. Starting in 2004, the preservatives used in pressure-treated lumber and decking were modified to eliminate the arsenic. However, the copper content was increased and the lumber became much more corrosive to aluminum, steel, and other ferrous metals.

      While, in theory, pre-coated aluminum might be OK, exposed edges at cuts and nail holes are vulnerable. Also the quality and thickness of the coatings vary and accurate specs are often difficult to obtain. Also, the PVC coating used on coil stock is really just a vinyl-based paint and is probably no more durable than other pre-finished aluminum stock.

      A number of variables determine the rate of corrosion here: the specific type of pressure-treated wood, the thickness of the coating and the underlying metal, the amount of moisture that reaches the flashing, and other corrosive effects from air pollution and salt air (in coastal areas).

      For critical flashings, such as over the deck ledger, where leakage could lead to structural decay and deck collapse, I would steer clear of aluminum. Better choices are bituminous membranes, copper, or high-quality PVC. In some cases it is possible to isolate the aluminum from contact with the pressure-treated wood by using a piece of membrane flashing or heavy felt paper. But I don’t recommend taking chances with the deck ledger or other structural connections.

      In fact, all fasteners and connectors in contact with pressure-treated wood, including lag screws, bolts, decking screws and joist hangers, should be made of corrosion-resistant materials. Stainless-steel is the first choice for screws, bolts, and connectors and is recommended coastal environments and other harsh conditions. Heavy galvanized connectors, rated G-185, are the next best choice for steel connectors. For screws, lags, and bolts, G-185 steel and proprietary coatings, specifically tested and approved ACQ and CA lumber, are also good choices.



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