Aluminum Flashing

What Is Aluminum Flashing

Aluminum flashing is roof flashing fabricated from aluminum sheet metal, typically at a thickness of 0.019 inches for standard residential applications. Aluminum is the second most commonly used metal for roof flashing after galvanized steel. Its primary advantages are light weight (roughly one-third the weight of steel), natural corrosion resistance, and excellent performance in coastal and corrosive environments. When exposed to air and moisture, aluminum forms a thin aluminum oxide layer on its surface that protects the metal from further corrosion. This natural oxide barrier makes aluminum flashing well suited for areas with salt air, high humidity, or industrial pollution where galvanized steel's zinc coating would degrade more quickly. Aluminum flashing is available in flat sheets, rolls, and pre-formed profiles for every standard flashing application.

Aluminum vs. Galvanized Steel

The choice between aluminum and galvanized steel flashing depends on the environment and application. Galvanized steel is stronger, more rigid, holds its shape better under foot traffic and ladder contact, and costs less. It is the industry standard for most inland residential roofing. Aluminum is preferred in coastal areas (within a few miles of salt water), tropical climates with high humidity, and environments with airborne corrosive agents. Aluminum is also easier to bend and form on site because it is softer, which is both an advantage (easier fabrication) and a disadvantage (more susceptible to denting and physical damage). Aluminum has a higher coefficient of thermal expansion than steel, so joints must allow for more movement. Aluminum cannot be soldered, so joints rely on mechanical seams, overlaps, and sealant rather than the soldered connections available with copper and lead.

Galvanic Corrosion Considerations

Aluminum sits in the middle of the galvanic series, which means it is vulnerable to accelerated corrosion when in contact with more noble metals like copper and stainless steel in the presence of moisture. The most common galvanic corrosion issue with aluminum flashing is contact with copper. A copper nail in an aluminum flashing piece will create a corrosion hole around the nail. Water runoff from copper roofing or flashing onto aluminum surfaces downstream will also corrode the aluminum. All fasteners used with aluminum flashing should be aluminum or stainless steel. Galvanized steel fasteners can also cause galvanic corrosion with aluminum, though the reaction is slower than with copper. When aluminum flashing is used on a roof that also has copper components, the two metals must be physically separated and their runoff paths must not intersect.

Insurance and Replacement Considerations

When aluminum flashing is damaged by a covered peril, the insurance estimate must specify aluminum as the replacement material. Aluminum costs more per linear foot than galvanized steel, and the fastener requirements (aluminum or stainless steel) add to the material cost. In coastal areas where aluminum was selected for its salt-air performance, substituting galvanized steel would not provide like-kind replacement and the homeowner is entitled to the same material. Hail damage on aluminum flashing is often more pronounced than on steel because the softer metal dents more easily. These visible dents can serve as strong evidence of storm impact. Adjusters should note the flashing material when documenting damage, and Xactimate line items should specify the aluminum material type and gauge to ensure the estimate reflects the actual replacement cost.