By John Chamberlin
Flashings are critical to ensuring moisture does not have an opportunity to enter wall assemblies. However, many designers cannot agree on flashings because what constitutes the products themselves is poorly defined. A flashing is a material put in place to prevent water penetration, or to direct the water flow away from the building. In order for a material to be defined as a flashing, it must also continue to function throughout the construction process and through the building’s life. Still, this broad definition allows for many different products, including metal pieces, various tapes, and even liquid-applied products to be considered flashings.
Typically, when a product is considered a flashing it must comply with standards or performance criteria such as ASTM D570, Standard Test Method for Water Absorption of Plastics, and American Association of Textile Chemists and Colorists (AATCC) 127, Hydrostatic Pressure Test. Although flashings are governed by standards, many of these are easy to comply with.
Historically, flashings were made primarily of metal. These products were often constructed from copper, aluminum, stainless steel, and many other types of corrosion resistant metals. These pieces were usually custom-made to fit the detail they were protecting and then mechanically fastened to the building with screws. Metal provides excellent water resistance and durability, but can be very difficult to install due to its inflexibility.
Metal flashings lost popularity because some designers were concerned the products might deform during building movement if they were improperly installed with expansion joints. There have also been various cases where the metal flashings corroded more quickly due to unfavourable chemical reactions when it was in contact with other building materials, including treated wood and other metal types.
Today, the most common flashing products are self-adhered flashing tapes. These tapes are made of a flexible sheet of material, sometimes called a ‘carrier sheet,’ that prevents water from getting past its defenses. The flexible sheet is joined by an adhesive on one side, ‘sticking’ it to the surface and holding it in place over time.
The concept behind self-adhered flashings is simple—covering a hole or tear in the building with water-resistant tape should stop water from entering the building through that point. Many self-adhered flashings come in different shapes and sizes, but are typically very manageable and lightweight. Rolls usually have a uniform thickness, making measuring material used on the building easy. Most varieties of carrier sheets are flexible, and can be bent, folded, and stretched.
These sheets can be composed of many materials to offer different performance characteristics, such as varying vapour permeance degrees, flexibility, or even different facings to allow other building products to interact within the wall assembly. The numerous different sheets may be combined with different adhesives featuring several chemistries, each with a set of pros and cons. Regardless of the building type or climate, there is a version of a self-adhered flashing tape to fit its needs.
Although self-adhered flashings remain one of the most popular options, they may be losing traction as this installation method requires more effort than others. These flashings come in pre-determined widths and lengths, which may not always perfectly fit the detail in question. Limited sizes and shapes of self-adhered flashings can be overcome by shingle-lapping tape pieces atop one another. This method is referred to as a ‘gravity lap,’ and it can work quite well as long as certain steps are followed.
In most cases, flashing manufacturers recommend priming the substrate before flashing installation. After priming, the self-adhered flashing can be laid flat onto the surface to protect either a vertical or horizontal joint or transition. Once the first piece is adhered, a second piece should be laid on the surface lapping over the first piece where they intersect. This second piece should also be adhered flat to the wall. Typically, self-adhered flashings bond well to one another—primer may not be necessary where the two pieces of tape meet. Manufacturers frequently recommend taking extra steps to seal the edges with a mastic or sealant.
Many buildings are geometrically complex, which can make sealing critical details with one or more pieces of tape difficult. Inside and outside corners, recessed windows, and any number of pipes jutting out of the side of the building may require splicing, folding, stretching, and counter flashing to make sure water stays out. However, applying tape in this manner can cause wrinkles or fish mouths—a phenomena that occurs when the tape is not lain completely flat creating a bend or loop at the edge of the tape—that may create later problems. When tape is puckered and improperly adhered, moisture can get behind the flashing, or water can be directed back into the wall assembly.