September 17, 2015
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.
Fitting a flashing to a building in this complicated manner can affect long-term performance. Even though most of these flashings have reputations as high-quality products, the intended installation process and the actuality of correct installation tend not to directly mirror one another. If any of the previous installation steps are bypassed, the products should function as they were designed, but the method of installation can affect the performance.
Each step has a unique importance to ensure the tape functions as intended. Tape alone ‘sticks’ to some wall substrates better than others, which is why manufactures recommend priming the surface first. If this step is skipped, it may not adhere well. Additionally, if the tape is not completely flat around the edges, it may allow water to enter the building. This is why a mastic or sealant is recommended as the final step.
A building will not remain perfectly still throughout its lifetime, as the ground below it shifts, or as it expands and contracts with the weather and temperature. Flashing tapes are typically designed to account for this movement, but if not installed exactly to the manufacturer’s specifications, problems could arise five, 10, or 50 years in the future. If the various pieces of tapes are improperly lapped, or if fish mouths and wrinkles are present, the tapes could lose adhesion and allow moisture to enter the building. This works in the same way a bandage does. A cut on the knuckle can be covered by a bandage, but if the hand is always in use, the product will start to peel away. Windows suffer from similar adhesion and leaking problems, but leaks are rarely caused by the defects in the window.
Self-adhered flashings require more time to properly install than other new methods, such as the fluid-applied flashing products introduced over the last few years, because additional installation steps—priming, installing, and terminating with mastic—are required with self-adhered flashings. These additional steps also require the use of more materials than a fluid-applied flashing and entail proper sequencing of laps to ensure water is directed correctly throughout the building’s lifetime. It takes a considerable amount of time to properly splice, bend, and fold in the appropriate areas. Although fluid-applied flashings are relatively new, they still conform to the same flashing standards. In fact, a study by Charlie Saul of Aquatech Consultancy suggested fluid-applied products actually exceed most code requirements for flashings.
Like their self-adhered counterparts, fluid-applied flashings have many different chemical formulations that allow them to perform in multiple types of construction and different environments. Instead of pre-determined sizes and shapes, fluid-applied flashings may come in sausages, cartridges, or even pails. Before applying these flashings, the surface must be dry and the materials must be applied at the proper thickness. Some will require a primer while others do not. Even if the material requires a primer, the fluid-applied flashing application is quicker than tape application.
Fluid-applied flashings are incredibly easy to install. The liquid material is applied to the substrate and is spread across areas requiring protection. Liquids take the shape of whatever substrate they are in or on—meaning wrinkles and fish mouths are no longer a concern. Further, there is no reason for pieces to lap onto each other. The liquid chemistry results in all the material applied becoming a single monolithic membrane. Generally, these types of products form a chemical bond with whatever substrate they are applied to—meaning long-term adhesion is less of a concern than before.
Like self-adhered flashings, there are some materials that may inhibit proper bonding of a liquid flashing. It is important to ensure substrates are clean before installation. Some liquid flashings have temperature requirements for surface and ambient conditions to make sure that these materials bond properly to the substrate. However, some of the new liquid flashing technologies allow for a much wider range of installation temperatures and in some cases can even be installed in wet conditions. Liquid flashings also tend to be highly flexible throughout the building’s lifetime, and normal building movement should not prevent them from protecting against moisture intrusion.
Self-adhered flashing tape ‘sticks’ to the substrate, or pieces of tape attach to one another to protect a building. A liquid-applied flashing forms a chemical bond with the substrate and any additional material it laps onto. Therefore, liquid-applied flashings can be viewed as fully adhered rather than self-adhered. Liquid-applied flashings become monolithic, and when they are tied in with liquid-applied air and moisture barriers, they become a component in a complete, monolithic system.
Flashings are an integral part of a building and should be given consideration during design and construction. Self-adhered flashings have been popular for many years, but these new products may be a better option in many cases. Liquid-applied flashings meet the same performance criteria, but are much easier to install.
Leaks are most commonly a result of installation error rather than product failure, therefore, it makes more sense to find a product that offers long-term performance and can be applied most easily. Keeping moisture out of buildings is a serious concern, but the products available today can easily overcome these issues.
John Chamberlin is product manager for StoGuard and StoEnergy Guard at Sto Corp; these divisions are focused on heat, air, and moisture management within the building envelope. Prior to this position, he served as product manager for StoCoatings and as associate product manager for StoPowerwall and StoQuik Silver. He serves on the board of directors for the Air Barrier Association of America (ABAA). Chamberlin earned an MBA at Atlanta’s Emory University and is a graduate of the University of Tennessee, with a science degree in marketing. He can be reached by e-mail at firstname.lastname@example.org.
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