Flashing Matters: Do it Right!
Water leaks in walls and roofs are commonly associated with flashing and detailing problems at penetrations and interfaces between wall and roof components. Despite the importance of flashing to the durability of buildings, building codes provide little practical guidance or defer to “manufacturer installation instructions” which may be incomplete or limiting in the choice of materials and methods used to construct a wall.
This section focuses on key flashing concepts to provide a foundation for appropriate practices that can be extended to a variety of applications. In addition, typical building department inspections often do not address flashing adequately because it is usually concealed once exterior finishes are installed. It is imperative that designers and builders consider this issue as a key element of construction plan detailing, construction trade coordination, and field quality control. Do not depend on caulk where flashing is feasible. Where caulking is unavoidable or necessary, refer to Section 4.2.9.
Flashing materials and methods generally fall into one of the following three categories:
Mechanical Flashing—Generally a rigid, preformed or formable, non-corrosive and water-resistant material (e.g., aluminum, copper, plastic, etc.) shaped to direct water away from joints or around components, mechanically fastened to a suitable substrate, and installed in an overlapping fashion to shed liquid water and prevent gravity-driven intrusion. Laps should be of a sufficient length to prevent liquid water intrusion due to capillary action or pressure differential. Mechanical flashings include preformed flashing components such as window and door pan flashings.
Self-Adhering Flashing—Flexible, water-resistant facing materials coated completely or partially on at least one side with an adhesive material and which do not depend on mechanical fasteners for permanent attachment; also known as flashing tape. They are used to bridge gaps and joints between components such as window flanges, mechanical flashing, water-resistive barriers, and other components to provide a water-tight and durable seal such that liquid water is unable to penetrate by gravity, capillary action, or air pressure differential.
Liquid-Applied Flashing—A type of flashing material that is a fluid or paste at the time of application and is applied by trowel, roller, spray, or other suitable method to provide a durable water-resistant coating or seal over compatible substrates and joints or interfaces between materials to prevent liquid water intrusion.
The above flashing methods may be used individually or combined together for a variety of joint and interface details between various types of WRB materials and wall or roof components. In each application, installation and flashing details required by the WRB, flashing, or component (e.g., window or door) manufacturer should be consulted and component compatibility verified. In many cases, approved applications must use a very specific set of materials because they act as a system.
The remainder of this section provides guidance and flashing concepts for a sample of applications with common materials. Regardless of the flashing method applied, the first step is to identify all the areas where flashing is needed. For example, refer to the house shown in Figure 4–11. As noted in Chapter 2 and shown in Figure 4–11, typical locations where flashing is very important (and often defective) include window and door heads, and step/kick out flashing at roof-wall intersections. While all flashing is important, these locations seem to be two of the most problematic and consequential.
Step flashing is illustrated in Figure 4–12. Step and kick out flashing should be used at roof-wall intersections, with the flashing extending at least 4” up the wall surface above the roof deck and integrated shingle-style with drainage plane above or sealed with self-adhering flashing tape approved for the application.
Window flashing details should follow the window manufacturer’s installation instructions or provide an equivalent solution, as appropriate given the type of WRB approach being used. One example of window flashing sequence is shown in Figure 4-13, for a wall assembly which includes exterior foam insulation (which is becoming increasingly common based on energy codes). While the figure details the steps in this process, the key concept is integrating the WRB layer and window units with a compatible and approved self-adhering flashing tape to drain bulk water away from the window opening. This practice as well as other flashing practices should employ a drainage pan at the window sill to remove water that may penetrate the window unit itself (see Step 1 in Figure 4–13).
Figure 4–14 illustrates a deck ledger flashing detail. This assembly often is created after the initial construction of a home, and is particularly important because it carries structural safety implications directly related to the long-term ability of the flashing to manage water. The flashing and fasteners used in treated wood materials must be compatible with wood preservative treatments for durable moisture protection (e.g., avoid aluminum or electroplate galvanized steel—see Section 8.2.2).
For additional guidance on flashing methods and details, several additional resources with more extensive flashing details are noted in Section 4.4. Key flashing references listed in this section include Keeping Walls Dry (CMHC), Weather Resistive Barriers (U.S. DOE), and Guidance on Taped Insulating Sheathing Drainage Planes (U.S. DOE Building America program). Increasingly, useful flashing installation videos are easily found online as well.
