Water is a mystery. It bends all of the rules, it baffles and confounds, it flows where it wants to flow, and although often used as a symbol of renewal, it is also associated with destruction. Particularly in the building industry, it is a constant potential threat to the window and door openings of the building envelope. Managing it through proper drainage is essential.

Flashing systems with proper fenestration drainage are central to water management in stone, stucco, brick, vinyl, cement board and EIFS claddings. The IBC requires the flashing be “installed in such a manner as to prevent water from entering a wall or be redirected to the exterior” (IBC 2018 §1404.4; IBC 2015 §1405.4). Rough openings can often provide the easiest path for water to enter the interior of a structure. When windows and doors leak, the area between the sill of the rough opening and the window/door is particularly susceptible. If water reaches the sill, it will most likely enter the interior.


Pan Flashing

Historically, talk of fenestration flashings focused little on sills—despite their location in one of most moisture vulnerable areas of the building envelope. Early editions of the IRC mentioned sill flashing, usually along with other building components, but gave no specific implementation details. Also, in the field, when sill pan flashing was installed, it was often an afterthought.

Today, however, the importance of protecting door and window rough openings with sill pan flashing has been recognized by the construction industry as crucial. In the IRC 2012, the ICC added “pan flashing” to its definitions. Sill pan flashing can no longer be an afterthought. It requires special attention and thoughtful design. The best building construction can quickly be nullified by inadequate sill pan flashing. Good sill pan flashing is designed to collect water and direct it away. It should be waterproof, durable and self-draining with a good slope.

The ICC made significant changes to window and door sill pan flashing requirements in the IRC 2012. For the first time, slope was specifically mentioned. The new prescriptions were also carried over to the 2015 and 2018 IRC editions. As the import of the changing industry standards has been noted, most states have responded with legislation adopting the ICC’s new requirements.



Slope is important generally in construction. In the context of sill pan flashing, it is intuitive for efficient drainage. Pan flashing is required to be “sealed or sloped in such a manner as to direct water to the surface of the exterior wall finish or WRB” (2018 IRC R703.4). The ASTM prescribes a slope for sill pan flashing in depths less than 6 inches and a required slope in depths greater than 6 inches.

Slope in a finished punched opening can be achieved through a variety of methods. Self-draining sill pan products are available on the market, specifically designed with a consistent slope. They can be quick and easy to install depending on the brand. Tape or Fluid applied flashing systems manage pitch by applying a beveled piece of wood siding to the sill or by shaving the top piece of a 2x4 or 2x6 wooden stud of a double subsill.

Also, sloping of framing members of the sill can be accomplished during construction. The time to install and the amount of pitch required will vary in all of the field methods. Although ASTM does not specify the degree or pitch, the slope should be substantial enough to be effective. A good pitch is a minimum of 1½ by 12 inch (slope of 7.1°). Depending on the type of method and products used, a well-sloped and professional sill pan flashing can take anywhere from a couple of minutes to a half hour.

Water remains a mystery. However, the building industry has made headway in prioritizing its management in fenestration openings. Properly designed flashing systems, particularly, well designed sill pan flashings with significant slope and pitch, allow fenestration openings to be less vulnerable to water’s odd and perplexing nature. As more builders and contractors implement the Code and Standards, water will continue to flow where it wants to flow, but also where we, in the building industry, want it to go. W&C