The fundamental principle of water management is to shed water by layering materials in such a way that water is directed down, out, and away from the building. The key to this fundamental principle is drainage. Drainage applies to assemblies such as walls, roofs, and foundations as well as transitions to those assemblies such as lower roofs, porch roofs, decks, balconies, railings, and dormers.
The most elegant expression of the concept of drainage is flashing. Flashing is the most underrated of building enclosure components and arguably one of the most important. Metal flashing and self-adhesive flashing tapes protect vulnerable building components and help direct water down and out of the building (see Figure). Flashing should be integrated with roof and wall drainage planes, such as house wrap or building paper, in a shingle fashion where the top layer of the weather-resistant barrier (WRB) or flashing laps over the bottom layer to prevent water draining behind the bottom layer. These water control details are shown in Figures 4 and 5.

When the roof covering is being replaced on an existing roof, new flashing should be installed at roof-wall intersections. This flashing may also be replaced as a stand-alone retrofit measure or as part of an exterior wall upgrade.

Flashing is required at all roof-wall intersections. Either continuous L-metal flashing or step flashing is used to prevent water intrusion into the wall where a sloped roof intersects a vertical wall. Step flashing is used with shingle roofs; continuous flashing is used with metal and rubber membrane roofs.

The metal flashing pieces are bent at a right angle, with half of the flashing against the wall and the other half interwoven with or underneath the shingles of the abutting roof. The upturned leg of the step flashing is behind the vertical drainage plane (house wrap) on the wall or sealed to it with an adhesive membrane and sheathing tape. Metal flashing must be corrosion-resistant and at least 0.19 inches thick (2018 IRC Section R903.2.1). For asphalt shingle roofs, the IRC requires the base flashing against the vertical sidewall to be continuous or step flashing that is at least 4 inches high against the wall and 4 inches wide above the roofing underlayment (2018 IRC Section R905.2.8.3). Note that step flashing is typically pre-formed individual metal pieces with 5-inch legs (5 inches high by 5 inches wide) and 7 inches long (referred to as 5 by 7 step flashing or base flashing). For metal or rubber membrane roofs, continuous flashing is an alternative to step flashing. The self-adhesive membrane should consist of a material like a butyl or acrylic (meeting AAMA 711). Refer to the manufacturer’s details for installation. See IRC 2018 for the testing requirements for all the materials used. (IRC 2018 Section R903.2 Flashing, Section R905.2.8.3 Sidewall flashing, R908.6 Flashings for reroofing).

Figures 2 and 3 show a front view of the integration of a new strip of fully adhered air control transition membrane, new step or continuous flashing, new roof underlayment, and new cladding with the existing house wrap and siding. Figure 4 shows continuous L-metal flashing integrated with underlayment at roof-wall intersections. Figure 5 shows flashing at a roof-wall intersection with a CMU wall. Note Figures 4 and 5 are from the Insurance Institute for Building and Home Safety (IBHS) Fortified Home program, which recommends that the metal drip edge be installed over the underlayment on roofs in hurricane and high-wind-prone regions. This is not consistent with shingle-fashion installation of roofing materials generally recommended as a good practice; however, IBHS reports that they have tested this with good results. Counter flashing could be installed to reduce disruption to the flow of rainwater off the roof here. Figures 6 through 11 show the installation of the step and kick-out flashing on a new home but many of the steps are similar.

Installation of kick-out flashing is a critical component of a step or continuous flashing. Kick-out flashing is a piece of flashing at the bottom of a section of the roof that adjoins a wall. It is installed to direct water runoff away from the adjoining wall and usually into a gutter. Kick-out flashing is sometimes fabricated on-site by the roofing contractor; however, seamless plastic flashing pieces specifically molded to serve as kick-out flashing pieces are available. These are sized to handle the larger volumes of water runoff that may be concentrated along the wall in rain storms. For more information, see the guide Step and Kick-out Flashing at Roof-Wall Intersec

How to Install Flashing at the Transition of a Wall to a Lower Roof

1. Remove roofing and enough siding on the wall above the roof-wall intersection to expose the existing house wrap or building paper.
2. Inspect the structural integrity of the wall and the overhang. Check the framing for any deficiencies, rot, insect damage, etc. Proceed only after needed repairs are performed. Based on the findings, revise the wall and overhang assembly and review specific detailing as needed. Follow the minimum requirements of the current adopted local building code.
3. Install a fully adhered air control transition membrane from the existing roof sheathing onto the face of the existing wall sheathing (Figure 6). Extend at least 2 inches above the future step flashing.
4. Install the roof underlayment as the water control layer. Extend it to the existing wall and over the transition membrane; the underlayment should wrap up the sidewall at least 8 inches.
5. Install new kick-out and step flashing, integrated with shingles as required (Figures 7, 8, 9). Best practice is to install counter flashing, using a self-adhered membrane, over the top edge of the step flashing (Figure 10). Integrate the vertical leg of the step flashing, and counter flashing, with the weather-resistive barrier of the wall. See the guide Step and Kick-out Flashing at Roof-Wall Intersections. Install the new shingles and step flashing over the roof underlayment.
6. Lap the existing house wrap or building paper over the top of the step flashing (Figure 11).
7. Re-install the siding over the house wrap or building paper. Ensure a minimum 1.5-inch reveal between the bottom of the siding and the shingles of the roof.