Preventing Water Intrusion with Through-Wall Flashing

By: Matthew Hodge, P.E.

What is through-wall flashing? through-wall flashing
According to the International Residential Code (IRC), flashing refers to a corrosion-resistant material that is installed in a shingle-like fashion so that it will prevent water from entering the building envelope or contacting framing members. More specifically, through-wall flashing is a corrosion-resistant material that extends through the exterior weathering surface (siding, veneer, etc.) and behind the weather-resistant barrier (WRB, housewrap, building paper, etc.) so that any water that has migrated behind the weathering surface is drained to the outside.

Technically, any flashing that extends from behind the WRB and daylights on the exterior of the siding can be called through-wall flashing. All types of siding employ various flashing details that may behave like through-wall flashing to prevent or drain water intrusion but these may be known by a different name (counter flashing, step flashing, apron flashing, etc.). However, the term through-wall flashing is typically reserved for veneer sidings like brick, stone, and stucco. Through-wall flashing is accompanied by weep holes or a weep screed that will allow any water that contacts the flashing to freely drain to the exterior. However, just because there are weep holes in a veneer does not mean that through-wall flashing is present. A contractor may install the weep holes without the through-wall flashing due to lack of knowledge or to avoid the hassle. Without through-wall flashing, weep holes can add little to no value to the performance of a siding.

Why is through-wall flashing important?
Water will inevitably migrate behind a veneer through a variety of means that most commonly include absorption through the permeable veneer and mortar, gaps from failed caulking around door and windows, and/or gaps and cracks from structural and thermal movements. When water does get behind the weathering surface, it will run down the wall until it finds a way into the building envelope or until it finds an exit, hopefully via proper detail and/or flashing.

Where is through-wall flashing installed?
Since the purpose of through-wall flashing is to drain water that has found a pathway behind the exterior weathering surface, such flashing should be installed wherever there are interruptions in the plane of the weathering surface. Through-wall flashing should be installed include but are not limited to above and below all wall penetrations (doors, windows, etc.), at roof-to-wall intersections, at ledger boards, at the top of the foundation wall and/or just above grade level.

flashing windows

Chimneys, above and below windows, and below porch roofs or bay windows where the continuity of the veneer stops are areas where a lack of proper through-wall installation most often results in unsightly interior water damage. The primary concern with chimneys occurs when chimneys are wood framed and protrude from the roof plane such that the exterior finish stops at roof level instead of extending to ground level. Imagine a scenario where a chimney that is clad with stone veneer stops at roof level, where the stone rests on wood and metal supports (common with newer construction). Water will inevitably penetrate behind the veneer via pathways through the chimney crown, at various decorative ledges near the top of the chimney, or through the stone and mortar. This water will run down the back side of the finish until it finds a pathway through the exterior sheathing or until it hits roof level. If through-wall flashing is not present, there is nothing to drain this water back to the exterior. Instead, the water will continue its downward movement below the siding and into the attic or wall cavity. That same scenario can be applied where veneer stops at a porch roof or bay window, and where veneer stops above doors and windows. The photographs below show interior water staining one may expect at a roof-to-wall interface where no through-wall flashing is present and where the caulking around the windows above the bay window has failed.

water intrusion

Again, water will penetrate and run down the back of the veneer. Once this water reaches top of a bay window (or a porch roof) the veneer stops and without through- wall flashing there is no place for the water to go except to intrude into the building. It is common to see water staining on the ceiling of a bay window where there is no obvious damage to cause such leaks; the cause of this staining is most often the improper installation or absence of through-wall flashing.

Gaps in the caulking alongside windows or window sills are typical origination points for water migration behind siding. When water staining is observed on the ceiling of a bay window (or near where a porch roof juts away from the house), look to see if there is a window above the bay window (or roof). Windows aren’t necessary for water staining to occur when through-wall flashing is not present, but experience shows that they can be a common source for water intrusion.


Who Should Install Through-Wall Flashing and When?
The timeframe and responsibility for installing through-wall flashing can vary, but is often completed by the mason installing the brick veneer, stone veneer, or stucco. However, because some through-wall flashing must be installed in a shingle-like fashion with other flashings for doors, windows, and roofs, it is necessary for contractors to develop a plan for how flashing will work together. When this collaboration doesn’t happen, through-wall flashing may be missed at critical areas, and counter flashing may be improperly installed through a saw-cut into the siding or caulked to the exterior surface. These flashing methods, though common, are in direct conflict with building code language that states that exterior wall envelopes must not only be constructed in a manner that prevents moisture accumulation within the wall, but they must also contain a means of draining water that enters the assembly to the exterior. If contractors developed a plan to work together, then the mason ideally would have been instructed to leave a protruding lip to the through-wall flashing in areas where counter flashing is needed.
Through-wall flashing plays a vital role in weatherproofing the exterior of structures, yet this step if often done improperly or skipped over because of lack of time, knowledge, or poor practices. Ultimately, the cost of proper flashing is minimal when compared to the hassle and cost to remedy leaks from poor flashing details, or contractors risking their reputation on a disgruntled homeowner that would otherwise be happy if there weren’t water stains all over their new ceilings. However, contractors are also aware that flashing issues may takes years to become visible, at which time the builder’s warranty may have already expired.

Given the headaches that can and often do arise, it is surprising how often through-wall flashing is not installed. Even with strict code enforcement in new construction areas, the importance of through-wall flashing is often misunderstood and therefore improperly installed or not at all. With the widespread adoption of green construction that focuses on energy efficiency by making a structure more air tight, problems caused by a lack of through-wall flashing may only become more prevalent and more destructive.