Keep Water Out
Robert Grupe / July 2017
Q: I’m doing a project where a low-pitched roof terminates into a cold-formed steel-framed wall that extends vertically for another floor. Is there any advice that you can provide for that interface?
A: Without proper detailing and installation this interface can be prone to water leakage. This issue was first brought up by Frank Laux, principal engineer at CTL Group, in a recent AWCI presentation on the exterior envelope. CTL is an “internationally recognized consulting engineering and material science firm.” Laux was part of a panel presentation to the Association of Licensed Architects. He mentioned that there is a common misapplication of flashing that bridges between the wall and the roof that is required to keep water from entering the building.
What makes this more critical today than in the past are the current code requirements for the inclusion of both an air and water-resistive barrier on the exterior envelope. The International Building Code has provisions for water resistive barriers, and the International Energy Conservation Code mandates the use of an air barrier. An air barrier is a material or system of materials designed to restrict the passage of air through the exterior envelope. Air leakage causes a breakdown in the wall’s thermal resistance and allows the passage of moisture-laden air. David Cook, principal architect at CTL Group, defines a water-resistive barrier, or WRB, as a second line defense against water intrusion. If the exterior cladding deflects 99% of the liquid water that impacts the exterior wall, then the water resistive barrier re-directs the remaining 1%. The common location for the WRB is behind the cladding and over any water-sensitive materials such as gypsum sheathing or plywood. The concept is that the WRB then sheds the water down the face of the exterior wall where flashing is used to divert the water away from the wall and outside the building. The WRB and the air barrier maybe the same material or system, but for this discussion they are left separate.
Although there are many building materials and products that are recognized as WRBs, the industry is trending to either a liquid or a self-adhered membrane. The liquid is either spray or roller applied over some type of substrate or sheathing. In our industry, it’s typically gypsum sheathing. The self-adhered, as the name implies, has the ability to bond directly to the sheathing that has been properly primed. This brings us to the crux of the issue. For the water drainage mechanism to function properly, there is a required sequence of installation. Working from the sheathing out, the flashing must be applied directly over the sheathing, with the WRB installed over the flashing. Any water that channels down the exterior side of the WRB is captured and re-directed by the flashing. If the flashing is installed over the WRB, then the water will get behind the flashing and enter the structure. This is a common problem where a low-pitched roof terminates into a cold-formed steel-framed wall that extends vertically to another floor or roof level.
In this case, the water may not enter the wall cavity, but it will penetrate the roof assembly. Potentially, the difficulty in identifying the source of the problem will be compounded by the water traveling within the roof assembly and entering the inhabited space at a location that will make it appear as a common roof leak. Determining the root cause of the leak is obscured by the fact that the leak appears to be at a location that is far away from its source.
One of the primary reasons for this leaking condition is sequencing. Ideally the flashing should be installed prior to the WRB. There have been instances where a self-adhered membrane is installed prior to when the roofing contractor installs the flashing for the lower roof, and a flap of the membrane is left unbonded at the location of the flashing. This was done so the roof contractor can “tuck” the flashing material behind the WRB. This flap was then inadvertently cut away, which then allowed water to flow behind the flashing and into the roof assembly.
Understanding the principles of WRBs, how they work and how they are installed are all prerequisites for a successful installation. The concepts that are covered in this article fall under what is called a “rain screen design,” and understanding how it works is essential. Specific to this roof/wall interface, Cook suggests that all involved contractors become aware of the exact plane or location where the required flashing should be installed. Then follow a proper sequence for installation. This sequence must be coordinated with all the appropriate trades.
That leads us to communication, coordination and documentation. There should be open communication between all the trades that have a role in the installation of the exterior envelope, or as termed by the IECC, the “building thermal envelope.” This envelope includes “the basement walls, exterior walls, floor, roof and any other building elements that enclose conditioned space or provide a boundary between conditioned space and unconditioned space.”
The code recognizes that the building thermal envelope is one single unit. The envelope is not seen as discrete different building elements (walls, roofs, foundations, etc.) with differing trades doing the installation. For the system to perform as a whole, there must be communication and coordination by all parties.
Robert Grupe is AWCI’s director of technical services. Send your questions to firstname.lastname@example.org, or call him directly at (703) 538.1611.