By Christopher Matthews
The title might seem to imply that designers and builders actually have a choice as to whether or not their wall and glazing systems will drain, and unfortunately, many buildings are erroneously designed and constructed under this misconception. The fact is that precipitation, wind, and gravity will inevitably combine to first force water into these systems, and then to cause that water to flow downward through them. Thus, our only choice is not whether we will allow drainage, but rather, how and where we will control and direct the water. Simply put, will we drain this water to the exterior, or will we allow it to drain uncontrolled into vulnerable areas where damage is caused. Either way, it will assuredly drain.
Most buildings constructed today include a weather resistive barrier (WRB) as part of the wall system, as is typically required by building codes and good construction practice. This WRB is intended to protect the wall system components from water flowing to the interior of the exterior wall cladding materials by providing a drainage plane against which the water will flow to lower locations. Following this method to its logical conclusion should lead to a wall system which includes integrated flashing at locations below, to route the collected water out through the wall to the exterior. Regrettably, although the codes also require flashing as necessary to protect the walls, we often see this critical component omitted. This results in a system where the water flows down the WRB, but has no ready path to the exterior. With no planned drainage locations, the water collects within the wall system and eventually flows through the path of least resistance, usually draining to the interior or being absorbed to the interior through the WRB itself.
The inclusion of through-wall drainage flashing may seem like a “no-brainer”, and it has been included in masonry cavity wall construction for hundreds of years. Sadly though, it is often overlooked. For some reason, through-wall flashing seems to be routinely omitted in single-family residences clad with brick veneer, and we are often requested to assist with residences constructed in this manner that subsequently experience water intrusion problems. As another example, we have also been called in later on multiple projects where exterior wall claddings, glazing systems, and WRB have already been replaced two and three times in an attempt to resolve water infiltration problems. Ultimately, we determine that in each expensive rebuild, through-wall drainage flashing was not included and the problems returned.
The same basic “contain and drain” principle has been employed in glazing systems for many years. Standard storefront systems, if properly installed, route water entering the system to a “pan” type sill flashing assembly where it drains through weep holes to the exterior. Pressure-bar curtain wall systems control water around each glass lite, and also drain to the exterior through weep holes located in each “zone”. Most competent glazing contractors understand and install these systems properly, but as with faulty wall systems, we sometimes see these and other glazing systems installed with improper drainage and flashing details. Once again, the water will drain regardless of whether the flashing is properly installed or not, but when it is not, we have no control over where the water ends up.
Obviously, the design and construction of a properly performing wall or glazing system involves many components and details, and it is not my intent to oversimplify the process. Various aspects must be carefully planned and implemented, and especially in wall systems, there is continuing research and product development toward providing improvements in moisture control and flow to lower locations. However, one basic concept remains constant – the water will enter and will then drain somewhere. Always keep that thought in mind, and in addition to all the methods and materials used to protect the building walls, be sure to provide the proper drainage details to route the water where ultimately intended.
Christopher Matthews is Vice President and a Senior Consultant, employed with Glazing Consultants International, LLC, since 2002. He has over 20 years of experience working with exterior glazing and wall systems, and specializes in the installation and water resistance of these systems. He has consulted with owners, architects, engineers and builders on hundreds of projects throughout North America and the Caribbean, and also serves as an expert witness in related matters. Chris can be reached at firstname.lastname@example.org or on Twitter @glazingconsult. Find out more about GCI and join our Building Envelope Matters LinkedIn group to discuss building envelope issues.