Essential but limited parts of the moisture protection formula

Flashing and sealants

As a member of a historic building preservation authority in Calgary, I have always felt that at least some of the present day construction problems that I see must have occurred in the past as well. I have spent many hours examining the construction details of older buildings to review how water was shed, and how water penetration was addressed. One of the first things I noticed is that sealant was seldom, if ever, used and that drip edges and flashings abound.
In contrast, after years of reviewing modern construction details, I have found a real lack of understanding of why flashing is installed, and how it is to perform. From my perspective, flashing can provide physical protection to a construction joint, but more importantly, flashing channels water away from a joint. Most of the problems I see relate to flashing that channels water back onto building components. A good example is window head flashing. As a rule, I specify a minimum 5% outward slope on a window head flashing. This allows for a little play in the installation. What I commonly find is head flashing that is installed practically horizontally or that has been pushed up providing a backslope toward the building. Typically, this moisture drains off each end of the flashing and results in water staining of the exterior finish around staining of the exterior finish around the window jambs. Patio deck flashing is also a common area where the flashing does not channel the drainage water away from the building. Many high-rise condominiums have large water stains flowing down the exterior walls from the edges of the decks.
Another common misconception is that flashing is watertight and provides an effective moisture barrier. Flashing is a ‘water shed’, only not ‘waterproofing’. Flashing must be backed up with an effective drainage plane underneath to channel any penetrating moisture away. A good example of this is roofing felt used in conjunction with step flashing on sloped roofs or a torched-applied roofing membrane that is installed up and over a parapet of a high-rise and finished with cap flashing.
It is this drainage plane that is most commonly missed in the apparent hope that the flashing will be watertight. And when it is found not to be, sealant is the standard fix.
Although the word ‘sealant’ raises the red flag for many building scientists, it does not have its place. It just should never be used as the sole source of waterproofing. Too many times have we investigated water penetration only to find multiple layers of sealant gooped into a poorly designed joint. Redundancy here is the key to effective water management. Sealant can be used to provide an effective ‘water shed’ over a joint but only if other appropriate materials are installed behind to provide a continuous ‘drainage plane’. If water does not penetrate the sealant, the underlying drainage plane will still channel it harmlessly away. In many cases, the sealant may provide the initial waterproofing. Over time though, it is the drainage plane that will provide most of the moisture protection. As such, sealant should be viewed as only providing a supporting or even architectural role.
In other words, don’t put all your eggs in one moisture protection basket. It is unrealistic to assume that all construction joints will be installed perfectly and will perform perfectly over the life of the building.
Therefore the redundancy of the water shed and drainage plane systems is paramount. As I have seen many times in historic structures, when properly executed, this approach will address the limitations in materials, installation, design, and life cycle and provide for a durable long term building.

Randy Smith, P.Eng.

The Trowel Vol. 54. Num. 1. page 10-11