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
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
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
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