Lisø PhD Dissertation Manuscript - NTNU

LisÖ et al.
Introduction
Background
The principal function of weather-protective flashings
is to serve as an outer rain shield in a two-stage tightening,
and to act as a mechanical safeguard for any
underlying tightening layer, e.g. roofing. Flashing
should be designed and installed so that precipitation
is directed away from the structure and not be
allowed to penetrate beneath the flashing with the
risk of leaks. The flashing material must withstand
the climatic loads to which it is exposed. Achieving
such results often proves difficult in practice. In
addition, the appearance of the flashing must satisfy
the aesthetic requirements demanded of the structure
of which it forms a part. Quite often, there is a conflict
between technical or functional requirements and
aesthetic demands.
The cause of most deficiencies connected with flashing
can usually be traced back to workmanship by operatives
other than skilled tinsmiths. Installation of flashings
by unskilled operatives often results in poorly
planned and executed work. The flashing can be incorrectly
designed, incorrectly installed and/or incorrectly
fastened to the base. A lack of understanding about the
correct design and construction of weather-protective
flashing can lead to water leakage and other damage
occurring soon after completion. In addition, many
architects tend to make flashings as marginal as possible
(e.g. reducing the turndown along the facade).
This often results in discoloration and staining of
otherwise fine facades.
Internationally, several scientific studies have focused
on building damage associated with weather-protective
flashings. McDonald et al. (1997) reported the results
of wind-tunnel and full-scale measurements of wind
pressure on metal edge flashings. Baker (1965)
focused on faulty flashings at interruptions and terminations
of roofing membranes as a frequent source
of leakage in a paper on flashings for membrane
roofing, and stated that a clear understanding of the
function of flashings, the forces they are subjected to
and the limitations of the materials commonly used is
necessary for successful design. The development of
flashing solutions and techniques in Norway are primarily
based on experience and field investigations of
unfortunate or ill-advised solutions (e.g. NBI, 1991).
Thus, there is a clear need to obtain further knowledge
on the performance and durability of the most commonly
used solutions.
Climatic impact and building damage in Norway
Climatic impact from precipitation, wind, temperature
and exposure to the sun causes extensive degradation
and damage to the built environment every year.
The understanding of how degradation and damage
can best be reduced is of significant importance in the
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design and construction of buildings (Lisø et al.,
2003b). Natural disasters caused by extreme weather
events are, of course, one of the major challenges confronting
the built environment. However, the amount
of building damage not covered by natural disaster
insurance is tremendous. The increasing demands
in the construction industry for economy, progress
and quality, combined with the existence of large
amounts of precipitation during the construction
process, prove to be a difficult circle to square (Lisø
et al., 2003c). Investigations carried out by the
Norwegian Building Research Institute (NBI) have
shown that the cost of repairing process-related building
damage in Norway amounts to 5% of the annual
capital invested in new buildings (Ingvaldsen, 1994).
Correcting faults and repairing damage in buildings
during the construction process is estimated to cost
roughly the same amount as repairing buildings in
use, e.g. another 5% (Ingvaldsen, 2001).
As a rule, building damage starts to develop shortly after
completion of the building. Poor planning, design and
implementation of flashing, with consequential moisture
damage to underlying or adjacent structures, are
the direct or contributory cause of much of this
damage. Complicated geometry makes great demands
on flashing techniques. Very often, the flashings are
not seen as an integral part of the building enclosure.
Climate change will entail new conditions for several
sectors of Norwegian society, including the construction
industry. The climate system is likely to undergo
changes, regardless of the implementation of abatement
policies under the Kyoto Protocol or other
regimes. A special issue of Building Research & Information
(31[3–4]) (2003) posed the questions of ‘what
policies, strategies, practical measurements and underpinning
research will be needed to adapt the built
environment to climate change?’ While the full range
of impacts resulting from these changes is still uncertain,
it is becoming clear that adaptation to climate
change is necessary and inevitable (Lisø et al., 2003c).
Norway’s climate is extremely varied, the rugged topography
being one of the main reasons for large local
differences over short distances. Climate scenarios for
Norway emanating from the project ‘Regional Climate
Development under Global Warming’ (RegClim;
http://www.regclim.met.no) suggest changes in mean
and extreme temperature, precipitation and wind.
Precipitation scenarios for Norway conform to those
at the global scale in that they assume increased precipitation
in existing wet areas and periods. Climate change
will have different climatic impacts on different types of
buildings depending on scale, use, design, construction
and location. The amount of building damage in
Norway clearly illustrates that it is not only the
extreme weather events that should be studied to
ensure long-term adjustments to a climate regime with