Lisø PhD Dissertation Manuscript - NTNU

LisÖ et al.
related to broken roofing tiles, especially in connection
with valley gutters. Short overlapping at jointing
has resulted in cases of leakage. The same applies
when the under flashing is missing. Another typical
problem is the tendency to use too narrow widths on
both valley gutters and abutment gutters. Experience
has shown that a width of 50 mm on abutment
gutters is too narrow.
Recommendations for further work
The work presented here is embedded within the
ongoing NBI Research & Development Programme
‘Climate 2000’ (Lisø et al., 2004). The present
authors’ ambition is to use the results from the
present investigation as a tool for further studies on
the performance of weather-protective flashing in
severe climates. Building damage records in NBI’s
archives reveal serious deficiencies in the construction
industry when it comes to knowledge concerning
the planning, design and construction of weatherprotective
flashing. The investigation presented here
has revealed a number of typical problem areas. The
results will be used as a basis for planning new field
studies and laboratory investigations.
The field study will be undertaken as the basis for a
systematic review and assessment of various forms of
flashing design. The study will be carried out in close
cooperation with the construction industry.
The laboratory investigations will be carried out to
analyse how various types of weatherboard flashing
can provide a shield against precipitation. The test
arrangement will encompass techniques that are
expected to be good, average and possibly deficient/
unsatisfactory. The tests will be carried out in a turnable
equipment for full-scale rain and wind tightness
testing (RAWI box). The main objective will be to
obtain a basis for the ranking of different flashing
designs according to their performance in different
climate situations.
Conclusions and implications
The transition from a prescriptive to a performancebased
building code in Norway has strengthened
the demand for supporting standards and easily accessible
design guidelines and best-practice solutions. The
widely recognized NBI Building Research Design
Sheets comply with the requirements in the building
code, and their main purpose is to provide solutions
and recommendations that encourage high quality in
the planning, design and construction of buildings in
a country with an extremely variable climate. The
presented investigation of NBI’s project archives
reveals serious deficiencies in the construction industry
52
with regard to knowledge about the correct design and
construction of weather-protective flashing, illustrating
the need to obtain further knowledge on commonly
used flashing solutions. The presented analysis clearly
shows that certain faults and deficiencies are recurring
items. Windowsill/weatherboard flashings comprise as
much as 41% of the examined building damage cases
associated with weather-protective flashing. Damage
in connection with parapet flashing comprises 27%
of all cases included in this investigation. With few
exceptions, instances of damage are on Norway’s
coastal areas.
The presented analysis calls for a redefinition and
strengthening of existing performance requirements
for weather-protective flashings in severe climates as
a basis for the improvement of existing flashing
design, guidelines and workmanship. Flashing should
always be designed and performed so that water is
directed away from the structure, and rain or snow is
not led underneath the flashing with the consequent
risk of leaks. Generally, a flashing should not be the
only tightening layer against water ingress. Flashing
should function primarily as a drainage covering or
external rain screen in a two-stage tightening
(Figure 13). It will also act as a mechanical safeguard
for any underlying barrier layer, e.g. roofing. The flashing
and underlying structure must always be designed
so that any possible water entering behind the flashing
will not penetrate the structure behind.
Simplified flashing solutions could be acceptable in
areas with low and moderate driving rain exposure.
However, the economic benefit from such simplification
is marginal. In light of a more severe climate in
parts of the country due to global warming, or rather
the uncertain risks of future climate change, it would
be a fairly inexpensive insurance to choose flashing
solutions with a higher climatic safety level.
The results from the investigation, revised and
improved high-performance flashing solutions for
typical problem areas, will be incorporated in the
appropriate Building Research Design Sheets.
Figure 13 Principle of one-stage tightening (left) and two-stage
tightening (right)