Lisø PhD Dissertation Manuscript - NTNU

Typical problem areas and
recommended solutions
Introduction
This section presents examples of typical problem areas
and causes of damage regarding weather-protective
flashing, based on the results from the investigation
of building damage assignment reports in NBI’s
project archives (Tables 1 and 2). Examples of ideal
high-performance model solutions for harsh climates
are also provided, based on the performance requirements
for flashings summarized below.
Weather-protective flashings should do the following:
. ensure that a negligible amount of precipitation
penetrates as far as the wind barrier behind
. direct water away from the facade
. allow airflow for appropriate ventilation of cavities
and air gaps
. resist the climatic and mechanical loads to which
they are exposed
. be designed and fastened so that they do not
damage, or become damaged by, the materials in
the adjacent building structures
Experience from other Nordic countries is used (e.g.
Pla˚tslageriernas Riksförbund, 2002). Typical problem
areas and recommended solutions are presented in
more detail by Kvande and Lisø (2002). In this overview
of problem areas and causes of damage, the
names of the flashing variants adhere mainly to the
terms and definitions given in Norwegian Standard
3420-S4 (1999). For corresponding English equivalents,
see the Appendix.
Jointing and fastening
The use of folded welts (or seams) is far preferable to,
for example, welding or gluing when jointing extended
lengths of flashing. Sealant should always be used when
fastening flashings in connection with sheet-metal
roofing. Folded welts can give leakage points at bends
in the flashing when poorly executed (Figure 1). It is
often found that attempts have been made to seal
leaks in flashing joints and transitions by using extra
screws or some sort of mastic. Using screws normally
makes matters worse. The use of mastic as a sealant
in connection with jointing is common practice, but
such sealing has only a short life span due to thermal
expansion/contraction of the sheet metal.
Folded welts facilitate the absorption of thermal expansion/contraction.
In order for the folded welt to absorb
temperature-induced movement, the spacing between
joints should not be too large (recommended lengths
High-performance weather-protective £ashings
Figure 1 Poorly executed folded welt on parapet £ashing. The
under-£ashing is missing. Source: Norwegian Building Research
Institute,Oslo
are given in Norwegian Standard 3420, 1999). The
use of overlap jointing is generally not advisable.
The strength of the flashing’s fastening must be sufficient
to prevent it from being blown off by locally
encountered wind forces. When securing flashing with
screws or nails, it is essential that fastening points
be positioned where the structure will not be exposed
to water pressure. Welted flashing (joints) should be
recommended as a principal rule. By anchoring the
flashing via the folded welts, perforation is avoided.
When using screws, one must always choose screws
with rubber washers to ensure that the seal will last
for as long as possible. Unless exposed nail heads are
suitably covered, the use of nails offers a very vulnerable
solution in relation to water penetration.
Flashing edge
Open-ended flashing edges, among other features,
must have a wrap-over to prevent corrosion. Besides
which, finishing off without a wrap-over leaves a
sharp edge that can cause personal injury. To lead
water from the flashing away from the facade, the
finished edge of the flashing must be given a drip
edge. Figure 2a shows examples of flashing edge
designs where the drip edge has been omitted and
where water is being directed to places where it is
unwanted. Recommended solutions are shown in
Figure 2b. Flashing edges lacking support or reinforcement
are also prone to damage when exposed to wind
loads or to possible mechanical loads.
Windowsill and weatherboard £ashing
In contrast to most other types of flashing, windowsill
flashings are generally installed with flashing as
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