From APRA’s Newsletter-The 1% Rule

Saw this in the latest APRA Advisor.
Don't forget to sign up for my webinar on decks 
with APRA, speaking of leaks…

One Percent Rule
Most building professionals would
agree that water intrusion problems at
exterior walls typically are found at
changes in plane and/or changes in
material, such as door and window
perimeters and wall intersections with
decks. The “One Percent Rule” states
that 99 percent of the sources of water
intrusion damage is found in 1 percent
of the building envelope.
Water infiltration usually occurs at the
transitions between the work carried out
by different contractors. In most cases,
successful long-term weatherproofing
of these transitions requires careful
design and installation of flashing
crafted from corrosion-resistant metal
or flexible waterproof membranes.
The International Building Code (IBC)
includes the following guidance for
flashing exterior walls:
• Flashing shall be installed at the
perimeters of exterior door and
window assemblies, exterior wall
intersections with roofs, chimneys,
porches, decks, balconies and similar
projects and at built-in gutters and
similar locations where moisture
could enter the wall.
• Flashing with projecting flanges shall
be installed on both sides and the ends
of copings, under sills and
continuously above projecting trim.