Window Fitters Guide to Window Installation 
126                ©2022 AWDI, LLC Do not reproduce without permission
WRB and exit the wall base at a weep screed or dissipate
through evaporation. There are limits, however, to the 
volume of water a stucco clad wall system can successfully
handle. 
Stucco can be applied over block (typical CBS construction)
or over wood frame (often used for the upper story of 
residential homes). In CBS construction, the block is the
Water-Resistant-Barrier (WRB), and in wood frame 
applications, stucco is placed over a WRB which is a 
membrane such as water-resistant building paper.
It is not the province of this manual to discuss various 
construction methods. However, the success of a window 
or door installation is dependent on the successful 
handling of the water that will penetrate the stucco. 
These instructions are dependent on certain principles 
to accomplish just that.
The role of Concrete Block
The concrete block used in residential housing, when 
covered with stucco, becomes the water resistant barrier.
What that means is that it retards the passage of the water
into the interior of the home.  Many mistake that to mean,
no water should be absorbed into the block, when in reali-
ty, the successful performance of the CBS wall depends on
a certain amount of absorption to handle the inevitable
water penetration of the stucco in severe weather.
When stucco claddings leak, the penetrating water is 
traditionally managed in two fundamental ways. The first is
direction to a water resistant barrier such as in a “drained
assembly where the water exits the wall downward, and the
second is a “mass assembly’ where the eater is absorbed in
non-water sensitive material and released to the interior
and exterior in a controlled way during “drying” periods. 
Concrete block serves this purpose well and it is expected
to do so in CBS construction. However, it is also expected
that the rate of drying will not be exceeded by water 
collection, so fenestration installation must limit possibility
of excess water getting inside the cavity.
Weep Screeds are Recommended
The second component of the systems design requirements
is a weep screed at the bottom of the wall and at all soffits.
Wherever the vertical wall terminates and/or changes
direction, water that may collect
behind the stucco veneer needs
to be able to drain out. This neg-
atively effects window and door
openings where the window unit
is recessed from the plane of the
wall. If no weep screed is
installed, the water will be
directed to the header of the 
window unit, and this is often
the case. Therefore, water needs
to be directed away from the
window/door header and back
into the drainage plane to be
able to escape from the behind
the veneer and not penetrate
into the interior of the building.
Proper Sill Pan
The third component of an effective stucco wall, is the
drainage of the sill of
the window or door unit.
As the illustrations
show, improper sill
shape, improper sill
slope, and improper
drainage path for the sill
will cause problems and
not allow the stucco
wall to perform as it was
intended.
The last consideration is
the creation of the win-
dow or door opening.
Too often it is out 
of square, improperly
sized, improperly
bucked, and improperly
prepared to accept the
window 
or door. Added to this is the reliance on other trades to 
prepare the opening before the window installer technician
arrives to fit the product.
These instructions will assume that the window fitter has
examined the opening and has assured that the rough sill
is proper, and that the dimensions are accurate within
1/8” in plumb, square and level. No fenestration product
should be installed in any other opening. In addition, the
window fitter should fabricate and install the wood bucks,
and the sill pan or equivalent(s) and seal them adequately
before mounting the new window or door.
Credits: Photos and Artwork courtesy of Building Science
Corporation; TM Windows and Doors; Builder Magazine, December
2005; and AWDI Standards, version 1.3.
Two ways water that gets behind stucco is handled
Drained Assembly
Drains to
Inside
Drains to
Outside
Mass Assembly
SAMPLE MANUAL

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