Window Fitters Guide to Window Installation 
95                ©2022 AWDI, LLC Do not reproduce without permission
If the special shape is ordered to be installed alone,
then modified, conventional framing methods need 
to be used.
Because of the variety of shapes that are possible,
standardized instructions are not really possible, 
or practical to follow.  
Circular windows are braced differently than oval
windows, and angular windows are braced  a third
way. In fact, the only basic, common feature with
special shapes is the need for jamb extensions to 
fit them to the interior wall. 
If these extensions must be cut from wallboard 
or wood it will require more skill than most 
amateurs have. 
The good news is that some manufacturers make
pre-formed jamb extensions that snap-in to the
groove provided for the snap-in mull strips. 
Using a hair dryer to warm the vinyl, this extension
system can be bent to accommodate curved windows
as well.
1. Special shapes, whether mulled or separate, must
be installed in a clear opening with the old window
(sash and frame) completely removed.
2. Integral fin should be used on special shapes as a
nailing anchor and to create a self-flashing installa-
tion.
3. All windows, rectangular, curved, or otherwise
require a proper sized header above the window to
support the studs above. 
4. All odd-shaped windows require similar studding
to a conventional rectangular window opening.
5. A Drip Cap, formed by shaping aluminum or vinyl 
J-channel (fan-cut for curved windows) should be
installed above all windows over proper flashing.
Flashing Curved Windows
Step 1
Step 2
Step 3
Step 4
Take care that the
sheathing is applied
with the correct arc
and that the proper
support for the curved
window is in place. 
Too often it is cut or
formed haphazardly
and the resulting gaps
invite leaks no matter
how well the opening
is flashed.
Place the window in
the opening. Nail just
enough to anchor it to
the opening, but not
too much to deform
the nailing fin and
allow the window to
slightly expand and
contract.
Do not nail across the
bottom.
Cut three pieces of
flashing and apply
them in overlapping
sequence over the
nailing flange.
Apply top piece last.
Use flexible uPVC 
or carefully notch 
aluminum to make a
drip cap if needed.
Just as you would 
in a rectangular 
opening, the sill 
flashing is applied
across the bottom
and slightly up the
sides. Make sure it
extends past the 
sides of the opening.
Place a bead of bed-
ding sealant on the 
sheathing or behind
the nailing flange. 
Caulk across the 
bottom leaving two
gaps for drainage.
SAMPLE MANUAL

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