If the decision is to leave the old frame intact, and
mount the new window to it or over it, then the
preparation of the old frame should include provision
for drainage, as previously described.
If the old frame is wood, 
then application of liquid
flashing can be made to the
remaining sill, “painting” it
along the bottom and up 4”
to 6” on each side. Apply the
liquid so as to seal all joints
of the sill to the interior stool,
and to the jambs on each
side. This will keep water
that may collect under the
new window from being
absorbed into the old wood
frame.
Further, application of 
adhesive-backed flashing to
the jambs and header will
provide a diversion path for
any water that comes into
the cavity. The adhesive-
backed flashing should be
applied to overlap the liquid-
applied sill flashing.
When the new window is
mounted, be sure that any
sill angles or starter strips
allow weeping of any 
collected water out from
under the new window’s 
sill. If the sill is capped, do
not caulk the underside to
prevent trapping of water or
condensation under the sill
capping.
For aluminum windows,
where the mounting fin is
under the siding (stucco or
other siding), you can 
remove the existing window’s
operating sash, and then 
the stationary glass panel.
Cutting the bar between the
sash and glazing panels 
creates an unobstructed
opening, leaving the old sill,
header and jambs in place.
As before, use liquid applied
flashing. The sill should be
coated, carefully sealing the
joints between the sill and
the jambs. Apply the liquid
up each side 4” to 6”.
If there are no weep holes in
the left-in-place sill track, drill
at least two to allow trapped
water to drain to the outside.
The installed window should
have a flush frontal fin to over-
lap the siding while the frame
of the new window rests on the
left-in-place frame of the old
window. Installing mounting
blocks in the channels of the
old sill will help support and
level the new window. 
If the total frame is to be
removed, start by cutting 
back the siding to expose the
old mounting fin. This will
allow the fasteners to be
removed from the fin, opening
the cavity to re-installation of 
a finned window. Molding is
then re-installed, with appro-
priate drip cap and flashing, 
to mate up with the cut-back
siding simulating brickmold
trim, as shown.
If the new window frame is
deeper than the old frame,
then the new window needs to
be smaller in width and height to “sit” inside the left-
in-place wallboard and stool, as shown. An alterna-
tive is to kerf a groove in the “brick mold” to fit over
the nailing fin which
will offset the new win-
dow towards the exteri-
or so the interior face of
the frame can sit in the
same space as the old
frame allowing the maximum width and height of the
new window, as shown. It is also recommended that
a waterproof trim be used.
Good
Better
Best
Window Fitters Guide to Window Installation 
37                ©2022 AWDI, LLC Do not reproduce without permission
Cutting
Back
to the
fin
Remove
the whole
window
Use pvc
trim boards
Important considerations are required when
leaving the old nail fin in place. See Page 44
for Guidance.
The
pieces
left can
make a
water
path at
the sill.
The pieces left 
can make water path 
at header and jambs.
SAMPLE MANUAL

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