The Contractor's Guide to installing Vinyl Windows                             43                 ©2022 AWDI, LLC Do not reproduce without permission
Universal Replacement
of Old Finned Windows 
New Construction finned windows have been used
extensively, and over the last 20 years the majority
have been uPVC. Bargain builder windows have
begun to fail and are in need of replacement.
There are considerations with the replacement, 
and most of them are due to differences in frame
depth, and existing interior and exterior trim.
The common technique, chosen solely for economic
reasons, has been to
use a reciprocal saw to
cut out the old window
and leave the old fin
behind the existing sid-
ing and trim. This is
not the best approach
because it destroys the
water management,
does not reconstruct it, and covers over the debris in
the opening cavity which then is difficult to 
properly seal. It will be only a matter of time before 
it will fail and leak. Similarly, prying the old window -
fin and all - out of the opening leaves a similar 
broken water management condition behind the 
old siding and trim, which is again covered over 
and difficult to properly seal.
The preferred technique is to cut or remove the 
exterior trim/siding to expose the old window’s fin
and flashing. The flashing is removed and the old 
window is removed, and a new finned window is
installed in its place and reflashed and re-trimmed.
This approach yelds the best long term result.
The new window, however, needs to be the perfect 
size - similar fin dimension, and similar depth, etc.
for a quality and weathertight finish. It often requires
a more expensive window than the typical replace-
ment product, and offers fewer style options.
There is a method that will yield a proper result, 
using a box-frame product. This universal approach
to replacing a finned window simply has the exterior
trim removed to expose the old fin, and new exterior
trim reinstalled over the repaired flashing before
installing the new window. This new trim is 3/8”
wider than the old molding. This allows it to extend
into the opening forming a “blindstop” to which the
new box frame window is installed in a familiar 
“pocket” install.
The new box-frame
replacement window 
can be mounted
against the new 
exterior trim using
pocket Install 
techniques. 
Regardless of what
style old window
was used in a home,
if it was mounted
with a fin, the fin
was covered with
flashing and then
either siding or trim
was used to cover
the flashed fin.
NOTE: It is important to repair the flashing in the
opening, around the jambs and header, and create a
drainage accommodation (effective sill pan or drainage
mat) on the sill. Also, placing 1/16” - 1/8” shims on
the sill, over a tapered piece of siding slanted towards
the outside and covered with flashing, will raise the
new window properly to allow drainage to the outside.
Two cap nails, installed on the sill overlap flashing
will complete the weep system. 
SAMPLE MANUAL

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