Window Fitters Guide to Window Installation 
101                ©2022 AWDI, LLC Do not reproduce without permission
Step 2:
Mount window using
nails along the jambs
and Simplex-type 
cap nails along the
top to accommodate
expansion as shown.
Place horizontal
flashing over 
top of header
nailing flange,
overlapping
jamb flashing.
Step 1:
Apply adhesive-backed
flashing along sides
and bottom of opening.
Apply bead of bedding
sealant on jamb flash-
ing and along header
to set the window. Do
not apply along sill.
There are more than one style of Integral J-Channel frame profiles.
Integral J-Channel Install
Most brands of uPvc replacement windows with 
mounting fins, are available with an integral J-
Channel (siding return) molded into the window
frame profile. These “built-in” J-Channels can be
wide and decorative, others can be thinner. However,
each variation serves the same purpose: to enable 
diversion of water from around the window without
installing a separate J-Channel and/or drip cap.
Installation of a window with a built-in J Channel 
is straightforward and can be done with either the
“Barrier Method” or Drainage Method” of flashing.
The diagrams here show “Method B” where flashing
is first attached to the wall, then a second layer over
the fin, and optional building paper layer installed
over the total wall, overlapping the flashing.
The goal no matter which method is used is to 
properly overlap the flashing on the wall and finally
overlap the second flashing layer over the mounting
fin. In this way, water is directed around and down
the sides of the new window, properly draining and
preventing water intrusion.
Remember: Do not seal the bottom fin to allow water
that may enter or condense inside the opening, to
have a proper patch to drain and eventually dry.
SAMPLE MANUAL

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