The basics for installing a sliding window are not very
different from the double hung. Head expanders and
sill supports can be used for many sliders exactly as
they are used for double hung windows.
Again, it is imperative to be sure the old frame is 
still properly integrated with the water management
system of the wall, otherwise it would be better to 
use a frame out installation described elsewhere in
this manual. The key to success, either way, is 
adequate support under the sill to avoid sagging.
There are two very important considerations when
installing sliding windows:
1. The Sill Must Be Level - and Supported!. 
Just as it is important not to bow-in the sides of the
double hung window, the lower and upper frame of
the slider cannot be bowed.  
Check the level of the sill. Read the section in this
book on Level and Square. The part of the opening
where the sliding window will rest must be flat. 
If it is “crowned” or concave, then the track on 
which the operating sash will “glide” or roll, will not
allow the operating sash to close properly, and the
interlock between the two or three sash of a slider
will not engage properly and there will be air and
possible water leaks.
Big Sliders, with multiple moving sash and heavy
glazing are prone to “sag” in the middle if not 
properly supported in retrofit applications. If the sill
can’t be assured of being level and strong, it would 
be wise to install a piece of exterior grade plywood
along the bottom of the opening to ensure that it is
flat in Frame-In applications, and fastening a water-
proof board along the sill to stiffen the tracks for
Frame-Out installations. Just shimming the bottom
of the window is often not sufficient to level the track
because the weight of the operating sash can deform
the track if the support of the under sill is not uni-
form (flat) and supported.
Shimming the window underneath the track may
temporarily solve the issue but over time, only a
structural addition of something akin to plywood, 
will assure proper operation and weather tightness
necessary for a sliding window.
Window Fitters Guide to Window Installation 
57               ©2022 AWDI, LLC Do not reproduce without permission
Installing Replacement
Box Frame
Sliding Windows 
The most important
aspect of proper
installation of a sliding
window is to have a 
level sill
If the sill is crowned or 
concave, place a piece of 
1/8” - 1/4” waterproof 
plywood, cut to the proper
length and width, on the
sill and make level,
to support the
sliding window
Properly insulate
the perimeter of
the window frame.
Sill Suport 
(If sill is sloped)
SAMPLE MANUAL

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