This condition is significant because it will indicate
that the upper corner of the window is not flat in
plane to the opposite lower corner. This will render
the new window contorted in installation. 
If it is a finned window, the fin will not rest flat on the
frame of the opening without pushing the window out
of shape. This can impede operation. Worse might be
leaving the out-of-plane condition opening up one or
more corners to leaks.
If possible, you can use the “Bulb Level” on the face
of the wall to determine if one side or both sides are
out of plane and by how much.  Record all these
dimensions on the specification sheet.
Crown – This condition will appear in the sill where
the center between the jambs is higher than at the
edges of the opening. Placing a window or door on a
crowned sill can cause the interlocks to not meet
properly; the panels to not slide properly on a sliding
window or door, or will impede the proper operation
of a hinged door or window.
Use a “Bulb Level” to determine crown, and record
location of the crown, and measure how much the
edge of the level needs to be raised on either side of
the high point to create a level condition. Record
these measurements to help in sizing and shimming
needs.
Bow – The opposite of Crown, a Bow will be a 
low spot between the jambs. As with a crown, a 
depression can cause the sill of the window or 
door to deflect, and impede proper operation and
sealing/locking. As with Crown, record location and
height differences on either side of the depression.
Record these dimensions on a specification sheet.
Window Fitters Guide to Window Installation 
29               ©2022 AWDI, LLC Do not reproduce without permission
Trapezoid – While most
openings have equal widths
and equal heights down and
across the opening, occa-
sional an opening can be
wider at the top, or wider at
the bottom. 
This condition might become 
evident when  measuring the
plumb and square, but a
trapezoidal effect can require
a change in the width and/or
height of the specified window, and therefore needs to
be recorded on its
own on the specifi-
cation sheet.
The specifier
should consider
the conditions 
of the opening 
to properly order
the appropriate
size of the 
new window. 
Then the installer
should consider 
all 8 conditions to
properly prepare 
to install the new
window in a
plumb, square and
planar condition – 
necessary for 
proper operation,
locking, and sealing.
CROWN IN SILL
BOW IN SILL
UNLEVEL SILL
CROWN
BOW
TRAPEZOID
SAMPLE MANUAL

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