Window Fitters Guide to Window Installation
117                ©2022 AWDI, LLC Do not reproduce without permission
1. Plumb, Level and Square
Proper Measurement and alignment of the opening 
is critical to proper install and operation
Plumb – Out of plumb can render locks inoperable
at worst, and difficult to operate at best.
Level -Out of level sills will distort the frame to an
out-of-square situation. This will make it nearly
impossible for the sash, which will be square at the
corners, to nest properly, lock properly, and seal out
window and moisture properly. 
Square -Determine square by measuring the corner
diagonals (upper left to lower right, and lower left to
upper right). Record both dimensions. The difference
between them is the out of square condition. More
than 1/8” may require re-sizing the new window.
Planar – Planar is measured by temporarily 
anchoring a string diagonally from upper corner to
opposite lower corner. The strings will touch in the
center where they cross — or they will not. Reverse
the strings. They should lightly touch either way. 
If not, this condition will indicate that one of the 
upper corners of the window is not flat in plane to 
the opposite lower corner. This will render the new
window contorted in installation. 
Crown – This condition is 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 the support blocks to level the support.
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. The sill support must be level.
Trapezoid – While most openings have equal widths
and equal heights down and across the opening,
occasional an opening can be wider at the top, or
wider at the bottom. A trapezoidal effect can require 
a change in the width and/or height of the specified
window or door.
Bottom line- The new Tilt/Turn or Tilt and Slide
product must be allowed to operate without being 
distorted to fit the opening in any manner. If you 
are using a wood buck, effort should be made to
make the buck (wood surround) perfectly Square.
plumb and level. Take this work into consideration
when ordering the size of the window or door to 
allow product undercut for support and shimming.
This will then allow the new product top be installed
and work properly.
LEVEL
PLUMB
SQUARE
PLANAR
CROWN IN SILL
BOW IN SILL
UNLEVEL SILL
SAMPLE MANUAL

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