Most installers plumb the jambs, and attempt to level
the header, but are often unsuccessful because the
sill is out-of-level (even in new construction) and they
don’t compensate for this with shimming.
Construction Standards accept out-of-level sills up to
1/240th of the span.  Translated into small dimen-
sions, this allows each 32” span to be out of level up
to 1/8”. An average 6 foot picture window or garden
window can be out-of level over 1/4”  in a new home.
In older homes, the situation can be worse due to
settling and softer building codes. Construction
debris, nail heads, knots and splits in the subsill can
create irregularities in the level of the sill on which
the new window rests.  
The first solution is to shim the bottom of the window
unit to level, as long as shimming doesn’t raise the
sill to a point where water can leak under the sill to
rot the subsill or leak into the house.  Because a
shimming a sill can cause part of the window bottom
to be unsupported, it is possible that the new win-
dow’s sill can bend under the weight.  And, in cases
where there is a crown in the sill, shimming becomes
complicated considering the size and weight of the
master frame. The only stable and long-lasting solu-
tion is to establish level across the entire sill prior to
installation.  Use some kind of shimming, hardening
filler, or a rigid “sub-sill”.
Plumb – Plumb is measured with a “Bulb Level”
placed on each jamb. Record how much one end
needs to be moved away from the jamb to create a
plumb condition – on each side. 
Plumb is often defined as vertical level. The sides of
the opening must be vertical so as not to distort or
skew the window or door frame. 
Out of plumb can render locks inoperable at worst,
and difficult to operate at best. As with level, 
construction often accepts out-of-plumb, so 
while measurements of width and height may be 
relatively equal in the 3 recommended locations, it
doesn’t mean the sill is level or the jambs plumb.
Record the condition of the plumb of the jambs on a
specification sheet.
Planar – Planar is measured by temporarily anchor-
ing a string diagonally from upper corner to opposite
lower corner. The strings will touch in the center
where they cross, or they will not. If they touch,
reverse which string is on the outside. If they cease
touching, the distance between the strings where
they cross will be the planar condition measurement.
Window Fitters Guide to Window Installation 
28                ©2022 AWDI, LLC Do not reproduce without permission
Careful Measuring and Checking is the difference
between successful installation and problems.
LEVEL
PLUMB
Tape
Cross Point
SQUARE
PLANAR
Other Important
Measurements
SAMPLE MANUAL

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