CONFIRM SQUARENESS OF EXISTING OPENING

Determine if the rough opening or existing window frame is square, level and plumb. You may need to remove some trim to properly inspect the opening. 

1. Measure the diagonals of the opening to determine if it is square. Measurements must be within 1/4". 

2. To determine if an opening is level and plumb, 1st place a level against the side jambs.

3. Determine if the plane of the window is vertical (plumb) by measuring the edge of the jamb, rather than the inside surface of the jamb. The opening cannot be more than 1/8" out-of-level or plumb.

Take three measurements of the opening for both the width and height. Report the smallest measurement when ordering. All three measurements should be within 1/8" of each other.

Open or remove the sash(es) and measure the narrowest part of the window frame in three places: 

  1. 6" from each corner (left- and right- sides, top- and bottom) 

  2. At the center (horizontally and vertically) 

  3. Report the smallest measurement.