The size of the finished opening should be the exact same size as the sectional garage door or down to 1” smaller on each side and/or the top. While technically the door could overlap the opening as much as the space will allow, the design on the outside of the door disappearing behind the jamb will tend to distract from the overall outside appearance. Furthermore if the opening is larger than the door the door may not seal properly against the elements and may also distract the overall appearance.
Although it is not necessary to measure the opening before requesting a garage door estimate the drawing below (Diagram A) shows the important measurements for the installation of your new overhead sectional garage door.
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