How to Select the Right Size Door Casing

Written by paul massey
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Door casing is an attractive finishing touch to a door installation, but it also serves a functional purpose as well. Besides framing the door opening, the door casing is applied to cover the shim space used around the door jamb to secure the door to the adjacent wall framing. Door casing is available in a wide range of shapes and sizes to complement any decor, and choosing the correct casing size depends on both aesthetic preference and functional criteria.

Skill level:
Moderately Easy

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Things you need

  • Tape measure
  • Hand square or straightedge

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  1. 1

    Determine the minimum width of door casing needed by measuring the distance between the midpoint of the jamb edge and the edge of the door's rough opening. This measurement must span the shim space between the outside of the door jamb and the face or edge of the wall framing. The minimum width of door casing required will be this dimension plus 1/2 inch.

  2. 2

    Measure the height of the door jamb on both sides and across the header. The total of these dimensions added together plus approximately 12 inches for mitre cuts or waste is the linear requirement of casing for each side of the door opening.

  3. 3

    Place the blade edge of a hand square or other straightedge against the wall surface, and extend across the shim opening to the jamb edge to determine if the jamb and wall surface are aligned. If the two surfaces are even, any square or shaped door casing type will be appropriate for the door. If the wall surface is not flush with the jamb edge, a casing shape with a concave back is recommended to ensure the casing rests flat on both surfaces when nailed in place.

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