Door masonry gaps are often too deep and a wood (or other materials) frame cannot cover the entire width. To economize the use of wood, door frames have to be of functional size (depth). The sides of the gap are covered by a Casing. Casing is a widened frame, or additional panelling that is placed on the remaining area of sides.
The casing usually is placed on the interior face of the door frame, but for interior space doors it can occur on any face. There are many types of casings:
1 a casing that covers the entire depth of the gap, and the door frame is mounted over it or is formed by placing a thin architrave that creates a door stop butt;
2 a casing is formed between two sets of frames (outer + inner door frames) that marks the edge;
3 a casing, or two sets of it are placed over the remaining portion, not occupied by a centrally placed frame.
4 a casing, is a different entity from an Architrave so far its placement and functions are considered. Architrave a door frame frontal masking border.
Casings are single panel constructions, multi division panels, or in the form of stripes or flutes. Casings are often confused with Architrave, Mouldings and other Trims used with opening systems.
Deep door portals are used for accommodating a set of doors, with an interim space. In case of double shutter set such as the set of shutters ‘solid + glazed’, ‘glazed + louvered’, ‘glazed + mosquito net’, etc. are hung on the inside and outside edges. In a multi shutter system the casings are often hollowed out to accommodate (third set) of sliding shutters.
Extra wide casings are created by having deeper door portals. Deep portals create a zone between the two shutters, to dampen the noise, provide insulation, create an air lock. To avoid deep casings the sides are architecturally or panelling wise chamfered or stepped (serrated as in Gothic doors).
Casings become termination points for the skirting, wainscotting, dado or wall panelling system. The chief tools for such termination or integration of two elements is architraves. (An architrave is a border that surrounds a door frame or door way. Often it covers the junction of a door frame and masonry wall, casing, panelling, etc. It is often a part of the moulding.)