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Last edited 13 Jul 2022
 Use of the term
The term structural opening (SO), sometimes also called rough opening. It refers to the size of an opening in a wall or facade where a window or door or other unit will be installed. It is normally slightly larger than the actual unit size to allow for tolerances in construction. The structural openings are called as such because their edges are most likely meet with structural elements of the building such as columns, bricks or blockwork.
 Finished opening
The term finished opening (FO may also sometimes be used, this is similar to a structural opening in that it is the opening that will house a unit such as a window in a facade or wall. Although in this case the opening may have been prepared in some way rather than being a raw structural opening, for instance with fixing of plywood boxing, plaster or render between the structural element and the location of the window or other unit.
By example the width of a structural opening to the edge of brick and blockwork might be 910mm, the finished opening may have a lining of 10mm creating a finished opening that is 900mm wide, whilst the unit size that is intended to be installed is 890mm wide, leaving <5mm tolerance on all sides.
NHBC guidance 9.1.4 Doors and windows discusses appropriate tolerances of <3mm on one side to the other of an opening: "Doors and windows shall be installed to appropriate tolerances, including openings in walls and external openings viewed from the inside."
It should be noted that how a window or any unit installed into a structural can have an impact on fabric performance, as the opening punctuates the building fabric from inside to outside. A Psi ( or ψ value) is a Greek letter that denotes heat loss over a given length rather than a given area. The thermal performance of a window should be calculated including an adjustment for the interface between the window unit and the structural or finished opening, this installation adjustment is often known as the window or installation Psi (ψw). It is an adjustment to the thermal performance of a window allowing for heat loss along the linear junction between the installed window and the edge of the opening.
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