Analysis of Window Installation Shim Space Tolerances

Abstract

Most windows are required by their manufacturer to be installed in rough openings constructed so efficiently that there is a maximum of 6.35-mm (1/4-in.) space between the window and the adjacent structure. This space is known in the industry as shim space. The minimal shim space requirement frequently results in installation defects that include damage to the window frame, damage to the flashing wrapped into the rough opening, and installation of the assembly out of level or square. The purpose of this paper is to determine the practicality of increasing the shim space in an effort to ease window installation. Also, the paper explores recommendations for alternative installation methods and materials that may allow the shim space to be increased. To do so, a structural analysis of both finned and box framed windows is conducted to study the stresses on the structural connections whose capacities are reduced by increasing the shim space. The capacities of several window frame configurations are compared with a variety of loading conditions. The analysis indicates that the window shim space may be increased in many circumstances to facilitate window installation. The analysis also reveals that there are several practical, cost-effective means of window design and anchorage by which the shim space can be increased.