Building tolerances and clearances
The terms ‘tolerance’ and ‘clearance’ are often confused. A tolerance is a permissible amount of deviation from a specified or nominal characteristic; in general, tighter tolerances equal higher costs. A clearance is a space or distance purposely provided between adjacent parts, either to allow for movements or for anticipated size variations, to provide working space, or for other reasons. Both are critical because curtain wall construction involves covering a site-constructed skeleton with a factory-made skin, involving the work of numerous trades, which implies numerous sources for variance.
The greatest quality issues related to glass and metal curtain wall installation are tolerances and clearances. Failure to properly control them is the reason for most curtain wall installation problems. Architectural detailing can fail to recognize the full significance of standard tolerances and may provide inadequate clearances for installing the wall, necessitating changes, and adjustments on site. This delays the work and usually results in unnecessary additional costs. Therefore, it is essential specifications require the proper alignment and location of all materials related to the wall. Four different tolerances must be considered: building frame, installation, material, and fabrication and assembly.
Clearances are essential to allow proper working of sealant joints for differential movement and for access for modifications. Clearance issues are most often noted at non-typical anchors. In general, these should be at least 50 mm (2 in.) plus outward tolerance provided, per the CMHC’s Glass and Metal Curtain Walls Best Practice Guide.
Industry-recommended curtain wall system tolerances (excluding installation and substrates) for the overall sizes of factory-assembled framing and trim or for singular rectangular curtain wall units cannot deviate from the specified dimensions drawings by more than the following:
(a) ± 1.5 mm (0.0625 in.) for all dimensions 1.83 m (6 ft) and under;
(b) ± 3 mm (0.125 in.) for all dimensions between 1.83 m and 3.66 m (12 ft); and
(c) ± 4.5 mm (0.1875 in.) for all dimensions greater than 3.66 m.
Recommended allowable maximum deviations during installation are called out for level, plumb, and true characteristics, as well as straightness and variation from plane. Typical Canadian tolerance recommendations for variation from nominal level, plumb, square (diagonal measure), and true (straightness) positioning of the overall unit is in all cases 3 mm/m, (~0.12 in./ft).
Guide specification offers roadmap
In North America, the comprehensive resource for manufacturers, architects, specifiers, contractors, and testing agencies is AAMA CWM-19, Curtain Wall Manual, which addresses the above summarized aspects of curtain wall design, specification, testing, and installation. Newly revised in 2019, it is the most current source of information and features a comprehensive guide specification to aid architects and specifiers in developing job-specific curtain wall specifications.
Formatted to be compatible with the three-part CSI Master Specification, it covers performance and testing requirements, plus fabrication and installation methods, referencing a broad range of accepted industry standards. In writing the specifications for a project, an architect or specifier fills in the appropriate blanks for specific performance indices, deletes inapplicable paragraphs, or adds paragraphs to meet special or optional requirements. Explanatory notes aid in use. The guide specification is laid out as follows:
Section 1: General
This section contains descriptions of product, scope of the system, and a list of work to be included in the specification. It references additional items that should be specified, such as qualifications, performance, and testing requirements (including possible use of mock-ups), required submittals (e.g., samples, structural calculations, and test reports), quality assurance measures, and warranties.