Meeting manufacturer specifications should
be of prime importance to the contract glazier. Indeed, adherence to specifications is a
crucial consideration for any contractor in any field.
Where glass size and tolerances are in question, minute deviations from manufacturer
requirements can cause a perfectly good system to fall prey to air and water infiltration.
Glazing contractors must take a proactive stance in the process of identifying and
assembling products for a given project. Both glass and aluminum system manufacturers have
stated requirements for the use of their products. The role of the contract glazier is to
understand all of these specifications and to ensure there is a correct match between
glass and aluminum systems.
The contract glazier must investigate requirements and choose materials that together meet
the criteria necessary for preventing air and water
infiltration.
Although glass specifications allow for an acceptable range of tolerances, storefront
manufacturers recommend glass thickness and glass bite (the amount of the edge of the
glass that must be captured by the frame and gaskets) for every framing system. Meeting
that specification isn’t merely a good idea—it’s crucial to the
system’s performance. Pittsburgh’s PPG Industries, for example, requires a
minimum of 1/4-inch bite for insulating glass units. Violate that requirement, and you
face potential misapplications that could cost you your profitability and your reputation.
Perhaps even more critical to the performance of the framing system is the thickness of
the glass, which creates compression against the gasket once all members are installed.
Compression, in turn, keeps air and water from entering the system and enables it to
perform as published in test reports.
Numbers Can Be Deceiving
Given the crucial and obvious requirement that the size and weight of glass fall within
the manufacturer’s framing system tolerances, why is this requirement so often
misunderstood or seemingly ignored? One reason we’ve found is that numbers
aren’t always what they seem.
Just as 2-by-4 lumber is not really 2- by 4-inches, 1/4-inch glass isn’t 1/4-inch
thick. If a system is designed for glass that is 1/4-inch thick and glass is installed
that is only .219 inches thick, more often than not, the system will not perform the way
its design indicates.
A similar scenario applies to the use of 1-inch insulating glass. Indeed, insulating glass
may compound the problem if the correct air spacer is not used in conjunction with the two
lites of monolithic glass.
For example, take a 1-inch insulating unit and subtract 1/4-inches twice, and you get
1/4-inches. If the insulating glass manufacturer uses a 1/4-inch spacer and then uses two
pieces of
1/4-inch glass that are really each .219 inches thick, the 1-inch sealed IG dimension is
now about .938 inches thick. The glazing contractor has a problem if the system gaskets
are designed for 1-inch. The solution is to know the tolerances of the manufacturer and
prescribe the correct combination of glass and spacer requirements.
Know the Standards
As a further investigation into the importance of standards, let’s look at PPG’s
thickness standard:
Thickness Tolerance Inch Tolerance Metric
6.0 mm .219 - .235 5.56 - 5.97
This is based on Federal Specification DD-G-45l (covering flat glass for glazing, mirrors
and other uses):
Nominal Decimal Thickness = 0.23 inch
Traditional Thickness = 1/4-inches
Metric Dimension = 6.0 mm
Thickness tolerance range:
Min. = 0.219 inch - Max. = 0.244 inch
Min. = 5.56 mm - Max. = 6.20 mm
The thickness tolerance range calculated is not an option to be chosen—it is a
requirement demanded.
In short, every time you install a window system, there is a thin line between success and
failure. Exercise care at the beginning of the job by making certain the designed glass
bite is adequate and the total thickness of each monolithic piece plus the spacer is what
it should be. It is very important that contract glaziers recognize their role in using
professional judgement and expertise to correctly match glass and aluminum system
requirements. In the long run, that professional judgement will be the criteria used in
determining the glazier’s
reputation.
