The Pitfalls of Glass Thickness Charts for Wind Loads: An Analysis Based on Canadian Standards

The Pitfalls of Glass Thickness Charts for Wind Loads: An Analysis Based on Canadian Standards

Attention Canadian Door and Window Manufacturers!

Many circulating glass thickness charts are not in line with Canadian standards. Let's consider the example below. According to the chart's supplier, the stated limits would apply to an insulated glass unit composed of two plies of annealed (non-tempered) glass. For a 4mm or 3.9mm glass, as mentioned in the chart, the limit would be 20 square feet for a ratio greater than 2.

What is the Ratio?

For example, let's consider an insulated glass unit measuring 1815 x 605mm (71'' x 23-7/8''). The ratio is 3 (1815 ÷ 605), and the area is 1.098 m² (11.8 square feet). We'll discuss later why we chose this size.

So we have a ratio of 3 which is higher than 2 (>2) which brings us to the right column of the table below (figure 1):

figure 1

Canadian Standards (CAN/CGSB-12.20-M89)

According to the chart in figure 1, the wind pressure to consider would be 40psf (DP40), equivalent to 1.92kPa. According to figure 2, a 4mm annealed glass of 1815 x 605mm could withstand approximately ±1.5kPa.

However, despite having two glass panes in the current insulated glass unit, we cannot simply double the resistance. According to the CAN/CGSB-12.20-M89 standard, the effects of external pressure and temperature must be taken into account. To do this, the standard considers that we can use 1.7 times the resistance of each glass to compare to the factored load when calculating a double glazed insulated unit.

figure 3


±1.5 kPa*1.7=±2.55kPa (±53psf). So, the 71 x 23-7/8’’ insulated glass unit composed with 2 plies of 4mm thick annealed glass could resist more than 40psf. We could verify with a larger size to find the maximum area for this composition…


In fact, the 71 x 23-7/8’’ insulated glass unit is at the limit with 40psf (1.92kPa):

figure 4

Differences Between the U.S. and Canada

It's important to note that many charts come from the United States, where the calculation method (allowable stress design or ASD) differs from ours (limit state design or LSD). In Canada, a safety factor is applied to the loads (factored loads) and a reduction factor is applied to the resistance depending on the material, while in the United States the safety factor is applied to the material resistance only.

The 40psf used for calculations in the United States is not the same as the DP40 (design pressure @ 40 psf) obtained from the Fenestration Canada calculator. Both represent 40 pounds per square foot, but how they are used in subsequent calculations leads to very different results.

The DP40 from the Fenestration Canada calculator is used to validate deformations (SLS or serviceability limit state). However, to verify the glass's resistance, a safety factor of 1.4 must be added. This safety factor is mandated by the building code. It's worth noting that there used to be another multiplicative factor to include, which made the 40psf become 40*1.4/0.75=74.7psf for the verification of the glass's resistance (with normal importance category). However, a change was introduced in the 2015 National Building Code of Canada (CNB) to compensate for the 0.75 factor. This was done to offset other changes made in earlier versions of the code. As a result, the 40psf (DP40) from the Fenestration Canada calculator now becomes 40*1.4=56psf (2.68kPa) for the verification of the glass resistance.

If you recall, when we did the calculation earlier with the chart, we had a resistance of approximately ±53psf for an insulated glass unit composed of two 4mm glass panes. The factored load divided by the resistance was 56psf/53psf for a result of 106%, roughly the same result as in Figure 4. It's important to note that the charts are less precise than the results obtained through our application, hence the small difference between the 100% in Figure 4 and the 106% calculated based on the CAN/CGSB-12.20-M89 standard charts.

To reiterate, the calculation done with the chart and the result in Figure 4 are valid for an insulated glass unit made of two 4mm annealed glasses with a ratio greater than 2 and an area of 11.8 square feet, whereas the chart in Figure 1 suggests a limit of 20 square feet.

This confirms that some charts are not consistent with Canadian standards. So, what are the options?

The Options Available

Hiring a specialized engineer in structural glass calculations (it happens that I know a very good one if you ever need help 😉), but it can be costly, especially if you're not sure you'll get the contract at the quotation phase. That's why we've developed tools to establish the dimensional limits of insulated glass units based on glass thickness and thermal treatment for wind loads.

We're proud to announce that one of our tools WLC (wind load charts) will be available for free on our website for an indefinite period. So you just have to provide us with your email address, and we'll give you access. You can contact us at

short demo of how WLC works


In any cases you will still need to have your calculations validated by an engineer before manufacturing, but you'll now at least avoid unpleasant surprises by being able to conduct preliminary verifications for free with WLC.

We also offer more advanced but paying versions like THV and ECV to further optimize glass thickness based on the exact wind loads of the project, depending on height and location of the building. You're likely to see considerable advantages by cutting down on the cost of materials. Don’t hesitate to ask, we’ll be happy to help you find the good one for you.

Contact us at


© Les Solutions de Verre et Mur-rideau inc., 2023. All rights reserved. Any unauthorized reproduction of this article or its contents is prohibited. This article is based on our interpretation of the codes and standards. This article and its contents do not constitute professional opinion and are provided for informational purposes only. This is an overview and does not necessarily cover all technical aspects and/or special cases. LSVM, MCi VSA or the author of the article cannot be held responsible for decisions taken in connection with this article. Reuse of this article or its contents for professional purposes, including engineering purposes, is prohibited. Please note that each project has specific conditions and must be validated by an engineer. Also note that this is valid for all articles published previously and subsequently.
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