How to Identify Defects in Glass and Manage Customer Expectations

Experienced project managers in the glazing business know that customer expectations of glass quality are often out of sync with the industry standards.

The following scenario arises far too often: The installation crew noticed the flaw and brought it to the project manager’s attention. The project manager, having years of experience working with glass, decides that the flaw is minor and the glass pane should be installed as is. Later, the customer finds the flaw and wants the whole pane to be replaced. At this point, the contractor would be forced to replace the pane (and bear the cost of doing so), even though its quality is well within the industry standards.

The project manager could have reached out to the glass fabricator to negotiate for a replacement, but usually this is only done if a flaw is in fact a “defect”. The best solution is to manage customer expectations upfront, to make sure everyone is on the same page.

The reality is that flaws come with the territory, and it’s impossible to get flawless glass. But sometimes glass comes with serious defects, and in that case it needs to be repaired or replaced. As a project manager, you need to make sure your installation team is vigilant and constantly on the lookout for defects.. You also need to communicate with customers to manage expectations. Doing this will help you avoid costly replacements after installation.

In the first part of the article, we’ll go over how to identify a defect from a flaw. And in the second part, we’ll briefly discuss how to manage customer expectations. For those interested in furthering their knowledge, we have an in-depth course for project managers that goes into a lot more detail.

Part 1: How to know when a flaw is actually a “defect”

Before we dive into the various types of defects and how to identify them, let’s define what we mean by defect. We need a precise definition, because everyone will have a different opinion about when a flaw becomes a defect.

Below is a quick way to assess whether a flaw that’s visible up close, but may not be visible from farther away, is in fact a defect:

  1. Stand about 10 feet away from the glass so that you are looking at it straight on. Do this during the day, but not in direct sunlight.
  2. Inspect the central 80% or so of the pane. If you can no longer see the flaw, it isn’t considered a defect.

This is a simple test you can use to explain the difference between flaws and defects with customers. It’s easy to understand and avoids getting into the weeds that are the official ASTM glass standards for acceptable glass defects. But as an industry professional, you should be well-versed in these standards and understand when the simple test above does not apply. For example, for very large units (more than 35 square feet), the glass pane is allowed to have up to three scratches smaller than 1”, even if those scratches are visible from 10 feet away, before being considered a defect.

ASTM International sets the standard for glass and coating quality, allowable defects, and visual inspection criteria. There are many different specifications and classifications for various types (flat, heat-treated, insulated, laminated, etc.) and sizes of glass.

For example, for a single lite (a separately framed pane of glass) up to 6 square feet, no more than one scratch up to 1’’ long or other viewable defect is allowed. For larger lites (6 to 35 square feet), no more than two scratches up to 1” long or other viewable defects are allowed. Finally, for lites over 35 square feet, the limit for scratches or other viewable defects is three.

You’ll learn a lot more about the ASTM standards in our course lesson, Common Glass Defects. We also show you how to identify the most common types of glass defects that arise from the manufacturing process, fabrication, and mishandling. These defects are summarized below.

Types of glass flaws and defects

There are three broad categories of glass defects and flaws.

1. Manufacturing defects and flaws
Flaws arising from the manufacturing process are inevitable. The ASTM standards can be used to help you decide whether a flaw is actually a defect and the pane should be discarded.

Examples of manufacturing flaws include:

  • Seeds: small bubble that forms inside the glass substrate
  • Rubs: abrasions on the surface caused by a foreign object or another pane of glass
  • Inclusions: small particles or stones that have been linked to “spontaneous breakage” — this important issue is discussed in detail in our course
  • Coating voids and pinholes: gaps in the coating material that exposes the substrate

2. Fabricator defects and flaws
There are many flaws/defects that can occur before the pane ever reaches the customer’s building.

Examples include:

  • Breakouts: waves along the edge of the glass caused by the cutting process
  • Miss spots: an indent in an otherwise flat polished edge
  • Bad aris: the aris sizes on both sides don’t match
  • Heat-treating defects: there are many kinds, including bows/warping, ripples, overcooking, and heat stains

3. Handling defects and flaws
Finally, there are those simple defects that arise from mishandling glass panes. Whenever a pane of glass is handled, there’s a chance it can be damaged.

Common examples include:

  • Chips: usually caused by a tool or other piece of glass, often along an edge
  • Shells: caused by a direct impact with a hard object
  • Scratches: the most common defect and hardest to avoid — classified as heavy/deep or light scratches

Our course provides practical advice for dealing with heavy and light scratches.

Part 2: Managing customer expectations

Sometimes a customer will reject a pane of glass due to perceived defects, even though the pane meets the ASTM standards. When this happens, the contractor usually bears the cost of replacing the glass.

To help avoid this type of situation, the customer needs to have a basic understanding of the industry standards for classifying defects before seeing the glass installed. They need to be warned that the standards may not be in line with their personal expectations, and that flaws are to be expected.

It may help to remind the customer that you can find flaws in any material if you look at it up close; it’s just that flaws may be more visible in glass because of its transparency. But it won’t necessarily be noticeable if you’re not specifically looking for flaws — and most building occupants don’t scrutinize glass surfaces too closely. After all, glass is meant to be looked through, not at.

In our course on common glass defects, we provide more details about how to manage customer expectations and give you several strategies for dealing with this type of situation. This is an essential skill for any project manager — a skill that can help contractors save thousands of dollars and avoid major headaches in the future.


One final note on dealing with glass defects: choosing the right fabricator is critical. You should ensure that their quality standards are in line with your standards. And, of course, they should also be in line with your customers’ standards.

As a project manager, you need to become skilled at dealing with flaws, deciding whether a flaw should be classified as a defect, replacing defective panes, and managing customer expectations. Replacing glass panes after installation can be very costly and frustrating. Anything you can do now to avoid this situation will be worth the effort in the long run. Your future self will thank you!

Be sure to sign up for our course to learn the best practices for glazing project management and strategies for dealing with glass flaws and defects.