What Happens When Glass Spontaneously Fractures in Buildings Above Head Height?

Modern buildings use more glass than ever before, but when glazing sits above occupied areas, strength alone is not enough. Designers, owners, maintenance teams and façade specialists also need to think about spontaneous fracture, falling fragments, post-breakage behaviour, compliance, and the importance of specifying the right type of glass from the outset.

Why this issue matters

Glass is a defining feature of modern architecture. It delivers light, openness and a premium finish across curtain walling, atriums, canopies, screens and high-level façades. As CIRIA’s Guidance on Glazing at Height explains, glazing at height includes glass that has the potential to fall on breakage and create safety concerns, particularly in façades, roofs, canopies and barriers.

This is why high-level glazing needs to be treated differently from ordinary low-level glazing. Once people may be present below the glass, the discussion changes from appearance alone to safety, specification, maintenance and risk management over the life of the building.

One of the most important issues in this area is spontaneous breakage in toughened glass, particularly when linked to nickel sulphide inclusions. In practical terms, that means a pane can look perfectly normal for months or even years and then fracture without obvious warning.

Why glass is so widely used in modern buildings

Glass offers architects and designers an exceptionally versatile building material. It allows natural light to flood interior spaces while maintaining weather protection, visibility and a contemporary aesthetic. It is used in curtain wall façades, shopfronts, atriums, skylights, canopies, balustrades and structural glazing systems.

CIRIA notes that the increasing use of large glazed areas at height has naturally increased the potential for glass to fall and injure people when breakage occurs. It also highlights that modern buildings demand more from glazing than ever before, including strength, environmental performance and safer post-breakage behaviour.

That is why glazed buildings need careful specification from the beginning. The wrong glass type in the wrong location can create a long-term safety and maintenance problem that only becomes apparent once the building is occupied.

What causes spontaneous glass breakage?

Glass can break for many reasons, including impact damage, thermal stress, poor installation, inadequate edge clearance, structural movement, surface damage or overload. CIRIA specifically identifies common causes of glass breakage such as thermal stress, edge and surface damage, loading issues and nickel sulphide inclusions.

However, one of the best-known causes of sudden unexplained fracture in toughened glass is nickel sulphide contamination. According to the technical note by John Colvin, small inclusions are an accepted reality of the float glass manufacturing process because raw materials are melted in bulk and tiny extraneous particles can remain within the finished glass.

Most inclusions have no real consequence. The problem arises when certain inclusions are present in thermally toughened glass. The toughening process heats the glass to just over 600°C and then rapidly cools it, creating high surface compression and internal tensile stress. If a critical nickel sulphide inclusion is present, it may later expand and disrupt that stress balance enough to fracture the pane.

This delayed failure is what makes spontaneous breakage so challenging. The pane may be in service for a long time before the inclusion causes enough local stress to trigger fracture.

What are nickel sulphide inclusions?

Nickel sulphide inclusions are microscopic particles trapped in the glass during manufacturing. In ordinary annealed glass they are generally harmless. The problem becomes more significant once the glass is toughened, because the inclusion can become locked into an unstable form during rapid cooling.

John Colvin explains that nickel sulphide occurs randomly in Western European float glass at around one critical inclusion in every 4–5 tonnes of glass. CIRIA similarly recognises nickel sulphide as a known cause of spontaneous breakage in toughened glass and notes that the risk can be significantly reduced, though not completely eliminated, by heat soaking.

The classic description is that the inclusion gradually reverts to its more stable low-temperature form and increases slightly in volume. Because toughened glass is already highly stressed internally, even a tiny increase in volume can create enough local pressure to cause fracture.

In some cases, nickel sulphide-related breakage may show a characteristic “butterfly” or figure-of-eight fracture pattern at the origin. However, many specialists believe that the only reliable way to determine the cause of breakage is to deglaze the pane, recover the origin and examine it microscopically, potentially with x-ray analysis to identify the chemical composition.

Why glazing above head height needs extra care

Toughened glass is often described as a safety glass because it breaks into many small blunt fragments rather than long sharp shards. CIRIA confirms that thermally toughened glass is considered safety glass for human impact situations. But it also warns that toughened glass is not necessarily safe in all situations, because the fragments can clump together and may fall en masse.

This is the critical distinction for high-level applications. In overhead or high-level glazing, the issue is not simply whether the glass fractures into safer particles. The issue is whether those particles, clumps or parts of the pane can fall and injure someone below.

John Colvin’s makes this point clearly. He notes that when toughened glass breaks due to a low-energy cause such as nickel sulphide, the broken glass may initially jam in the frame but can later relax and fall out. He also notes that even when the majority of broken glass stays in place, some small fragments and splinters may still detach immediately.

That is why high-level glazing has to be considered in terms of post-breakage behaviour, not merely in-service strength.

UK regulations and industry guidance

A number of UK regulations and guidance documents influence how glazing should be specified where safety is critical. CIRIA’s appendix on UK glazing legislation highlights Approved Document K, Approved Document N, BS 6262, BS 5516 and related guidance on glazing at height.

Approved Document K addresses protection from falling, collision and impact. It is relevant where glazing forms part of a barrier or where people could come into contact with glass.

Approved Document N, although now largely superseded in practice by later regulatory structure, historically addressed glazing safety, impact, opening and cleaning, and remains a familiar reference point in the industry.

BS 6262 provides detailed guidance on glazing for buildings, including safety-related matters, while BS 5516 deals with sloping glazing and is particularly relevant to roof and canopy applications.

CIRIA’s overall message is that modern glazing at height should be subject to structured risk assessment and careful glass selection rather than assumptions based solely on standard safety-glass classifications.

Why laminated glass is often the better option

Laminated glass uses two or more panes bonded together with an interlayer. If the glass breaks, the interlayer helps retain the fragments. CIRIA explains that laminated glass may be used to enhance safety and post-breakage performance, which is exactly why it is so important where people may be present below the glass.

In practical terms, laminated glass can prevent a fractured pane from immediately losing its integrity and shedding fragments into occupied areas. This makes it particularly valuable in roof glazing, canopies, atriums and other overhead applications.

For many high-risk positions, the safest specification is often toughened laminated glass, combining the strength of toughened glass with the fragment-retention characteristics of a laminated build-up.

What about heat-soaked toughened glass?

Heat soak testing is one of the glazing industry’s main control measures for nickel sulphide-related breakage. CIRIA describes heat-soaked toughened glass as toughened glass that has been subjected to sustained elevated temperature so that a large proportion of unstable nickel sulphide inclusions will cause failure during the test rather than later in service.

Importantly, CIRIA also makes clear that heat soaking significantly reduces the likelihood of future nickel sulphide breakage, but does not eliminate it entirely. That is why heat soak testing should be viewed as a major risk-reduction step, not an absolute guarantee.

For glazing at height, particularly in monolithic toughened applications, CIRIA states that the use of non-heat-soaked toughened glass is discouraged where the consequences of falling glass could be more severe.

Best practice for glazing above occupied areas

For buildings with overhead or high-level glass, the safest route is a specification-led approach supported by practical maintenance planning. In simple terms, that means using the right product in the right place, backed by proper design, risk assessment, inspection and replacement strategy.

• Use laminated or toughened laminated glass where broken fragment retention is important.
• Consider heat-soaked toughened glass where toughened panes are being specified at height.
• Review occupancy below the glazing and assess the real-world consequences of failure.
• Ensure the glazing system details are suitable for safe post-breakage behaviour.
• Inspect damaged or suspect glazing promptly and replace failed units professionally.
• Maintain accurate operation and maintenance records so future risks can be managed properly.

CIRIA also stresses the need for maintenance, inspection and repair to be considered during design. In other words, the glazing system should not only be safe when first installed, but also safe to inspect, maintain and replace over the life of the building.

Conclusion

Spontaneous glass breakage caused by nickel sulphide inclusions is a recognised phenomenon in the glazing industry. Although relatively uncommon, its unpredictable nature means it must be considered whenever toughened glass is used above head height or above occupied areas.

Understanding how toughened glass behaves, how nickel sulphide inclusions work, and why laminated or heat-soaked glass may be preferable in safety-critical positions helps reduce both risk and long-term maintenance issues.

By following good practice, complying with relevant regulations and selecting the right glass type for the application, building owners, architects and façade specialists can ensure that modern glazed architecture remains not only visually impressive, but safe and robust over the life of the building.

References

1. CIRIA C632, Guidance on Glazing at Height, 2005.
2. John B. Colvin, JCGC Limited, Risk assessment related to toughened glass used in the atriums, Report JBC3495/01, 14 September 2016.
3. JCGC Limited, Technical Note 02: Inclusions in Toughened Glass, 8 March 2006.
4. Approved Document K, Building Regulations – Protection from falling, collision and impact.
5. Approved Document N, Building Regulations – Glazing safety guidance.
6. BS 6262, Glazing for Buildings, and BS 5516, Patent glazing and sloping glazing.

Need help with glazing at height or specialist glass replacement?

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