Why Does Tempered Glass Shatter Years Later?

Spontaneous tempered glass breakage years after install traces back to one of two causes: a nickel sulfide inclusion trapped inside the glass when it was manufactured, or edge and face damage introduced during installation that took months to finish the job. Neither one gives you a warning, and both are manageable — but only if you decide how to handle them at spec, not at callback.

That distinction matters more on a $2M custom home than almost anywhere else, because the panel that lets go at 2 a.m. is usually the 10-foot stair railing or the wine cellar wall, and the builder's name is the one on the punch list two years after closeout.

What Actually Causes Tempered Glass to Break With No Impact?

Nickel sulfide inclusions. Exponent's managing architect Misael Rojas, AIA, writing for IIBEC's Interface, describes them as microscopic impurities formed when trace nickel and sulfur combine during float glass manufacturing. The particle gets trapped in the glass. In annealed or heat-strengthened glass it mostly sits there harmlessly. In fully tempered glass, it sits inside the tension core — the high-stress center layer that gives tempered glass its strength — and over time it can change phase and expand just enough to rupture the panel from the inside out.

How often? A 2001 University of Bath / Centre for Windows and Cladding Technology study, sponsored by Ove Arup and Bovis Lend Lease and cited in Pilkington's technical bulletin ATS-165, found that an NiS stone capable of causing spontaneous breakage occurs roughly once in every 4 to 12 tons of glass produced globally. The same study found that about half of NiS-driven breakage happens within the first 800 days — roughly 26 months — after installation. Pilkington's bulletin is blunt about the rest: breakage can occur with no applied load, at any time, including five or ten years after install.

So a shower enclosure that's been fine for three winters isn't automatically in the clear. It's just past the statistical peak.

Construction Specifier has covered the high-profile version of this — balcony and window glass letting go and falling from high-rise buildings — which is why spontaneous breakage now reads as a liability topic to architects and owners, not a manufacturing footnote. In residential work the stakes look different, but the phone call is the same.

Does Heat Soak Testing Actually Prevent Spontaneous Tempered Glass Breakage?

Partially. Heat soak testing holds tempered glass at roughly 290°C for a sustained hold per EN 14179, which forces unstable nickel sulfide inclusions to fail in the oven instead of in a finished home. It's a destructive screening process, not a cure — it breaks the bad panels early so you don't find them later.

The limits show up clearly in a documented U.S. case published on glassonweb.com. A developer on a project identified as "Project Alpha" heat-soaked all of the tempered glass and still experienced a high breakage rate in the field. Lab analysis of the failed specimens found nickel sulfide evidence in less than 12% of them. The rest were attributed to impact damage or were too damaged to determine a cause.

That single number should change how builders think about this. Most glass that breaks "for no reason" had a reason — it just happened at install, and took two years to finish.

Sources

Exponent: Nickel Sulfide Inclusions Are a Hidden Cause of Spontaneous Glass Breakage — Exponent (Misael Rojas, AIA, RRO, NCARB), republishing IIBEC Interface article, Apr 2026

Heat-Soaked Glass: Requirements, Implications, and Case Studies in the USA — glassonweb.com, 2025

Spontaneous Glass Breakage: Why It Happens and What to Do About It — Construction Specifier, Feb 2025

Spontaneous Breakage of Tempered Glass (ATS-165) — Pilkington North America Technical Bulletin, citing University of Bath / Centre for Windows and Cladding Technology study, 2013 (foundational, still cited)

ASTM C1048 — Standard Specification for Heat-Strengthened and Fully Tempered Flat Glass — ASTM International, standard (C1048-18)

Recommended Applications for Heat-Treated Glass (FB45-14) — National Glass Association (NGA) Glass Technical Paper, 2019/2021 (current NGA guidance)