Why Tempered Glass Fails — And How to Avoid It

8 6 minutes read

A practical, real-world guide for homeowners, installers, and commercial users

Tempered (toughened) glass is chosen for one main reason: safety. It’s heat-treated so it becomes much stronger than standard annealed glass and, if it breaks, it usually shatters into small blunt pieces rather than long sharp shards. That said, “strong” doesn’t mean “unbreakable.” Tempered glass can and does fail—sometimes immediately after installation, sometimes months later, and sometimes seemingly “for no reason.”

The good news is: most failures are predictable and preventable. If you understand how tempered glass behaves (and what it hates), you can dramatically reduce the risk of breakage.

This guide explains the most common reasons tempered or toughened glass fails and gives clear steps to avoid each one.

How tempered glass behaves (the key to understanding failure)

Tempering works by heating glass and cooling it rapidly so the surfaces go into compression and the core goes into tension. Compression on the surface makes the glass more resistant to bending and impact. But it also creates an important reality:

The edges and corners become critical
Tiny defects can become big problems
Local “point loads” can trigger sudden failure

In simple terms: tempered glass is very strong across its face, but vulnerable at edges, corners, and contact points, especially if there’s damage, tight fitting, or uneven support.

1) Edge or corner damage (the #1 cause)

What happens

Most tempered glass failures start at the edge—not the middle. Small chips, “shells,” or micro-cracks at the edge act like stress concentrators. Under normal use (vibration, door closing, shelf loading, temperature shifts), those tiny cracks can grow until the panel suddenly releases its stored energy and shatters.

Typical real-world sources

Glass bumped during delivery or carrying
Edge knocked against tile, concrete, metal, or stone
Installed with the edge rubbing against a hard surface
Fasteners/clamps biting into the edge
Sliding doors hitting stops without cushioning

How to avoid it

Specify arrised or polished edges for exposed edges.
Inspect edges before installation. Reject panels with chips or “flake outs.”
Protect corners during handling (corner guards, cardboard, foam).
Never rest glass edges directly on hard materials; use rubber setting blocks or neoprene pads.
Ensure hardware has proper gaskets and isn’t metal-on-glass.

2) Incorrect installation: point loads and uneven support

What happens

Tempered glass is strong when loads are distributed. It fails when force is concentrated at a single small contact point.

Examples of point loads:

A shelf supported only at two tiny points
A clamp tightened unevenly
A panel resting on a screw head
A small stone or tile spacer trapped under the glass edge

Even if the panel doesn’t break immediately, repeated stress can cause delayed failure.

How to avoid it

Support shelves with continuous channels or properly spaced brackets designed for glass.
Use setting blocks at correct positions (often near quarter points, not corners).
Ensure surfaces are level; don’t “force” glass into an uneven frame.
Tighten clamps evenly and only to manufacturer torque guidance.
Keep all contact surfaces clean—no grit, screws, or tile shards under glass.

3) “Too tight” fitting: no clearance for movement

What happens

Glass expands and contracts with temperature. Buildings also move slightly (timber swelling, floor deflection, vibration). If glass is installed tight to the frame with no clearance, it can bind and build stress. That stress often concentrates at corners until it fails.

This is common in:

Shower screens set tightly between walls
Balustrade panels in channels with no allowance
Glass in timber frames that swell
Splashbacks wedged between cabinets and worktops

How to avoid it

Leave correct edge clearances in frames and channels.
Use compressible gaskets, setting blocks, and packers (EPDM/neoprene).
Never wedge glass rigidly with hard shims.
For splashbacks: avoid “pinching” between rigid surfaces; use suitable adhesive dots/lines and allow movement.

4) Thermal stress and thermal shock

What happens

Although tempered glass handles heat better than ordinary glass, it can still fail if one area heats or cools much faster than another. That uneven temperature creates stress.

Common scenarios:

Cold water splashed onto hot glass (or vice versa)
Direct heat source close to an edge
Sunlight heating one part while another is shaded or cooled
Cooker/hob heat affecting a poorly installed splashback

How to avoid it

Use correct glass type for hot zones (tempered where required; sometimes other specialist glass is needed for extreme heat exposure).
Keep heat sources at appropriate distances.
Avoid directing boiling water/steam jets directly onto a cold panel.
Ensure splashbacks are installed with correct clearances and adhesives rated for temperature.

5) Nickel sulfide inclusions (rare, but real)

What happens

A small impurity called a nickel sulfide (NiS) inclusion can exist inside some glass. During tempering, it can be “frozen” in a form that later changes slightly in structure over time. That tiny change can create enough internal stress to cause spontaneous breakage days, months, or (rarely) years later.

This is uncommon, but it’s one of the reasons “spontaneous” tempered glass breakage is discussed in the industry.

How to reduce the risk

For higher-risk commercial applications (large panels, overhead glazing, façade, etc.), specify heat-soaked tempered glass where appropriate. Heat soak testing can reduce the likelihood of NiS-related failure.
For typical household shelves and small panels, the risk is low, but understanding it helps explain rare “no impact” breakages.

6) Drilling/cutting mistakes and fabrication errors

What happens

Tempered glass cannot be cut, drilled, or notched after tempering. Any attempt will cause immediate breakage.

But failures can also happen if:

Holes are too close to an edge
Notches have sharp internal corners (stress risers)
Edge finishing is poor
Dimensions are wrong and glass is forced into place

How to avoid it

Ensure all holes and cut-outs are finalized before tempering.
Follow proven design rules: keep holes and notches a safe distance from edges and corners.
Use rounded internal corners for cut-outs.
Double-check measurements and tolerances before ordering.

7) Wrong thickness for the job (excess flex = excess stress)

What happens

If glass is too thin for the span or load, it flexes more. Flexing increases stress at supports and edges. Over time, repeated flex + minor imperfections can lead to failure.

This often appears as:

Shelves that bounce or visibly sag under load
Large screens that wobble
Table tops with poor support

How to avoid it

Choose thickness based on span + support method + expected load, not just aesthetics.
If using point supports or long spans, increase thickness or change support design (continuous channel/support rails).

8) Hardware problems: metal-to-glass contact and over-tightening

What happens

Shower hinges, clamps, stand-offs, and channels are common failure triggers when:

Bare metal touches glass
Tightening creates a point load
Gaskets are missing or degraded
Hardware is misaligned, twisting the glass

How to avoid it

Use the right hardware for the glass thickness.
Ensure all gaskets are installed and in good condition.
Don’t over-tighten. More tight does not mean more secure.
Align doors properly so they don’t rack or slam.

9) Impact where tempered glass is weakest: edges and corners

What happens

A tempered glass panel can often take a strong hit in the center, yet shatter from a small tap on the corner. That surprises people, but it’s normal behavior.

Common causes:

Metal belt buckles or tools hitting the corner
A bottle or mug bumping a shelf edge
Vacuum cleaner or mop handle tapping a shower screen edge
Door edge contacting tile or a metal stop

How to avoid it

Install bumper stops and soft-close mechanisms.
Protect vulnerable corners (where possible) and avoid exposed sharp corners in high-traffic areas.
Keep hard objects away from glass edges during use and cleaning.

10) Chemical and surface damage over time (less common, but important)

What happens

Some chemicals can attack glass or coatings, and abrasive cleaning can create micro-scratches. While this usually affects appearance first, it can also contribute to weakening—especially at edges.

How to avoid it

Clean with non-abrasive cloths and mild cleaners.
Avoid scraping with blades on coated or painted-back glass unless manufacturer-approved.
In commercial settings, train cleaning staff on correct glass-safe products.