Short definition
Tempered glass is glass that’s been heat-treated to roughly four times the strength of standard annealed glass. When it does break, it fractures into small relatively safe granular pieces rather than dangerous shards. Required by code for all shower and tub-shower glass enclosures (IRC R308.4 / IBC 2406; WA via WAC 51-50 / 51-51).
What it is
The manufacturing process heats the glass to roughly 620°C and then rapidly cools the outside surfaces — the cooled surfaces compress while the still-hot interior retains tension. The resulting stress profile makes the glass much stronger and changes its breakage pattern.
Code applications in residential wet areas:
- Shower enclosures, tub-shower glass, shower doors. Always tempered.
- Doors and adjacent panels (within 24 inches of door, less than 60 inches from floor).
- Low windows (less than 18 inches from floor in walking areas).
- Stair side-walls and guard panels.
Common shower-door thicknesses:
- 3/16 inch (5mm) tempered. Framed shower doors.
- 3/8 inch (10mm) tempered. Frameless doors.
- 1/2 inch (12mm) tempered. Heavy frameless doors and panels.
Why it matters to a homeowner
The code rule is non-negotiable for any shower or tub-shower remodel: all glass in the enclosure must be tempered. Permit inspectors verify; manufacturers etch a small certification mark in a corner of each panel.
If a remodel reveals non-tempered glass in an existing shower (occasionally found in older custom DIY installs), replace it. Standard annealed glass in a shower is a safety hazard — it breaks into long sharp shards if shattered.
Spontaneous shattering of tempered glass is rare but real (typically from a nickel-sulfide inclusion in the glass batch or edge damage during install). The breakage pattern — sudden complete granulation with no impact — is dramatic but the small pieces are unlikely to cause severe cuts. Replacement is straightforward; the panel is gone, not repairable.
Replacement cost for a single tempered panel varies widely with size and finish: $100 to $1,500.
Common variants
- Tempered glass (heat-treated) vs. laminated safety glass (plastic interlayer holding shards). Both qualify as safety glass for code purposes; tempered is dominant in shower enclosures.
- Tempered (full strength) vs. heat-strengthened (about 2x annealed). Different products; heat-strengthened is not a substitute for tempered.