What Is Tempered Glass and Why It Matters
Tempered glass is glass that has been heat-treated so it breaks into small, blunt granules instead of long, sharp shards. It is roughly four times as strong as ordinary annealed glass, and building code requires it in the places where a person could fall into or push through a pane.
If you are specifying glass for a door, a shower, a storefront, or anything low to the floor near a walkway, the answer is almost always tempered. This page covers what tempering does, what it looks like when it breaks, and where the code requires it.
What tempering actually does
Tempering is a heat treatment, not a coating. The glass is cut, drilled, and edged to final size first, then heated to roughly 1,200 degrees Fahrenheit and quenched with high-pressure air. The surface cools faster than the core, which locks the surface into compression and the core into tension.
That compression layer is why tempered glass is stronger, and it is also why it fails differently. When a tempered lite finally does break, the internal stress releases all at once and the whole pane lets go into small pieces instead of cracking into a few large, knife-edged shards. We run tempering under the same roof as our cutting and edgework.
One consequence matters when you order: tempering is the last step. Once glass is tempered it cannot be cut, drilled, or edged. Every hole, notch, and edge finish has to be right before it goes in the furnace. That is covered in can you cut tempered glass.
What it looks like when it breaks
A broken tempered lite does not shatter into shards. It comes apart into thousands of small, roughly uniform pieces with blunt edges, often described as looking like coarse sugar or pebbles. There is no sharp blade left to fall on, which is the entire point of the treatment.
It is also worth knowing how it sounds and behaves before it fails. Tempered glass holds together under load and then releases suddenly, rather than cracking gradually the way annealed glass does. You get little warning.
Where tempered glass is required
Building codes call these "hazardous locations." The logic is simple: if the glass is somewhere a person could fall into it, push through it, or sit beside it while wet, it has to be safety glazing. In practical terms, tempered glass (or laminated glass) is required for:
- Doors. Any glazing in a swinging, sliding, or bifold door, including a glass door and a storm or combination door.
- Glass next to a door. Sidelights and fixed panels within about 24 inches of a door edge, where the bottom of the glass sits less than 60 inches above the walking surface.
- Large windows low to the floor. A pane larger than about 9 square feet, with its bottom edge under 18 inches off the floor and a walking surface within 36 inches of it.
- Bathrooms. Shower doors and enclosures, bathtub doors and enclosures, and glazing around hot tubs, whirlpools, saunas, and steam rooms.
- Guardrails and stairways. Glass used in a guard or railing, and glazing near the treads and landings of a stair.
- Vehicle glazing. Side and rear windows in cars, trucks, buses, and equipment cabs are tempered to meet the federal motor vehicle glazing standard.
Glass furniture is a separate case. Table tops, shelves, and display cases are not regulated the way a door is, but tempering is still the right call on anything people lean on, sit at, or reach across.
The standards behind the requirement
Three references come up over and over:
- ANSI Z97.1-2015 is the industry standard for safety glazing, with performance classes A and B. Class A is the higher of the two, tested at a 48-inch drop.
- CPSC 16 CFR 1201 is the federal safety standard for architectural glazing, with Category I and Category II impact classes. Category II is the equivalent of ANSI Class A.
- IBC Section 2406 is where the building code lists the hazardous locations and requires safety glazing in them.
We temper to ANSI Z97.1-2015 Class A and 16 CFR 1201 Category II, the higher classification, so we can supply any hazardous location. Every tempered lite we make carries a permanent mark in one corner identifying it as safety glazing and naming the standard it meets. That mark is how an inspector confirms the glass in an opening is what the drawings called for.
This is general information, not a code ruling. Local amendments vary, and the authority having jurisdiction has the final say on your project. Confirm the requirement with your local building official, and we will make the glass to match.
Where it is not required
- Framed windows and doors where the glass edge is fully covered and the pane is small or high off the floor.
- Small decorative lites, roughly 3 inches or less in their smallest dimension, which a person cannot pass through.
- Glass fully backed by a continuous surface, such as a mirror or panel mounted flat against a wall.
Annealed glass is fine in those spots and costs less. There is no reason to temper a pane that will never be reached.
How to tell if glass is tempered
Look at the corner of the pane. Tempered glass carries a small, permanent mark, usually etched or fired into the surface, that names the standard it meets. If you see the mark, the glass is safety glazing. If you do not, and the glass is in a door or a low window, it is worth checking before you assume. Tempered vs annealed glass covers the rest of the differences.
Frequently asked questions
How do you know if glass is tempered?
Look for the permanent mark in one corner of the pane. Tempered glass is required to carry a small stamp naming the safety glazing standard it meets. If there is no mark and the glass sits in a door or a low window, verify it before assuming.
Is tempered glass stronger than regular glass?
Yes. Tempered glass is roughly four times as strong as annealed glass of the same thickness. The bigger difference is how it breaks: into small blunt granules rather than large sharp shards.
Can tempered glass be cut after tempering?
No. Tempering is the final step. Once glass is tempered it cannot be cut, drilled, or edged without shattering, so every hole, notch, and edge finish has to be completed first.
Where is tempered glass required by code?
In doors and glazing near doors, in large windows low to the floor near a walking surface, in shower and tub enclosures, around hot tubs and saunas, and in guardrails and stairway glazing. These are the hazardous locations listed in IBC Section 2406.
Is tempered glass the same as safety glass?
Safety glazing is the broader term, and it covers both tempered and laminated glass. Tempered glass is one of the ways to meet a safety glazing requirement, and it is the one most common in doors, showers, and furniture.