What tempering does to a sheet of glass
Tempered glass starts as ordinary glass. It gets reheated until it's nearly soft, then blasted with cold air so the surfaces harden while the core is still hot. That locks the outer skin in compression, and the result is a panel several times stronger against impact than the same glass untreated.
The other half of the story is how it breaks. When tempered glass finally gives, the whole panel goes at once, crumbling into small blunt pebbles instead of long sharp pieces. That breakage pattern is why it's sold as safety glass. The trade-off is that it can't be cut, drilled, or trimmed after tempering, so every tempered panel is made to its final size before it goes into the furnace.
What regular glass does well
Annealed glass is cooled slowly at the factory, which leaves it easy to work with. A shop can cut it to size the same day, it costs less, and for a window sitting at chest height in a bedroom wall it's a perfectly good material that will last for decades.
Its weakness is the way it fails. Annealed glass breaks into shards with real edges, and those shards are the reason building codes draw firm lines around where it's allowed.
Where the code insists on tempered glass
The pattern behind the rules is easy to hold onto: anywhere a person could fall into glass or walk through it, the code wants safety glazing. In a house that generally means glass in and around tubs and showers, glass in doors, panes right beside doors, large panes that sit low to the floor, and glass alongside stairs.
The precise rules are written in inches and square feet, and we won't recite measurements here, because the exact numbers belong to the current code edition, not to a blog post. The working rule for a homeowner is simpler. If the glass is low, near a door, near stairs, or in a wet area, assume it needs to be tempered until someone who has checked the code tells you otherwise. When we measure for a replacement, checking the location against code is part of the visit.
Where laminated glass fits
There's a third kind worth knowing about. Laminated glass is two sheets bonded to a clear plastic interlayer, and when it breaks it behaves like a windshield: the glass cracks but stays in the frame, held together by the layer inside.
That staying-put quality gives it jobs tempered glass can't do. It slows a break-in, because the pane stays in the opening even after it's hit. The interlayer also damps sound, which makes laminated a real option for bedrooms facing a busy street. It qualifies as safety glazing in many of the same locations as tempered, and it costs more, so for most rooms in most homes tempered covers everything you need. Laminated earns its place when you specifically want glass that stays in the frame after it breaks.
How to check what you already have
Go back to that stamp, which glass people call the bug. Walk past your shower door, your patio slider, and any glass in or beside a door, and look in the corners. A stamped panel is tempered and exactly where it should be. A shower door or door pane with no stamp at all is worth replacing, whatever the story of how it got there, because that's a spot where the breakage pattern genuinely matters.
Getting the right glass when something breaks
When a pane breaks, sorting tempered from annealed isn't your homework. Matching the glass to the location is part of any replacement we quote, the written estimate says which type you're getting, and a broken window is the most common version of that call. Call (503) 887-8329 and describe where the glass is, and we'll take it from there. Clear Works Glass, based in SE Portland. Estimates are free. Monday through Friday 7 to 5, Saturday 8 to 2, and a 24/7 line for emergencies. Oregon CCB #261682, bonded and insured.