Tempered Glass

Tempered glass starts as standard glass. It is heated to about 1,200 degrees Fahrenheit and then blasted with cold air on both faces at once. This rapid cooling puts the outer surfaces under compression. The interior stays in tension. That built-in stress makes tempered glass four to five times stronger than ordinary glass.

When tempered glass breaks, it fractures into hundreds of small, blunt-edged cubes. These pieces are sometimes called "dice." They can still scratch skin but are much less likely to cause deep cuts than the long, sharp shards of ordinary glass.

Why it is required in garage doors:

U.S. building codes require safety glazing in hazardous locations. Windows in doors qualify. Garage door lites are subject to impact from tools, balls, and backing vehicles. They fall under this rule. The standard is ANSI Z97.1. Tempered glass is one of the most common materials that passes it.

For example, a 9-foot by 7-foot garage door with four windows in the top section must use tempered glass in each insert. If a ball hits one of those windows, the tempered glass breaks into blunt pieces. Standard glass would send sharp shards into the garage.

Tempered glass works with other upgrades. Low-E glazing coatings can be applied to it. Tempered panes are also used inside insulated glass double-pane units. One limit: tempered glass cannot be cut or drilled after the heat treatment. It must be ordered to its final size before tempering.