Four kinds of glass that look identical until they break. Which one the code requires, which one stays in the opening, and which one is the cheaper right answer.
If code says safety glazing — a door, a sidelite, a shower, anything within 18 inches of the floor — you need tempered or laminated; annealed and heat-strengthened do not qualify. If the glass is overhead, a railing, a skylight or anything where a broken pane must stay put, you need laminated, often laminated tempered. If the glass is a big fixed lite in a facade that will see wind and thermal stress but nobody can walk into, heat-strengthened is usually the right and cheaper answer. Everything else is the reasoning.
| Feature | Annealed (untreated) | Heat-Strengthened | Tempered | Laminated |
|---|---|---|---|---|
| Strength vs annealed | 1× | ~2× | ~4× | Depends on lites; interlayer holds the pane |
| How it breaks | Large sharp shards | Large pieces, stays in frame | Small blunt cubes, falls out | Cracks but stays in one piece on the interlayer |
| Safety glazing (ANSI Z97.1 / CPSC 16 CFR 1201) | No | No | Yes | Yes |
| Stays in the opening after breaking | Somewhat | Yes | No | Yes |
| Can be cut after processing | Yes | No | No | No (each lite pre-cut) |
| Spontaneous breakage risk (NiS) | None | Negligible | Present; heat-soaking screens it | Present in tempered lites; contained |
| Security / sound | Low | Low | Low; loud shatter | Best; interlayer resists penetration and damps sound |
| Optical distortion | None | Slight | Roller wave, anisotropy visible in polarised light | Same as its lites |
| Typical use | Interior fixed lites | Spandrel, high-wind fixed lites, overhead with laminate | Doors, showers, railings, sidelites, storefront | Overhead, railings, skylights, security, acoustic, hurricane |
| Relative cost | Lowest | Middle | Middle | Highest |
Annealed is ordinary float glass, cooled slowly. It can be cut, drilled and edged after manufacture, which is why it is the starting point for everything else, and it breaks into long sharp pieces, which is why it is not allowed where people can walk into it.
Heat-strengthened glass is heated to about 620°C and cooled at a controlled rate, leaving surface compression of roughly 3,500 to 7,500 psi. It is about twice as strong as annealed, resists thermal stress well, and when it breaks the pieces are large and tend to stay in the frame — which is exactly what you want in a fixed lite forty feet up. It is not safety glazing: the shards are still sharp. See tempered and heat-treated glass.
Tempered (fully tempered) glass is cooled fast, leaving surface compression over 10,000 psi. About four times annealed strength, and it breaks into small blunt cubes — the “safety” in safety glass. Two consequences follow. It cannot be cut or drilled afterwards, so every hole and notch is done first. And it can, rarely, break on its own: nickel sulphide inclusions in the glass expand over years and can trigger a spontaneous break. Heat soaking is the screening test that provokes those breaks in the oven instead of on the building; specify it on any tempered lite that is hard to replace.
Laminated glass is two or more lites bonded with an interlayer — PVB for most work, SentryGlas (SGP) where stiffness and edge stability matter, as on frameless railings. The lites can be annealed, heat-strengthened or tempered; the interlayer holds the broken pieces together whatever they are. Laminated glass is safety glazing, resists penetration, damps sound, and blocks almost all UV. It is the only option where a broken pane must stay in place: overhead, railings, floors, skylights, hurricane glazing.
Entrance doors, storefront lites at sidewalk level, shower enclosures and interior partitions: tempered, because the code requires safety glazing and nothing must stay put. The glass on Coney Island Avenue and at 3915 Flatlands Avenue is tempered insulated units for that reason.
Balcony guards and stair railings: laminated, and usually laminated tempered with an SGP interlayer if the railing is frameless. A tempered-only railing that breaks leaves a hole; a laminated one leaves a cracked but standing panel. The Emmons Avenue balcony guards and the UGI staircase are laminated for that reason. See glass balcony railings.
Large fixed lites on a facade, spandrels, anything with a lot of sun on one side and shade on the other: heat-strengthened, which handles thermal stress and stays in the frame, at less cost than tempered and without the spontaneous-breakage risk.
Windows facing a busy avenue or a school playground: laminated for sound, because the interlayer damps the vibration — the church window on Amsterdam Avenue is two lites of laminated glass with an air space between them. See acoustic glass.
Overhead glass, canopies, skylights: laminated, and the inboard lite must be laminated even if the outboard is tempered, because a tempered pane that breaks overhead falls as a shower of cubes.
Tempered glass is one kind of safety glazing; laminated is the other. Both meet ANSI Z97.1 and CPSC 16 CFR 1201. Heat-strengthened and annealed glass do not, even though heat-strengthened is stronger than annealed.
A single tempered lite is stronger than a single annealed lite of the same thickness by about four times. Laminated glass takes its strength from its lites; laminated tempered is both strong and holds together when it breaks. For impact and security, laminated wins because it does not fall out.
Rarely, yes: nickel sulphide inclusions can expand over years and trigger a spontaneous break. Heat-soaking is the screening test that provokes those breaks in the factory. Specify it for tempered glass that is hard or dangerous to replace.
Because when a railing panel breaks it has to stay in place until it is replaced. Tempered glass leaves a hole; laminated glass leaves a cracked but standing panel. Most codes and every engineer we work with require laminated glass in guards.
Laminated, in an asymmetric build, because the interlayer damps vibration. Tempered and heat-strengthened do nothing for sound that annealed does not.
Tempered Glass Laminated Glass Heat-Soaked Glass SGP Laminated Glass Balcony Railings All Glass Types
About this page. Written by Alex GC Inc. d/b/a Alex Glass Construction and reviewed before publication by Aleksandr Chernovolenko, Managing Partner, with our design and estimating lead and a field foreman. Our foremen hold the NYC DOB Site Safety Training supervisor card and our crews the 40-hour worker card; we are certified to fabricate, install and service Alutech and Everest aluminum systems. Facts are drawn from our own projects, manufacturer test data and the applicable codes; where we are giving an opinion, we say so. Last reviewed September 2026. Credentials
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