I participate in several home inspector forums and lately there have been lots of questions about safety glass. I thought I might take the opportunity to share what I’ve learned over 20 years as a window guy.
Safety glass. The words we use to describe glass that has some kind of magic properties that keep us from getting hurt if it breaks. In residential buildings, I’ll call them houses from now on, there are basically 3 kinds of glass; float glass that makes up the vast majority of the flat glass in our homes, tempered glass, and laminated glass. If you live in areas prone to hurricanes you may have impact glass, but that is an entirely different subject.
Float glass is pretty much what it sounds like. It’s made by floating the molten glass ingredients on top of a pool of molten zinc. The perfectly flat surface of the pool yields a perfectly flat sheet of glass. We can make this in various thicknesses, commonly single strength (slightly more than 1/16”), double strength (1/8”), and nominally 3/16” and 1/4”. It is common for larger pieces of the thinner material to be annealed which is a heat treatment that hardens the glass making it slightly tougher to break. These are not safety glass.
Tempered glass is float glass that has been treated so that it breaks into tiny little pieces with blunt edges. The process starts with a piece of glass cut to the desired size. That piece is then put onto a roller conveyor and into a furnace that heats it until it begins to soften, in technical terms – really hot. When the temperature is right the glass is conveyed out of the furnace and immediately blasted with cold air, like a blacksmith quenches his iron. The result is an incredibly hard piece of glass with tremendous tension across it’s surfaces. That tension dissipates stress from blunt impacts making the glass hard to break, hit it with a sharp object though (or enough force) and that tension causes the entire surface to fracture, breaking into tiny little pellets. The side windows on your car are tempered. You can hit them with a hammer and they won’t break. Turn the hammer around and strike with the edge of the claw and the sharp point will break the glass. There is tempered glass in almost every house.
Laminated glass is, well, laminated. Two pieces of glass are sandwiched around a plastic film to create a composite that is not only hard to break, but that stays together when it does. The most common example of laminated glass is your car windshield. We’ve all seen the guy driving around with a crack from edge to edge, yet the window doesn’t fall out. When one layer fails, the others stay together. There are lots of uses for laminated glass in houses. It can be substituted for tempered glass in almost any instance where safety glazings are required. It can also be used to enhance security.
So now that we know what safety glass is, where is it supposed to be?
Doors.
Doors with glass in them and any windows within 24 inches of the sides of a door are required to have safety glass. That’s the entry door, any sidelights, the storm door, the French door to the parlor, The glass doors on your closets, and your patio door. It’s a good idea to have it in cabinet doors and overhead garage doors but it’s not required.
Windows.
Windows that are in what we classify as Hazardous Locations require safety glass. Windows in bathrooms with any part of the glass less than 60” above the standing surface of your tub or shower. This also includes any windows next to doors if any part of the glass falls within 24” of the edge of the door and is 60” or less above the floor. All glass in walls or fences that enclose pools or spas where any part of the glass is less than 60” above the floor and less than 60” from the water’s edge. Glass within 36” horizontally from stairs, or within 60” of the bottom tread. These are all required to be safety glass.
The trickiest one is windows adjacent to walking surfaces. There are 4 conditions that must all be met in order for these windows to require safety glass; The exposed area of glass must be greater than 9 square feet, the bottom edge is closer than 18” to the floor or ground, the top edge is more than 36” above the floor or ground, and within 36” horizontally of a walking surface. This rule applies from either side of the window. Let’s imagine a window that meets all of these requirements but is 2 feet off the floor inside, but there is a sidewalk outside that is only 12” away from the building and 16” below the glass. From inside the building only 3 conditions have been met so the glass could be exempt, but all 4 conditions have been met from outside so safety glass is required.
Other Glazings.
We don’t just put glass in doors and windows. We like glass. There are rules for safety glass in other locations too. Shower enclosures are required to be tempered glass, as is all glass in railings. Oddly, things like glass table tops and shelves are exempt.
Exceptions
We can’t have rules without exceptions, right? Here they are. Transoms, the little windows you see above doors, are not required to be tempered. Jalousey doors, the ones that have the little crank that opens all the little louvered pieces of glass are also exempt. Doors with windows smaller than 3” on one side. Glass blocks, because they are masonry units and not glazing. Mirrored closet doors are not required to be safety glass as long as they have a continuous impact rated backing applied (there will be a sticker that says it’s there). We can also forego any safety glazings if we protect these areas with guardrails, but then there is a whole other set of rules, and design implications, and practical concerns. Who wants to climb over a railing to get out the door?
So how do we know it’s safety glass?
Products that are designed for locations that require tempered glass are themselves required to be labeled. The most common label is an etching in the corner of the glass describing it as tempered and listing the standard to which it was manufactured. Glass guys call it a bug. High-end products are allowed to be identified via affidavit, for example a custom 6’ patio door with true divided lites in a traditional French door design would have 30 little square pieces of glass that would each be required to have a bug (one on each side if it were insulated glass). This would be a glass door the cost of a new commuter car that you wouldn’t be able to look through because of all the labels. The manufacturer produces the door without the bugs, and provides a letter attesting to the fact the door is tempered glass. The document is required to transfer with the door. Don’t lose it.
Have a piece of glass that you think should be tempered but can’t find the bug or the affidavit? There’s a few simple tests to determine if it is tempered.
First, tap on it. Tempered glass has a different sound to it, almost a metallic ring. This test takes some practice and tends to work best on glass that is loose, or not installed.
Second, look for the ripples. Remember the conveyor used in the tempering process? The gaps between rollers allow the glass to sag as it is being quenched. As a result, almost all tempered glass will have a very slight, very regular ripple pattern. These ripples are typically parallel to the short side of the glass. This makes them hard to see in a door since you are looking along them rather than across them. The easiest way to learn what to look for is to go out to your car, place a knee against a bumper and look down the side windows. Ripples! These ripples have their own use as well, like putting tempered glass in large picture windows prone to bird strikes. Birdie sees the ripples and knows not to fly into the glass.
Lastly, break it. The failure mode of tempered glass is unmistakable.
These tests can also be helpful in identifying laminated glass. It will have a unique sound when tapped. You can sometimes see the laminations from a close angle. And, it breaks in a unique fashion.
Well there it is. One mystery ingredient identified. Want to know what else is in the hotdog we call a house? Send in your questions, and check back often.