Volume 37, Issue 5, May 2002
Run, Chicken Little
Are You Sure That Overhead Glazing is Secure?
by Dez Farnady
The sky may not be falling, but I sometimes feel like Chicken Little when I see some overhead glazings. During all of my years in the business, there has been a bit of paranoia deep in my heart about very large pieces of glass in overhead applications. As a supplier, I could put my customer on notice while protecting the liability exposure of my employer. But now that I am in the skylight business the issue is even closer to home and I still don’t know who makes the rules.
Where’s the Info?
Codes are ambiguous or altogether non-existent. Manufacturers’ specifications are few and far between. The software for wind loads and overhead glazing, which is available free on the Internet, is probably the only reasonable resource for calculating glass requirements. It allows for special size, slope, load requirements and offers a variety of design possibilities to meet the circumstances in a reasonably user-friendly format. I feel the old AAMA formulas are too conservative for most applications. And the automatic use of heat-strengthened laminated, as preferred by the “big guys” in the skylight business, gets to be too expensive on smaller, single-lite skylights. Heat-strengthened lami more than doubles the glass costs of an insulating unit made with tempered, over-annealed lami.
The only minor problem I find with the load-calculating computer program is it sometimes seems too liberal because it doesn’t deal with the material, only the math. A piece of lami may be perfectly capable of meeting the load requirements in a computer model, but the computer can’t anticipate some of the handling problems.
The two most common products used in overhead glazing are nightmares. My grandmother loved wired glass in the bathroom window or even in the shower door. Well, maybe not. But, just about every old sheet metal or putty-glazed steel skylight is glazed with the stuff. Just don’t look too closely because you will find that many of them are cracked. Lami may be no better if you consider that a two-by-four is not a two-by-four, ¼-inch lami is not ¼-inch either. It used to be 7/32 inches but it may sometimes be as thin as 3/16. The vinyl is still the same .030, but one piece of lami glass is so thin that it isn’t good for much more than being glued to another piece.
Wired and laminated glass used in overhead applications is there to keep the glass, if broken, from falling out and raining on Chicken Little’s head—the two do this admirably. So while both wired glass and standard-commodity ¼-inch lami are safe enough in most applications, they are still a major item of grief when they start cracking or the lami starts to run. This is especially fun in a skylight that is a mile up in the air on somebody’s expensive tile roof. Unfortunately, Spiderman is not on your glazing staff.
I won’t even discuss the idea of tinted, annealed lami overhead, that stuff will break when the sun smiles on it and I don’t want to get into the heat-break issues at this point. Size problems are enough to deal with in this column.
The true test of sloped glazing is similar to evaluating a friend. He may be a good guy to drink beer with, but would you want him to marry your sister? A big 20- or 25-square-foot piece of lami may be OK in a vertical application, but would you want it over your head?
With all of the cracked lami and wired glass with which I have had to deal, I have discovered my own thumb and made my own rules for it. My rule for wired glass is the old UBC rule—about 24 inches in width is enough for me. I will live with both wired and commodity- stock lami up to 12 to 16 square feet, with sunshine loading (no snow load). This means that for overhead glazing, 2-feet-wide wired glass is probably OK up to 8 feet in length. Seven thirty-seconds lami is pretty standard in four-by-four skylights. But 16 square feet is usually my max, whether the slope glaze is nearly flat or a steep pitch. With those two products, anything bigger may crack easily. And while you will not have to run around crying “Run, Chicken Little! The sky is falling,” you will probably be looking for Spiderman.
Dez Farnady serves as general
manager of Royalite Manufacturing Inc., a skylight manufacturer in San Carlos, Calif. His
column appears monthly.
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