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Tempered or Non-tempered Glass

Question

I am building a desk and want to sandwich glass between two wooden ends. Should I use tempered or non-tempered glass? What thickness? How to attach glass in wood sandwich (glass panel in frame)?—Anon

Answer

Dave Frechette replies: If you go to a glass dealer with your measurements and with the weight you want the glass to hold, they will tell you what to buy. I would suggest that the wooden ends be about twice the thickness of the glass with a space routed out for the glass to rest in. The more glass that is in the routed section, the stronger the joint.

I am not an engineer so there are no guarantees, but I built a coffee table using about the ratios below and it is solid.

For instance, if I were making a six foot long desk and using 1/2″ glass, I would make each end of two pieces of 1/2″ or 3/4″ plywood about 1-1/2 feet long with a 1/4″ deep routed section the width of the glass and extending into the end piece about a 15″. I would caulk inside the routing on both pieces, lay the glass in and glue the pieces together, repeating the process on the other end. The more glass that is inside the wood, the more stability of the center exposed glass.

You can then veneer or treat the plywood anyway you want. If you are veneering, you might want to veneer the edges that will butt up to the glass before the glue up. You also might want to color the routed plywood near the edges so that the white will not show when the joint is examined.

Myrl Phelps replies: I’m no expert, but, I don’t think that you can use tempered glass in a horizontal application. I believe it will break. And if I needed to use an adhesive to connect it to wood I would use silicone.

Bob Oswald replies: First, in a desk or table application, I would highly recommend tempered glass. Tempering creates balanced internal stresses which cause the glass, when broken, to crumble into small granular chunks instead of splintering into jagged shards. The granular chunks are less likely to cause injury.

Tempered glass is commonly used in vehicle windows, shower doors, architectural glass doors and tables.
Thickness is best recommended by glass manufacturers. A table would be subject to someone leaning on it or dropping an object, possibly heavy, from an unknown height with high impact load possibility. Single strength which is 1/16˝—definitely not. Double strength, 1/8˝, might be ok for a small contact area perhaps 12˝ in the narrowest dimension. Safest would be 1/4˝ tempered glass for the typical table application. For a commercial coffee table, a glass top would be 3/4˝ thick although probably more for appearance than strength.

One attachment would be to set the glass into a rabbet with no hold-downs. Another would be to insert it into a groove cut with a tongue and groove type bit. Replacement might be a problem however.
I’ve built a couple of living room end tables using 1/4˝ tempered glass with beveled edges. The glass is simply resting in a rabbet, cut close on the sides to minimize glass movement, dust collection and better appearance. The best cutting option is to make the table top and take it to the glass shop. You’ll get a better, tighter fit if you let them take responsibility for the size. I have a heavy table lamp on this top and have no reservations about the strength.

Bob LaCivita replies: In NH building code any glass below 21″ from the floor must be tempered. Your desk does not need to comply with building code but I feel the 21″ rule is worth sticking to. The benefits of tempered glass is safety. Tempered glass can take a significant blow before it shatters. I would recommend a minimum of 1/4″.

Brooks Tanner replies: Use of glass is dependent upon the load expected in use. Permissible load limits are specified as pounds per square foot between supports. Load carrying capability is a steep curve, and is not a straight line ability. As an example, in 1/2″ glass if the distance between supports is 2 feet, the load capability is 81 lb. per square foot. Extend that to 5 feet between supports, and the load is diminished to 7 lb. per square foot. Using tempered will increase the strength by a multiple of approx. four (permissible load x4).

Tempered glass is stronger in blows to the face, however the edge is fragile. An accidental blow to the edge will leave you with many small cubes of glass all over your floor. The plus is the safety factor in not having a large shard to lacerate. I have found that 1/2″ plus glass gives a richer, more substantial look.

If any of the edges are to be exposed, have the glass factory polished or sanded. Glass that is sanded or polished at a local glassier is less than adequate for any reasonable project (local is not a commercial or factory supplier). I have used many shops both large and small and now only order factory edges.

I have found silicon to be the best adhesive for glass to wood or metal. It provides a bond that will allow movement of the substrate without breaking. Use of the black silicon will give a nice transition from the glass to the wood or metal. I recommend the wood be finished before the silicon is applied. This allows you to clean up excess silicon without penetration into any of the pores, and leaves a crisp pencil line transition. Be sure to use sufficient silicon to smoothly cover the entire interior surface of the joint since this surface will be visible through the glass.

Tags: Glass