When to use tongue and groove?
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When making a top for a table or other platform, when should one just use edge glue and when do you use tongue and groove joints?
I have only used tongue in groove a few times so I could get this wrong. I am not mistaken, tongue in groove joints are not generally used for table tops. Tongue in groove is most common in wooden floors, but is also common for the backs of cabinets. It can, of course, be used elsewhere as well. Unless I am mistaken, its main function is to is to hold boards together mechanically and without edge gluing them together in a way that allows for expansion and contraction. A wooden floor, for example, can be very wide and hence have a lot of expansion and contraction, maybe enough to break your walls! The tongue in groove joint allows each expanding and contracting board to change size easily without the entire floor expanding and contracting. Of course, tongue and groove can also be decorative and you can even highlight the joint to make it more visible, which is common when you use it for the backs of cabinets. (Obviously, expansion and contraction for the backs of cabinets can be controlled by other methods as well. For example, you can make them using ship lapped joints. You can also use frame and panel. Today, plywood, which does not expand and contract much, is widely used for the backs of cabinets.)
Table tops can generally be allowed to expand and contract as they want. With a table top, the main issue is keeping the top from breaking away from the table apron as the top expands and contracts. This is generally controlled by using turnbuttons to hold the top to the apron rather than by using tongue in groove. I suppose you could make a table top with with a tongue and groove joint, but it might have some unpleasant results. For example, since tongue in groove joints are not glued together, and are left to expand and contract freely, food particles could find their way in there.
I wonder if you are thinking about using splines to help align table top boards to help with gluing. That is fairly common.
Thanks for the response. I was thinking about strength and durability. I have two table tops I am making – one is a 460mm x 350mm from two 460mm x 175mm and 30mm thick boards (walnut) with no apron – it will have four pillars to support it (part of a HiFi stand), the other is a 2m x 700mm top supported by two side frames only, 30mm thick, made from eastern maple. It will be a combination of 4 x 175 mm boards.
I haven’t seen T&G in table tops that often.
At an antique dealer several months ago, I did notice the use of T&G on the top of a bureau. This was from a piece circa 1850 or so I think.
I don’t see a problem with using it per se. I imagine it would be very helpful in aligning long boards for glue-up.
Hej Waldo,
If I understand Prof. R. Bruce Hoadley’s “Understanding Wood” correctly, splines / tongue-grooves actually weaken edge joints. The tongues were clearly the least strong parts when I laid out 1¾” thick floor boards at our croft. The tongues were 3/8″ thick, but they did not contribute in keeping the boards flat.
Loose tenons, and possibly biscuits, apparently do add strengths, as has been shown for the Festool Domino System; but neither have contributed to make my table tops and panels easier to glue up or kept them flatter. Admittedly, my clumsiness reaches its peak when it’s time for glueing.
Please accept a link to the Wood Database. With the modulus of rupture given for maple, it seems unlikely that a 30 mm thick table top will bow from gravity.
Sven-Olof Jansson
London, UK; Boston, MA
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