Width Limit of a Glued Tenon

but what are the limits? March 12, 2006

Question
I would like to know how wide (across the grain) a tenon can be before wood movement would weaken the glue joint. What if quartersawn wood is used?

Forum Responses
(Furniture Making Forum)
From contributor J:
On wide joint you don’t use glue. If you do, only glue up the middle of the tenon. Use pegs as in breadboard ends. The tenon needs to be cut narrower than the mortise to give room to expand.

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From contributor P:
If you are going to peg it you need to make sure that you allow for movement with the pegs. Otherwise it is no different than putting glue in there.

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From the original questioner:
How wide is the widest tenon you would glue?

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From contributor T:
I have made a few tables with breadboard ends and I typically have a centered through (exposed) tenon of 3 and 1/2", the rest of the "breadboard" is stepped back. Sometimes, I place small wedges in that tenon, for show purposes but it is always glued.

There are no pegs involved. It looks good and after 15 years of having one in my home I can only detect the smallest splitting in the center of the table. So I’m guessing 3-4" is the width you seek, but that is for breadboard purposes. I have always wondered about passage doors with full tenons.

A 6-10" rail glued to a stile (even with only a 3-4" tenon) will likely show some cracks during heating season, especially if there is a center, vertical stile. The good question is how wide can you go with mortise and tenon construction before you go to wedges, staves (with glue), etc. and eliminate straight glue.

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From contributor P:
3" should be safe, 4" is the most we ever do, and that would only be joining two very thick pieces.

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From the original questioner:
Three to four inches seems to be the accepted limit.