Mortise and Tenon Questions

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Forum topic by TechnoBabble28 posted 05-20-2015 06:28 PM 770 views 0 times favorited 10 replies Add to Favorites Watch
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4 posts in 522 days

05-20-2015 06:28 PM

Topic tags/keywords: joinery mortise tenon question

Hey guys,

I will be building an outdoor table in a couple weeks and just had a question or two about Mortise and Tenon Joinery. I have attached two pictures that will hopefully help visualize my question.

This is the overall project (minus the top):

And this is what my question is about:

As you can see from the second photo, I want to use a Mortise and Tenon Joint for all of the corners. I am, however, unsure as to whether or not my approach to the matter is the best one. The legs (and all other base pieces) are 4×4 nominal. As such, I have sketched Mortise “A” to be 2.5” x 2.5” (.5” shoulder) with a secondary Mortise, “B”, perpendicular to this at 1.5” x 1.5” (1” shoulder). My thought was, that by interlocking the two rails inside the leg, I would maintain the strength of the joint. I was also planning to drive a lag bolt downwards through both tenons and into the leg.

I have spent some time trying to find reference to a similar joint online but have come up empty. It is for this reason that I appeal to those of you with experience to tell me if I am being silly or if this is a sound concept.


10 replies so far

View bandit571's profile


14063 posts in 2103 days

#1 posted 05-20-2015 06:47 PM

Just cut both ends at a 45 degree angle so that the meet. Really don’t need the lag from the top of the leg. Maybe add some metal corner brackets, and use screws to fasten the brackets with?

Note: You can “pin” each tenon before it meets the 45 angle…

-- A Planer? I'M the planer, this is what I use

View jdh122's profile


878 posts in 2237 days

#2 posted 05-20-2015 06:48 PM

First of all, I would forget about using the lag bolts – glue inside a reasonably tight joint will hold for a very long time, and a bolt down into the endgrain of the leg will only weaken things IMHO.
There are two traditional ways to deal with the meeting up of tenons in a leg mortise – the first (and most common) is to bevel them both at 45 degrees, and the second is to cut small notches on both tenons. Both should work, although personally I’ve only ever done the bevel method.
I suppose your method would work. It might offer slightly more strength (although you substantially reduce the glue surface on mortise A along the strongest part of the joint, as the top of A is sidegrain to endgrain and so very weak), but I would go with the more traditional methods.

Edit: Looks like bandit beat me to it…

-- Jeremy, in the Acadian forests

View HornedWoodwork's profile


222 posts in 634 days

#3 posted 05-20-2015 07:23 PM

That’s an incredibly ambitious joint to try and cut. It can be done but the miter as has been suggested is a more traditional approach and is plenty strong. One major disadvantage to the joint your propose is all of that short end grain making up the wall of mortise B against the narrowest part of the end of tenon A. There are some Japanese three way joints that would work in this scenario but they are difficult to cut and probably a fair bit of overkill in this scenario.

I do pin my M&T when I can to reinforce them, however a lag bolt in this scenario would do more harm then good I believe. If you are uncomfortable with the M&T miter joint from a strength perspective you might consider wedged stub-tenons, or alternately, through-tenons that half lap each other.

-- Talent, brilliance, and humility are my virtues.

View TechnoBabble28's profile


4 posts in 522 days

#4 posted 05-20-2015 07:36 PM

Thank you for the quick reply guys! So if I am understanding this correctly you are saying the best approach would be two beveled tenons like so?

View jdh122's profile


878 posts in 2237 days

#5 posted 05-20-2015 07:47 PM

Exactly. Except make the tenons thinner so that the walls of the mortises are thicker. People often suggest that the mortise should be about a third the thickness of the board, although you can make it slightly larger. So with 2.5 inch thick boards I’d suggest making the tenons no more than an inch thick.

-- Jeremy, in the Acadian forests

View darinS's profile


676 posts in 2287 days

#6 posted 05-20-2015 08:03 PM

Just throwing out this thought for you. Instead of one big tenon, make two smaller ones on each member. On the plus side, you end up with more gluing surface.

-- They say many people die because of alcohol. They never realized how many of them are born because of it.

View MrRon's profile


3891 posts in 2663 days

#7 posted 05-20-2015 08:03 PM

Thank you for the quick reply guys! So if I am understanding this correctly you are saying the best approach would be two beveled tenons like so?

- TechnoBabble28

You could also add a half lap to the tenons to increase the glue surface. A lag bolt won’t do any good. Screws don’t hold well in end grain (legs).

View TechnoBabble28's profile


4 posts in 522 days

#8 posted 05-20-2015 08:24 PM

I have been on a lot of forums over the years and I have to say you guys have been, by far, the most helpful so thanks again!!

I have two more renderings of what you guys have suggested so hopefully I am getting it right (not that I mind making renderings, they are rather fun).

Reduced the tenon down to 1.5”H X 1.5”W x 2.5”L:

Using those same smaller dimensions I then added a half lap per MrRon’s suggestion:

Which gives me a final version (potentially) looking something like this:

View jerryminer's profile


497 posts in 861 days

#9 posted 05-20-2015 09:29 PM

You’re getting close, but I would keep the tenons tall (maybe 2 3/4” in a 3 1/2” rail) but narrow ( 1—1 1/4), and offset towards the outside (for more glue surface. I also don’t think the half-lap as drawn is beneficial (reduces the glue surface on tenon face)

you could draw-bore these, too, for extra security

View TechnoBabble28's profile


4 posts in 522 days

#10 posted 05-22-2015 07:17 AM

Jerryminer, thanks for the rendering! Your approach is fairly straight forward. I think I have a good idea of how i am going to proceed so I just wanted to thank all of you again for the quick responses and all the help!

I’ll be sure to post up some pictures of the project when i get started. I know it is a fairly basic one but hey, I have to start somewhere.

Thanks again!

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