# Tenon size when members are different sizes



## AwlThat (Mar 1, 2018)

I've been reading up on the proper ratios for tenons and while I think I understand it pretty well, I have a question regarding the joining of two specific member sizes.

I want to use a 4"x4" post, which is actually 3.5"x3.5" and a 2"x4" rail, which is actually 1.5"x3.5".

The post is going to be a corner piece and I want to mortise and tenon all the joints plus, I want the rails to be flush with the top of the post but inset 1/4" from the faces of the post.

How big should the tenons be on the 2×4's and still not interfere with each other inside the post?

If I make the thickness of a tenon 1/3 the thickness of the 2×4, it would be 1/2" thick. If the tenon length is to be 5 times the thickness of the tenon, it would need to be 2.5" long. Using those rules, the two tenons would "overlap" inside the post.

To avoid the "overlap" I could either shorten the tenons by 1/4" or 3/8" or mount the rails flush with the faces of the post.

Which would be better? Also, is it a bad thing to be removing so much material from the post where the mortises will be so close to each other?

By the way, I'm making the base of a workbench that will have a Paulk-style top on it. The generall size of the base will be 6ft long by 40-42" wide.

I am only in the designing stages and exploring the different joinery possibilities. Maybe a shop project doesn't need this kind of attention, but I want to use the workbench as practice for other things I want to start making.


----------



## rad457 (Jun 15, 2013)

I usually go by how much shoulder I want to leave on the Tenon?


----------



## pintodeluxe (Sep 12, 2010)

One more rule I'd add is to keep the mortise 1/2" or more away from the top of the post. I see some cut too close to the top, which weakens the joint.

Rendering models in SketchUp has been a great help for me. It will help you predict tenon length, and you'll know when to miter intersecting tenons.

I'd make the tenons 2-1/2" or 2-3/4" wide and 3/4" or 1" thick. Make the tenons as long as you can, so long as they'll fit once mitered.

You have a lot more leeway when mortising into a big post. You can make the tenons bigger than you think.


----------



## AwlThat (Mar 1, 2018)

Good point, pinto. I wasn't even thinking about how close the tenon would be to the top of the post.

If I leave a 1/2" shoulder on the top and bottom of the 2×4, I will have a 2.5" tall tenon (or wide, using your reference, pinto).

If I inset the 2×4 .25" from the face of the post and leave a .25" shoulder on the sides of the 2×4, I'm left with a tenon that is 1" thick and 2-1/2" wide. That leaves the mortise 1/2" away from either edge of the post, i.e. top and side.

I never thought about mitering the end of the tenons to allow them to touch.


----------



## jeffdeluca56 (Dec 28, 2018)

Personally I would lean toward a slightly heavier tenon, maybe 5/8" thk. The thinking being the strength will be the weakest part of the joint which is the tenon at the joint line. Anything inside that joint after the glue cures will be in effect… solid wood. I would also peg the joint for added strength in a workbench as well as aesthetics.


----------



## 55woodbutcher (Nov 16, 2018)

I second the pegs. If you draw-bore for them you can almost get away with using no clamps.


----------

