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I will be creating a floating side table for my wife and am wondering the best way to join two pieces of wood.

They will be joined in an “L” shape with the back of the L hanging against the wall and the leg of the L used as the table.

My question is what is the best way to join these two pieces so the table portion can bear a good deal of weight? Mortise and tenon? I’d like the two pieces, once joined and painted, to look like one piece (seamless).

Also any idea of the best way of mounting it to the wall so it sits flush? I was thinking of recessing a French cleat in the back.

Type of wood suggestions are appreciated as well. Something that will provide a good deal of strength.

Please see this quick drawing to get a better idea of what I’m talking about:

hand drawn diagram of my idea

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  • How far from the wall will it extend and what will make up the downward force?
    – Dave D
    Sep 4 '20 at 19:29
  • The piece hanging on the wall is 16in tall and the table section jutting our from it will also be 16in long.
    – bongerama
    Sep 4 '20 at 20:18
  • It will be used as a side table so typical things that go on a side table(water bottle phones, occasionally heavier things up to around 30Lbs) will make up the downward force
    – bongerama
    Sep 4 '20 at 20:32
  • 2
    Hi, welcome to StackExchange. A 16" projection with that kind of load is likely too much for a M&T joint. You'd be better off going with one of the mounting ideas used for floating shelves, where metal rods project from the wall and the 'table' will slip over them. There's a lot of guidance on this online because of the current popularity of floating shelves, and a series of vids on YouTube covering how strongly they can be made mounted just to drywall (assuming you can't find a stud to mount to which would be better, or are working off a block wall).
    – Graphus
    Sep 5 '20 at 6:58
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    There are basically 3 questions buried in this one question. Related, maybe, but 3 distinct questions. Maybe edit this one to make it clear that you are interested primarily in the mechanical strength of a floating shelf. Installation would probably be a separate question and the type of wood is nearly irrelevant with any floating shelf technique you use.
    – jdv
    Sep 6 '20 at 13:39
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[Edited completely because I didn't understand that this appears to be a design that requires a 16in. cantilever shelf supported only by some sort of single joint, with no bracing.]

TL;DR

A mortice and tenon joint will never work. No simple joint would ever work.

You need to choose one or more of:

  • Engineer a proper cantilever design
  • Use bracing
  • Use a different material

Cantilevers

Think about how cantilevers are constructed for houses. There are engineering rules-of-thumb that says how much you can cantilever out for a given joist size, and the joists are extended (either as longer members, or by sistering up joists) to allow for that. Well, that's hard to do here, but it is how the Ikea floating shelves work; a metal frame is screwed with approximately 1003 screws into the wall which makes it part of the wall onto which a shelf is slid. (Not to mention that cantilever designs often incorporate bracing of some sort so that the cantilevered part is supported to support joist deflection.)

You could construct a similar design using only wood, but the idea is the same: you make a frame of some sort that extends the "wall" into the air, providing the stiffness along the length and at the joint. This might get very chunky, but the idea is that you combine members in two directions to support each-other. Think of how all-wood lumber racks are often constructed: there is a vertical stud onto which cantilevered brackets are mounted; often glued-and-screwed so we take advantage of the geometry of a "joist" meeting a "stud" not allowing easy rotational movement of the bracket. The rotational force has to overcome the shear strength of plywood cheeks, glue, and screws (and the screws are there mainly to prevent sudden failure and gives us good clamping for the glue joint).

(This is how my all-wood rack I built is made, and it supports hundreds of kilograms across three brackets.)

There are examples of this sort of floating shelf made entirely out of wood. A nice wood "stud" member is attached the wall, and shelf/brackets are attached to that via a notch or even clever plywood cheeks that lock the joint in place so it cannot deflect vertically. You could even hide this construction with larger "floating" shelf members, but there is a limit to what you could do with only wood. The more "brackets" you create, of course, the smaller the overall load is on each bracket.

Bracing/Trussing

Bracing reduces the clean look, but even braided cable (with or without a turnbuckle) going from about 2/3rds of the way out on the shelf to at least an equal distance up on the wall would work. The idea is you add a triangle to the design that actually supports the cantilevered mass.

Use Metal

As seen in the Ikea (et al) designs is that the actual load-bearing parts are metal. There is a reason for this.

Final Comments

All of this assumes you would be fastening the whole thing to the wall in a manner that resists the significant mass that wants to lever the fasteners out over time, and offers acceptable resistance to shearing.

As for how robust to make this design, that is an open question. You could consult civil engineering tables for various material and adapt those. But I think you'd be on your own with some help from the designs you find. Also, you need to determine your material first. Honestly, plywood is your best choice, at least for much of the actual joinery. And good, clean glue joints clamped down with hidden or pocket screws is your best friend in the long run.

To recap: this requires some clever wood engineering, but no simple joint will suffice. Either research some of the common floating wood shelf ideas out there that either display or hide the "triangles" necessary to support such a joint, add some bracing, or switch to a commercial or home-made metal load-bearing part.

On a personal note I'll warn you that you will probably never get a clean, straight look. You can make it strong, but every floating shelf made by mere mortals always sags, or looks like it sags. Walls are never true enough in any direction, and fastened brackets always come to rest at non-ideal angles (which look larger the longer the length of cantilever). So consider shims and other techniques for making the outside faces of your bracket mounting construction as true as possible. Never assume the wall is true enough for your design. And try to allow for bracket sag in your design by installing them a little high at the end.

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  • Butt joint reinforced with screws for a 16" projection from the wall?? Man, I thought I was confident in the strength of that joint ^_^ Seriously though, the tough requirements (the lack of obvious support plus an estimated max load of 30lb) led to the follow-on suggestion of treating it like a floating shelf, where the 'shelf' would slip on to steel rods. It's basically the only solution I could envisage that would give the look the OP wanted with the required strength.
    – Graphus
    May 25 at 8:34
  • I don't know. At those dimensions with those loads I'd be more concerned about how you are mounting it to the wall. A reinforced butt-joint using decent plywood would be plenty strong enough for static loads. That load is going to act more on the fasteners into the wall and only be shared along the lengths of that joint. I mean, I've got a cabinet full of records that relies only on reinforced mitre and butt-joints made out of relative soft wood and it's fine.
    – jdv
    May 25 at 11:31
  • Yes but surely those shelves with records on them are attached at both ends? This has to entirely support a cantilevered load (assuming it went ahead as planned needless to say). "I'd be more concerned about how you are mounting it to the wall" Yes, and by treating the projecting part like a floating shelf that's taken care of. The vertical part, in essence, becomes merely decorative.
    – Graphus
    May 25 at 14:15
  • Oh, I just looked closer at the sketch. The M&T joint is intended to support the whole cantilevered mass? I see what you are getting at. If this was a U-shape on the wall any joinery would do. But a 16in. cantilevered floating shelf is an engineering problem well beyond joinery.
    – jdv
    May 25 at 14:53
  • Updated with what I hope is relevant advice. This was nearly unanswerable in terms of this specific design because we have few details. But as an engineering problem there are obvious directions we can go. Honestly, the OP is on their own here.
    – jdv
    May 25 at 15:26

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