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I'm looking for a style of joinery to attach the mattress boxes in a bunk bed to the supporting posts. I tried inventing my own here and was correctly dissuaded.

The basic bed structure is from Ana White's Modern Bunk Beds and other online designs. The main difference is I want to build the mattress boxes as permanent units and be able to knock this down by removing them from the posts by unscrewing bolts.

Here's what I'm almost certain I want to keep:

  • Material is pine.
  • Posts are 4x4, mattress box sides are 2x6, cleats are 2x2, slats are 1x4, everything else is 2x4.
  • Mattress boxes should be permanent (i.e. 78x38 boxes with glued sides, cleats, and probably glued slats).
  • One side will have a ladder so guard rails can't be counted on to hold the posts apart or together.
  • No cuts more complicated than a half lap or dado. I also think I need to use bolts or screws rather than tusk tenons or the like.
  • Boxes are attached to posts by bolts.

This is the rough structure and the joint in question:

rough bed structure

It seems entirely possible that the posts will attach (at least partially) inside the frames. Or maybe they should be turned into a pair of 2x4 in an L arrangement.

I've looked at many online designs but I haven't found any that both do this and (given the importance of this joint) I trust.

What would you use?

Edit: added explicit mentions of bolts

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  • I say this to everyone who posts a link to an Ana White plan, please do yourself a favour and never go there again looking for a how-to :-) Very early the site became infamous among woodworkers for not paying any attention to certain key facts of wood construction (zero knowledge of and hence accommodation for wood movement) and, of course, for its overuse of pocket-screwing — as if somehow they were the only joinery method one could aspire to? Every time I checked back on the site in the years subsequent to that, nothing had changed.
    – Graphus
    Commented May 8, 2023 at 6:02
  • "...and other online designs" It may not be relevant here but in case you want to use the site again for a build query as a rule you should list some, if not all, of these others so that respondents don't make suggestions that you've already considered and dismissed for whatever reason. Anyway, "The main difference is I want to build the mattress boxes as permanent units and be able to knock this down by removing them from the posts by unscrewing bolts." You can absolutely build this using bolts and/or beefy screws (and in fact it's one of the strongest options while keeping it KD) [contd]
    – Graphus
    Commented May 8, 2023 at 6:07
  • ...but the plan illustrated doesn't have mattress boxes per se, so I'm a little confused. If you want permanent mattress boxes then I think you need to make actual mattress boxes, and then it's just a matter of figuring out how you will attach them to your uprights. If you go this route I would strongly recommend looking at the huge variety of metal connectors e.g. those by Simpson (no affiliation) as this sort of thing allows for a strong, very secure/stiff connection of legs to shelves and other horizontal elements but can be taken apart easily in due course using nothing but a screwdriver.
    – Graphus
    Commented May 8, 2023 at 6:19
  • Unrelated to the other points, have you bought wood yet? If not you might want to read one or more guide about how to buy (how to be selective) starting with one of my previous Answers. Please particularly note the lead time! Many populist guides are not realistic about the time you should wait because they don't want to put people off, but the reality is what it is — wood as wet as 2x stuff can take weeks to get to equilibrium with the interior of a home or workshop.
    – Graphus
    Commented May 8, 2023 at 6:30
  • Just a couple of thoughts: You'll want some kind of headboard and footboard structures. Not only to add some strength, and stability, but also to keep things like pillows and children in the bed. You may want to reconsider gluing in the mattress slats. Those need replaced sometimes. Also, people have been making beds for 100s of years using rail hardware and bed bolts, etc. You might reconsider that approach rather than making permanent mattress boxes.
    – gnicko
    Commented May 9, 2023 at 3:21

2 Answers 2

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I just typed this whole answer up and then realised I'd missed your "no cuts more complicated than a half-lap or dado" requirement... but figured I'd post it anyway, it may still be useful - perhaps the second, half-lap joint will be doable?:

When it comes to knockdown joinery, it seems that wedged through-tenons, or tusk tenons, or keyed mortise and tenon joints (all names for the same thing) are king. Basically it's where you have a mortise and tenon joint, but rather than the tenon stopping inside the timber (with a blind mortise hole) or flush with the outside of the mortised piece (with a through-mortise hole, the tenon travels fully through the mortised piece, sticks out the other side, and has a hole in the tenon through which you drive a wedge to hold it in place:

Keyed mortise and tenon joint (Image from here)

This kind of joint can also be adapted to make a "three-way" joint as you're asking for. You'll need to cut two tenons where one passes through the other:

Constructed: Three-way knockdown joint with wedged through-tenons

Deconstructed: Deconstructed three-way knockdown joint with wedged through-tenons

There's a video of somebody using this exact method here.

Alternatively, there's a simpler (possibly, depending on the tools and woodworking experience you have) half-lap version of a half-lap joint with wedged through-tenon on wikifactory here.

See image for reference: Half-lap three-way wedged through-tenon joint

Alternatively, you may wish to look at bolted connections. A similar question has been asked here, with a good answer illustrating one method for a bolted connection.

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    This is a beautiful joint but I think it's a little over my head right now. Some day. I'd also like to leave the box as one piece and this requires the box to disassemble any time it is knocked down. Thank you for the beautiful answer, though! I aspire to do this in v2.
    – LoftyGoals
    Commented May 6, 2023 at 4:31
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I think the problem, is that you have the sides of the mattress box butting up to the posts. If you redesign as follows (crude ascii art)

xx       xx
xx       xx
|=========|
|         |
|=========|
xx       xx
xx       xx

where the xs are the posts, and the | and = are the sides of the mattress box, you can then easily bolt through the posts into the mattress box.

This does mean all the weight is being taken on the bolts - so don't be stingy with them. (You may be designing this for some sweet young toddlers, but they don't stay toddlers. It's always a good idea to design beds for two overweight people having vigorous sex).

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  • If somebody want's to draw a better image than that, feel free to add an answer, and if you comment here, I'll withdraw my answer. Commented May 11, 2023 at 12:18

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