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How strong is a castle joint?

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Flyover

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Pretty sure one of my next projects in the coming year will be a DIY bed frame. I'm envisioning the main part being 4x4 posts with 2x6s or 2x8s as rails/connectors. For some reason, in my mind I always picture the corners as castle joints. Provided I have bought or can borrow a table saw by then (because that's how I'd cut the joint), that's my default joint option right now.

But how strong are castle joints really? If the kids climb onto the bed to jump or wrestle (jumping is officially not allowed but I can't guarantee its prevention) will a castle joint be enough to withstand the shearing forces? It's easy to imagine those four skinny little 1" pins (what's left of the 4x4 which surrounds the 2x6s) snapping -- especially since I know myself well enough to know I probably won't engineer the joints perfectly. Is this fear unfounded? Would simply using a 6x6 pretty much alleviate any issue?

If not, what are the best alternatives to a castle joint for this type of application? (And don't say mortise and tenon, because I can't cut mortises.)
 
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zannej

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I don't know what a castle joint is but I'm about to google it....
Ooh, neat. It looks like how they would do the half-lap on corners of log cabins. Reminds me of the log blocks I used to play with only this has a receptor to hold them. Vertically it looks strong. The half-lap looks pretty sturdy to begin with. I think it would be pretty cool to try that. I don't think the strength lies in the 1" piers-- I think those are just extra stabilizers. The half-lap connection should work to hold it tight and the post with the + cut out is just to hold them in place a little more. You could also glue stuff to the bottom part of the inside of the + to make sure it stays in place. You could also use double nails (there are nails that have two pointed ends or those sort of wavy metal pieces that are sort of like biscuits but also act like nails. I hope that makes sense... It's like a blade on two sides and is used to join pieces of wood. I don't know what it's called. If you wanted extra security you could use biscuits or dowels.
Found a video on how to do it with a table saw (probably one you saw already)
Ana White's website has all sorts of bed plans (link is to the bed plans section). Not sure if she has any with castle joints, but you could check it out and see if there are any plans you like that could be modified to have the castle joints.

If you're really worried about stability (or the 4 points breaking off) you might be able to chisel out a spot to have small angle brackets on the inside where they would be hidden as extra reinforcement. Just recess them a teeny bit inside and don't bring them all the way to the ends. I hope this is making sense.

My only concern with the castle joint is that the bits sticking out the end might be something the kids could hit their heads on, but they could hit their head on any wood really. You could also do a post with an L notch (cut with table saw, oscillating tool, & chisels) & do the half-lap technique on the boards & have them set inside the L. Then you could sand/plane the edges to be a bit more rounded so you don't have any sharp points.

Guy in the video used a screw down the center to hold the laps in the middle. That would keep them from exerting pressure on the little nubs sticking up on the ends I think.

Editing to add-- looks to me like a 4x4 would work for posts. 6x6 might make it too heavy to move.

Any pictures of the style of bed or type you're planning to make? You know I love pictures. I also love learning new things so now I know about the castle joint. I've seen them before but didn't know what they were called. I think my brother had a loft bed with some castle joints on it for the loft. The rail that kept people from falling off the top was like a half castle but could be removed. Notched ends fit into notched parts on the posts to make it slide down in place. Could be removed to change bedding.
 
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Flyover

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Thanks for that thoughtful response as usual, @zannej!!

I realize now the shearing forces I'm worried about depend on the length of the legs. My concern is the "piers" (as you call them) snapping off the 4x4s, and it occurs to me this is a function of leverage: the longer the legs, the more shearing force might be exerted on that pivot point.

So it's a question of how tall I want the bed. I'd like there to be at least 8" or so of clearance under the 2x6s because I plan to use that space for storage (haven't decided yet whether to use the frame as a "carcass" and install drawers, or just use those low plastic tubs with wheels), but it'd be nice to be able to go taller so I could use even bigger storage tubs (or double them up) to make better use of the space. So, the 4x4s will be a minimum of 11.5" long, but I'm open to making them >20" if it would be structurally sound.

I suppose in any case one way to mitigate the shearing forces would be to add another set of 2x4 or 2x6 connectors at the bottom, in an I shape so it's open at the sides so the tubs/drawers can slide in and out.
 

Flyover

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*meant to say the 4x4s will have to be a minimum of 13.5" tall, not 11.5".

Also, I just looked at the 4x4s I used for my kids' bunk beds; I don't think they're thick enough for strong castle joints unless I use 1xs for the rails/connectors. I'll probably go with 6x6s.
 

zannej

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If you're worried about sheering force on the piers, the screw down the middle should help to prevent pressure on the piers themselves as the joint will be held in place vertically. I hope that makes sense. You may want to peruse the beds on Ana White's site. She has a few loft beds there although a lot of her stuff is made from plywood.
While plastic drawer sets on castors are waterproof on the outside, I've found them to be very flimsy & if they get squished even a little (say from a child climbing on them) the drawers won't open and shut properly. You could do storage stairs up to the loft part. Instead of having pull out drawers, you could have hinged tops that flip up. On the side of the stairs you could have either open cubbies or drawers.
I did some crappy drawings of some suggestions for my friend to build his daughter a loft bed.
loftbedplan1.png
The leg supports would obviously be thicker. I didn't finish the drawing. LOL. Power had gone out.
The idea was to have a tufted/padded headboard with drawers above. Stairs up and the light gray would be either cubbies or drawers.

Here's a bed I saw as reference that shows cubbies.
berg-furniture-utica-twin-dorm-loft-bed-with-desk-and-storage-3.jpg
I'm trying to find the one with the lid that flips up instead of drawers on the steps & one that has drawers on the side.
But I think the castle posts would be better than this plywood.
Another thought: If you add something sort of flared at the bottom of the posts to stabilize them more it might also reduce movement-- making bigger feet so to speak.

I forgot to mention that you can make mortise & tenons using a table saw, handheld circular saw, oscillating saw, & chisel. But if the small piece in mortise in tenon can resist breaking off, I think those piers should be ok. Using good glue to hold them to the half-lap should also help. So you don't need mortise & tenons.
 
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Flyover

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I think pictures would help me explain. Here is the original, simple idea:
Screen Shot 2020-11-21 at 20.09.20.png


Unless the legs were really short, I was afraid a shearing force along the mattress could snap the legs right at the base of what we're calling the piers. (Is that the actual name for them?) So this is how I thought I could reinforce it if the legs were long enough to provide 14" of clearance below the side rails:
Screen Shot 2020-11-21 at 21.03.58.png
 

Steve123

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Your design (second photo) looks pretty strong. A kid can break just about anything --- but that looks pretty good.
 

zannej

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It looks good to me. Will there be horizontal slats under the mattress or are you going with floor-grade plywood spanning over with supports maybe 24" on center?
Just to be clear we're on the same page-- are the piers at the top? The 4 prongs that stick up around the half laps?
 

Flyover

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are you going with floor-grade plywood spanning over with supports maybe 24" on center?
I have slats I might use. Depends whether the design accommodates them easily.

Just to be clear we're on the same page-- are the piers at the top? The 4 prongs that stick up around the half laps?
Yes.
 

zannej

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Ok, because if you make it as a platform bed you won't need a box spring. You can design it so the mattress won't slip. You can put rug mats under the mattress that serves two purposes: keeps it from sleeping and prevents mildew/mold.

There shouldn't be any concern of piers sheering off if you secure the half-laps in the middle either with nails or long screws. They will be tethered to the middle if you pop a nail or screw down the middle- although you need to be careful not to split the wood. You could also use corrugated joint nails under the lower half-lap. Maybe not that particular one that I linked, that's just an example. It had lousy reviews.
 

Flyover

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If the rails are 2x6s that'd have to be a really long screw, almost like specialty screws, right? The longest ones I have in my shop right now are like 4, maybe 4.5 inches.

Unless you're saying the screw is just meant to go through one of the half laps and terminate in the other? I thought you're saying it was supposed to go all the way through and into the end grain of the posts. Even with 2x4s that's a really long screw.
 

tomtheelder2020

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If I were doing that castle joint I could be confident the fit would be sloppy. My solution would be to glue the joint with epoxy. That way even the side-grain to end-grain part of the joint (bottom of rail to upper surface of leg) would be as strong as the wood. A few weeks ago I tested and end-grain to end-grain butt joint using TotalBoat Thixo Flex (high viscosity, slightly flexible epoxy) and was very surprised how strong it was.

FYI, Thixo Flex is a slightly flexible epoxy designed for joints in wood boats where some flex will occur. My first tube was directly from TotalBoat and worked great but took a week for shipping. I ordered my second tube through Amazon, for next day delivery, and it was defective. I returned it for a replacement - which was also defective. I have returned that tube for a refund and will get my next one from TotalBoat.
 

slownsteady

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Just feels like so much overkill to me. You could have some kinda orgy on a bed like that :eyeballs: 😎
 

zannej

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If the top of the castle joint will be hidden (or even if not) you can counter-sink a screw through the half-lap in to the post (putting it a bit deeper in) and if you are worried about the hole you can put a wood plug in it.

When it comes to safety, I don't think it's overkill. Kids can be destructive.
 

Flyover

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@zannej

Guess I'd have to go to the hardware store and buy four of those 8" lag screws for $3+ apiece.
 

zannej

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Honestly though, the half-lap joint should hold it securely enough with some good glue. A coat of lacquer or something on the piers after assembly might also help strengthen them. When my dad was trying to build a keyboard tray, it was flimsy at first (he used plywood). I had him reinforce it with some small horizontal pieces of wood on the underside and then lacquer it. The Lacquer made it firmer.

If you're using 2x6s & doing a half-lap then you'd still have 6" of wood to go through. Sink a screw about 1" to 2" in & you can use a shorter screw and plug the hole. But, I doubt it would even need that if you use the right kind of lumber. Use a harder wood (heavier but sturdier) & it will be less likely to break. Also mind the grain pattern. Vertical grain will be less likely to break off horizontally.
 

Flyover

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I think I'm going to not glue it; I want to be able to disassemble. And, let's assume I'll be using pine studs.

I just found out one of my new neighbors does woodworking, and more seriously than me by the looks of it. If he has a planer, joiner, and table saw w/dado stack he'd let me use, then maybe I can get the castle joints precise enough to tap in nicely with a mallet and call it a day.
 
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