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I'm planning on making a loft bed, and I now got a plan and I'm working on the details.

The bed

The measurements I'm going for is 1700(start of the top beam)x1700x2100mm. The main beams are 98x98mm, middle beam 98x48mm and the crossbeams are 48x48mm. I will probably use pine for the beams and spruce for the rest to save some money. What I'm wondering is if this will hold the weight that gets on a double bed. I looked at this and some other, but they where all single beds and as i understand 98x98mm should hold a single bed just fine. Will these plans hold a double bed and does anyone know of a ballpark of how much weight it can hold?

So next question, I must say I've fallen in love with the thought of just using wood for the joints, no glue or screws. So I looked about and wonder if a 3-way Miter joint will hold? I understand it will be much stronger if I use glue and reinforce with screws. I will also add a desk and shelves to the bed, but before I get ahead of myself I wanted to know if my main plan would work. I mention the desk since it opens up for the possibility to add support crossbeams on the legs if that should be needed.

  • Everything you need to know is referenced in the question to which you referred. When using Sagulator, be sure to set the end conditions to free, or floating, not fixed. Note that all of your beams must be treated as 49 mm deep because you are cutting notches which reduces the effective depth in half. Use the total maximum load for all beams, because it certainly possible for two people to be sitting on any beam simultaneously. You need to be aware that you have done nothing to make the structure rigid (no cross bracing). Even with the 3-way miter, the legs can easily be kicked out from under. – Ast Pace Aug 5 '16 at 20:02
  • Will the mattress lie directly on top of this or will there be a box spring or flat board in between? – rob Aug 6 '16 at 0:25
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    No glue or screws?! That would be a massive mistake. Don't let romantic ideas get in the way of sound construction. Unglued connections throughout is for massive timbers that rely on friction, gravity and mechanical advantage (joints that self-lock), along with judiciously placed pegs to actually fix certain joints in place where needed. And perhaps most important of all, the joints need to be cut to incredibly tight tolerances — impossible to get a piece of paper between any joint surfaces. – Graphus supports Monica Aug 6 '16 at 6:43
  • @rob I was planning on putting the matrass directly on top yes, or add this in the middle. – Bjerke Aug 6 '16 at 11:32
  • @Graphus thanks for this advice, I had planed to buy some 2'x2' to practise makeing the joint with, but I'll drop the whole no screw/glue idea(will still make some of those joints though, seems fun). – Bjerke Aug 6 '16 at 11:33
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I see three inherent weaknesses, two of which critically compromise the strength of the bed. I'd suggest looking at commercially-produced solid wood bed frames for some design ideas (don't look at cheap futons, etc.).

illustration with callouts for weaknesses

The support beam running down the middle is inherently weak at the ends because the top half transfers the load to the outer frame, but the top has notches cut into it. You want a single contiguous piece from end to end for maximum load-bearing capacity. This weakness is not present in the other question you linked. You may be able to somewhat address it by using a box spring or sheet of plywood across the top, but if the mattress is going to lie directly on this frame, I'd be concerned about that center beam.

Similar to the other question, your design's legs appear to be very long but they don't have anything to reinforce them against racking. You may want to consider using wider but thinner lumber, as opposed to more square lumber for the outer frame. Although you will still want some additional reinforcement against racking with such long legs, a wider board will give you a longer and stronger joint between the legs and the frame. It will also be more resistant to deflection than a square beam if you orient it with its narrower edges facing up and down, and it will make your design stronger since you won't have to cut halfway through them in order to accommodate the cross bars in your design.

The third weakness involves all those cuts into the load-bearing parts. Some might argue that they are not very visually appealing, either, but that judgement is up to you or whoever will be using this bed. Instead of making those cuts in the outer frame, consider installing cleats to bear the load without compromising the strength of the frame.

  • Is this this what you mean by cleats? And how much wider does the main beams need to be, a 1 to 2 ratio? or will 1 to 1.5 hold? – Bjerke Aug 6 '16 at 11:54
  • That's a joist hanger, which would also work well. A cleat is just a piece of material mounted horizontally such that it supports the edge of something resting on top of it. Often cabinets are hung by mounting a strip of wood like a 1x2 to a wall. A variation on this is a French cleat, which has a slanted surface that also pulls the hung item toward the wall. (You would just need a normal cleat, not a French cleat.) – rob Aug 6 '16 at 13:42
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Rob's concerns are important. When you calculate the load that the main beams will carry you must ignore the depth of the notches and calculate the sag/load based only on the depth of the continuous wood below the notches. I would also caution you regarding the use of the 3-way miter. This is not a joint for beginners. It will need to be tight, so unless you have been at this for a while, experiment first for practice and to evaluate the joint's strength. Rob also point out that this joint must resist racking forces. The constant lateral loads on the joint will encourage movement and a softwood will give over time making the joint sloppier, which will increase the racking. In the link for the 3 miter joint you provided it shows that the joint is held in place by the three small square tenons that extend into mortises in the intersection pieces. These pins are all that is used to resist the racking forces. I am not sure this joint is up to the task using softwoods and I certainly would not use a joint like this at all unless it was glued.

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