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I decided to make a bunk bed for my room as it is small and I need a desk. I'm a beginner at woodworking and I need some help.

I have made some "plans", I will put 7x2m stakes, 4x1.40m each around 10cm x 10cm wide. Each horizontal stake will have a angle bracket under. Each horizontal stake next to another will a a square between.

The bed dimensions are 140x200cm.

Is spruce wood a good choice ?

How can I calculate if it supports 2 people (about 70kg each)?

Material :

My plans :

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  • Sorry I'm having a hard time understanding your plans and material sizes
    – bowlturner ♦
    Commented Oct 13, 2015 at 13:07
  • The wood I will use is 100x100x3000cm (or less if possible), the angle brackets I will use are 150x150x65cm. There will be two angle bracket under each horizontal wood, and one when two horizontal wood meet. Look at the last image link I put.
    – Servietsky
    Commented Oct 13, 2015 at 13:26
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    @Servietsky, mm not cm in those dimensions you wrote! I'm just trying to imagine using a piece of wood 100x100x3000cm :-)
    – Graphus
    Commented Oct 13, 2015 at 13:47
  • OMG, how could I write centimeters. It's in millimeters !
    – Servietsky
    Commented Oct 13, 2015 at 13:49
  • I made a new design in this thread woodworking.stackexchange.com/questions/2568/make-a-bunk-bed-v2
    – Servietsky
    Commented Oct 15, 2015 at 16:12

3 Answers 3

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Is spruce wood a good choice ? Or maybe I should choose a stronger one ?

Wood species
Spruce is one of the strongest woods in the world for its weight. Beds of a similar design to yours have been made from other softwoods, including pine and fir, which are not nearly as strong, so I think spruce is a fine choice and you're lucky you have a source of it.

If you wanted to go with a stronger wood you would be looking at a hardwood, e.g. oak, which would be significantly heavier and very likely much more expensive.

Dimensions
In addition to the inherent strength of the species of wood you use, the size and thickness of the pieces you use is very important. I think you've chosen material that is the right size for each component so nothing to worry about there.

Stock selection
One additional factor to bear in mind is the presence of knots in the wood. Small knots, relatively far apart are not a big problem but a very large knot in the centre of a piece can weaken the wood there significantly.

When buying the wood, if possible:

  • select pieces with straight, uniform grain;
  • don't buy boards with many knots (where you have to buy knotty boards the knots should be small and ideally not numerous);
  • prefer boards where the growth rings are close together, not wider apart;
  • don't buy any boards that are bent or twisted.
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  • Thank you very much ! Do you think the steel angle brackets 150x150x65cm are enough ? I made a little design update imgur.com/yOthh1b. Also, do you think 80x60cm wood stake would work too ?
    – Servietsky
    Commented Oct 13, 2015 at 12:15
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    @Servietsky, yes those steel brackets are immensely strong. You could use them to reinforce the structure of a shed or garage so they're more than strong enough for this. "Also, do you think 80x60[mm] wood stake would work too?" do you mean instead of 100x100? Yes probably strong enough. On the verticals that size would almost certainly be fine, not certain when used horizontally.
    – Graphus
    Commented Oct 13, 2015 at 13:52
  • Knots close to the ends are bad, because they mess up how the wood attaches to the next piece. Knots along edges are bad, because the reduce the effective height of the wood. Knots in the middle (of the length) are worse than knots closer to (but not too close to) the ends, because the bending moment is greatest in the middle of the length. For a given size of knot, knots part-way along the length that are not close to the edges are the least bad. (The previous sentence also describes the parts of a framing joist that are allowed to have holes in them, for the same reasons.)
    – Jasper
    Commented Oct 14, 2015 at 5:23
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I am personally not a bit fan of spruce (or pine, or anything the like) for anything but a tool shack in my garden, or similar. But sure enough, spruce is quite resilient. You can most certainly certainly make a bed from that material, which will easily support two people.
For the most part, this is a matter of personal preference. For someone who is unexperienced, it's probably a good choice because it makes your life a lot easier when it comes to working in general, driving screws, or fitting pieces together. Because, well, it's soft wood. Hard wood is a lot harder to work with, and less forgiving to the unexperienced worker.

10x10 on the stakes is indeed pretty massive, you could probably get away with 2/3 or one half the thickness for the stakes as well (but better be on the safe side, and the look is much better that way too -- not worth bothering the 1.50 CHF difference).

Hornbach is one of the "better" home improvement stores, but still you will have to expect that 90-95% of what they sell as building wood is total crap (at least if they are the same in Switzerland as they are in Germany).

Do invest some time to get some proper pieces together, look at every single one, and hold every single one against an even surface, or you will regret it later. Really, take your time. Do not haste.

This is one of the main reasons why I dislike spruce: While you can get it virtually everywhere, you usually need a lot of time sorting until you have a couple of usable pieces together. The same store usually sells douglasia in virtually will-buy-blindly quality at +20% the price. Or beech, if you want a hard wood.
Oak is actually pretty much impossible to get in my region unless you go to to specialized dealers (but I guess in Switzerland, that will be very different, in southern Bavaria you can get it anywhere at almost the same price as I'm paying for spruce... but even though it looks a lot better, I wouldn't recommend oak to a beginner, you'll have no fun!).

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The main structural elements (the columns and the beams) are big enough to support a horse. However the slats are inadequate. The first time that you and your partner decide to occupy the center of the mattress simultaneously and apply a dynamic load you are going find yourselves and the mattress occupying the desk.

Normally a double bed would have another beam running lengthwise in the middle which should give the slats a generous amount of spring without breaking.

In addition the type of wood, sizes of wood, and the load, one must consider what is known as the end conditions of the slats. In Sagulator the default end condition (Shelf attachment) is fixed. In a fixed end condition, the end is restricted from rotation. In the case of a floating support the ends rotate when the load is applied - the extreme ends actually of the shelf lift from the support. The condition in this bed is floating. Even if the slats were to be firmly nailed to the supports, the nails would eventually loosen, same would be true of wood screws. I'm not so sure of what might happen using glue or even screwed and glued connection for the ends of the slats.

Using the floating end shelf and the 17 mm thickness specified by OP the slats are going to sag excessively (The sag is four times as much in the floating as in the fixed attachment) and quite likely could reach the point of catastrophic failure. (That's what it is called when the members actually break.)

The additional longitudinal support will solve all of the problems.

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    It's not just the size of the slat material but the spacing too. Those look numerous enough to me, but obviously adding a central supporting beam is a trivial addition to the bed so well worth doing if Servietsky wants to err on the side of caution.
    – Graphus
    Commented Oct 14, 2015 at 9:57
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    Valid and well worded concern! However, it seems that, as @Graphus said, there are enough slats, and from the drawings, they do look substantial. I'd recommend a trip to the Sagulator (I need to bookmark that!) to determine if the slats are sturdy enough.
    – FreeMan
    Commented Oct 14, 2015 at 20:16
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    I plugged in Black Spruce, 140kg total load, 1400mm total span, 1000mm depth (10 @ 10cm wide boards), 20mm thickness. It claims that's an acceptable load. Of course, it's calculating for a static load, so you may want to err on the safe side for dynamic values.
    – FreeMan
    Commented Oct 14, 2015 at 20:21
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    I assumed they'd be fixed since they're not in a rebate of the frame. I think this is a safe assumption as if they were just resting on the top edge they wouldn't need to break for a major problem to occur, they could be knocked out of position or wiggle out of position over time and eventually fall through on their own!
    – Graphus
    Commented Oct 15, 2015 at 8:53
  • @Graphus, even if nailed in place to prevent movement end to end or side to side they are still considered to be floating. The concepts of floating and fixed come from structural engineering and refer to whether the ends are restrained from bending.
    – Ast Pace
    Commented Oct 15, 2015 at 15:29

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