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I want to build a plateau for my child (for sleeping, playing, whatever - except romping), around 1m x 1.6m. For that, I want to use a plywood plate of the given dimensions and put it partly onto cabinets, partly onto heavy duty brackets screwed to the wall.

Here is a drawing (true to scale, might be a bit sheared due to photo angle):

enter image description here

The orange-ish and greenish area together are the plateau, the greenish are cabinets and wall brackets that support the plateau. Grey lines means wall.

The red corner has no support underneath but it has indirect support from the ladder next to it (because the bars are connected; the ladder hits the ground), hope you understand what I mean from this drawing:

enter image description here

My preferred thickness of the plywood (in order to bear the load of the child and sometimes even an adult (for cleaning)) is 35mm-45mm, but where I live it is impossible to get such a thick plate in the desired size without buying a whole plate (size around 3m x 1.5m) and thus having (and paying!) a lot of excess. This is no option.

Thinner ones can be bought in the needed size without having to buy the whole plate; the thickest I found is 21mm.

So I was thinking of buying two 21mm thick plates of the needed size and stack them. They will be attached to each other by the screws that I have to use anyway to attach the plate(s) to the cabinets and wall brackets.

Is this a viable solution? Will two seperate thin plywood plates stacked on top of each other be (around) as stable as one thick plywood plate?

I think the only problematic area in my setup is the rectangle in the bottom right area (where the red corner is); it is ca. 50cm wide and 38cm long.

Note: I do not want to glue them together, only screw. And not in 100 places, only in the ones where I have to screw anyway (for attaching to what is below them, i.e. cabinet or wall bracket) and maybe a little more. Reasons: Would need a lot of glue, costs money and worktime, might not even work without proper equipment (which I don't have) and makes it harder to install the plate due to weight. Two plates are easier to handle, considering moving to another place in the future.

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  • Can you add wooden supports under the plywood? Most floors do quite well holding weight with just 13mm or 19mm thick plywood. The thickness you want is usually only for tile floor where any flex will crack the expensive tile, not to just hold weight.
    – crip659
    Jul 7, 2023 at 14:08
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    Why not glue them? I mean… that's how they make it anyway.
    – Tetsujin
    Jul 7, 2023 at 14:48
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    A photo of the situation and a better use case description would help us provide better answers.
    – isherwood
    Jul 7, 2023 at 16:49
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    You might also be able to use the thinner piece of plywood with a board at 90° attached underneath to add strength. That can be done with just screws. A little more of a picture of what you're trying to do could help us to help you solve the problem, rather than just to answer your question.
    – KMJ
    Jul 7, 2023 at 17:07
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    Ok I, and I think everyone, were assuming that the platform is mostly unsupported. In fact you have at most 20cm unsupported. So long as those brackets are strongly attached to studs you can use pretty much anything. If you're worried about the one corner with the red dot, you could just saw it off diagonally. That will be safer anyway as you won't hit your hip on the corner when on the ladder. I'm not sure how safe a ladder is for a small child but that's not your question.
    – jay613
    Jul 7, 2023 at 21:56

3 Answers 3

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21mm (~13/16") plywood will carry a person up to say 200 lbs. reliably in a sheet that size (about 40x60 inches for the Yanks), assuming proper edge support. The question is one of flex. You haven't told us what the "plateau" is for or how it will be used, so beyond that there's not much to say.

Adding a second layer will certainly reduce flex, but whether that's enough rigidity is unclear. Strength isn't a concern, though. An adult would have to work pretty hard to break through the sheet. Sag and detachment are your bigger issues.

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  • Yes, sag IS my issue. I might not have chosen the right words, but by saying "...in order to bear the load of the child/adult...", I meant that the plate should not bend. I never assumed that a person would "fall through" 2cm of plywood...
    – Kjara
    Jul 7, 2023 at 21:06
  • Having now seen the newly added diagrams, I assumed incorrectly (and I think this answer did too) that this platform would be mostly unsupported. In fact, the unsupported spans are less than a foot. I no longer think there is anything to worry about. So long as the cabinet and the wall brackets are strong enough, you could use a good collapsed cardboard box for the platform. :)
    – jay613
    Jul 7, 2023 at 21:58
  • @jay613 "the unsupported spans are less than a foot" - not the rectangle next to the red corner; this is an area of 50cm x 38cm. I don't know how much the indirect support from the ladder will help, but do you think that even there 21mm is enough?
    – Kjara
    Jul 8, 2023 at 7:09
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Your concerns with the thickness of the plywood are misdirected.

A 21mm thick piece of plywood would be able to carry the weight of as many adults that could fit on a 1m x 1.6m slab. Therefore it will be fine for a child or 2 or 3 and an adult.

The issue will be what is supporting the plywood? Are the supports structures that can withstand the type of weight? We don't know how they are constructed or what materials, so no one can answer that for you. I suggest a new question regarding the supports needed to carry the load of an adult and 3 children on a 1m x 1.6m plywood 21mm thick.

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There is certainly the option of laminating several pieces of plywood to effectively create a thicker piece. May take a lot of glue (and probably some screws as a way to firmly clamp the pieces together until the glue dries), but quite doable, and can be almost as rigid as a piece manufactured to that thickness.

Also to be considered: For rigidity, you might want to look up stressed-skin construction. The middle layers can be surprisingly sparse yet still impart rigidity.

But, ues, I think the first question is how you're going to support this. I've seen a preteen kid pack herself into a shelf of a high cabinet when playing hide and seek, but I wouldn't trust cabinet not to rip free from it's supporting backplate if overloaded, and your proposal increases the lever arm.

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