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I have some cabinet units that I am wanting to put a topper on.

[top down view]

enter image description here

I am planning on using 18mm Birch plywood, however since this is relatively expensive I am wanting to make the most out of it/stretch its usage as I'm needing some shelves out of Birch as well.

Therefore, I am planning on making a faux double-thickness sheet by adding another thin strip of the 18mm birch to the edge and to fill in the rest of the area with something like regular plywood, or MDF as this will be cheaper, plus I already have plenty of MDF in the shed.

My question would be around the best (I know "best" is a contentious word on here, so let's say "most optimal") way to go about this.

For context, I will be placing some bookshelves on top of this - When I get around to making them.

Would I be better off laminating the same size sheet of MDF/ply on the bottom or would I be wasting material and be better off simply laminating strips that follow the structure of the cabinet?

Something like this, where the speckles represents the Birch ply and the white represents the laminating material enter image description here

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  • Using other words that mean exactly the same thing as best doesn't sidestep the issue LOL Now (assuming I'm understanding the problem correctly which I'm not 100%sure of, starting with the confusing scale of parts of the first drawing) are you familiar with the torsion boxes? Given the nature of the way they're constructed, and how strong they turn out, I think it gives an idea of what you can get away with in terms of the unsupported portions of something roughly similar — especially if there won't be any loading specifically onto the unsupported spans, which I think will be the case for you?
    – Graphus
    Oct 16, 2023 at 17:21
  • If you were to go ahead with the idea of completely filling the area with another material I suspect you'll run into the issue that board materials' thicknesses are very much nominal, not actual. It's widely known that plywoods take the described thickness as very much a suggestion :-) And actually boards from of the same stuff can even vary. While in theory MDF is more uniform because of the way that it's made, this still relies on the manufacturer taking steps to ensure uniformity to the stated thickness, which in practice seems to be beyond the industry.
    – Graphus
    Oct 16, 2023 at 17:24
  • @Graphus Yes, I did consider the potential difference in thickness of sheet goods, let's just hope that it's not an issue XD I will have to maybe experiment a little. The bookshelves going on top will be a config of 2 x ~750 to fill the width as a full 1500 would clearly buckle under any weight, so 2 of the verticals will land in the middle of this topper, which won't have any stretcher or divider support under it... So I'll definitely have to have something a bit more substantial for thickness there...
    – physicsboy
    Oct 17, 2023 at 7:14
  • This isn't a freestanding unit is it? If it's not, attachment to the wall could take most, potentially even all of the weight of the bookshelves.
    – Graphus
    Oct 17, 2023 at 8:13
  • @Graphus - It's a kitchen-style cabinet in my office sat on carpet. I've screwed it into the plasterboard with a few Fischer rawlplugs and one inadvertently found stud. So I hope that it's secure enough!
    – physicsboy
    Oct 17, 2023 at 10:00

1 Answer 1

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The "best" way is entirely up to you, but here are some things to consider mostly as frame challenges to your plan.

  • Plywood, of any grade or quality, is usually edge banded to hide the plys in any exposed edge. Assuming you'll be doing this, it doesn't matter what the second, hidden layer is made out of - the edge of it will be completely hidden behind the edge banding.

  • 18mm (~3/4") is very thick, sturdy plywood. Why do you want to double up the thickness? It appears that your top will have support every 500mm which should be more than enough to support an 18mm thick top against most loads.

  • If you want a 36mm edge just for looks, consider just applying an extra 18mm layer around the edges and not worrying about the middle at all.

  • Consider using a solid wood board at 36mm (or whatever thickness you choose at this point) for your edging.

    • This wouldn't need any material behind it as it would attach to the edge of the plywood (and possibly under if you rabbet the banding) and serve as the edge banding for the plywood and the added thickness you desire all at once.
    • It has the added benefit that you could route both the top (a small round over or chamfer on the top to relieve the sharp edge) and a decorative design on the bottom to add some style (as you desire).
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  • Thanks for the advice freeman. Just for context, the doubling up of the thickness and exposed ply edges will be for decorative purposes. I have a double thickness birch ply desk, so just want to keep it cohesive in that room - I will have a thin strip of the birch double up around the edge to make it look like it's doubly thick - There won't be any edge banding here. I would have done just a skirt as you suggest, but the doors and drawers are already done, so don't want to cut them down.
    – physicsboy
    Oct 21, 2023 at 19:19
  • Wanting to see the edge of the plywood is an unusual style choice, @physicsboy, but it's just that - a choice. If it works for you, then go for it!
    – FreeMan
    Oct 21, 2023 at 20:46
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    What can I say? I'm an unusual man :-P
    – physicsboy
    Oct 23, 2023 at 7:41

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