Plywood Thickness for Shelves?

paperclap

Hi all,

The bedroom wardrobe build continues!

I'll have a number of shelves throughout the wardrobe, all of which will span 1 metre, with a depth of around 40cm.

18 MDF just ain't gonna cut it. It'll sag. So, what about plywood?

Would 18mm plywood suffice? Or perhaps 24mm / 25mm (depends where I shop) plywood?

Further to that, would a glued dado joint (only at the sides) add any meaningful amount of strength to it? Or would standard shelf pins suffice?

Thanks!

aliby21

conti board?

ThisIsWeird

Plywood is stronger, but I suspect will still sag to some degree. Possibly not to a point of visual concern, tho'.

If you fit 'quadrant' beading on both sides and along the back of your unit, the shelf sitting on there will have the max amount of support. It'll still likely sag to some degree along its unsupported front, tho'. So, make the shelves fully symmetrical, so you can just flip them over every 6 months or so...

"Further to that, would a glued dado joint (only at the sides) add any meaningful amount of strength to it?" I suspect not - the shelf will effectively stretch to accommodate the sag. Having said that, if you were to have sturdy battens ('dado' as you suggest) along the sides and back, and glued the shelves down with a decent contact area, it is bound to help. No idea how much, tho'.

The other way to hold back the sag is to have a front rail fitted as part of the shelf, something less inclined to sag - perhaps a beech batten, or even box steel! You were considering up to 25mm thick ply, but I would stick with 18mm, but add that 25mm (or more) thick batten to the front, blended in with the shelf's top surface so it all looks flush from above - ie, you cannot see the added section (not that it matters much).

Or, since your wardrobe tops will be study, mounted to the ceiling by the filler panel?, the other solution is to use wire 'ties' to suspend upwards the shelf fronts in the middle. Or struts coming up from the 'robe floor. You want unbroken 1m spans, tho'?

The simplest is the flip-overable shelves :-)

FreeBear

Block board ?

Gavin83

paperclap said:

Hi all,

The bedroom wardrobe build continues!

I'll have a number of shelves throughout the wardrobe, all of which will span 1 metre, with a depth of around 40cm.

18 MDF just ain't gonna cut it. It'll sag. So, what about plywood?

Would 18mm plywood suffice? Or perhaps 24mm / 25mm (depends where I shop) plywood?

Further to that, would a glued dado joint (only at the sides) add any meaningful amount of strength to it? Or would standard shelf pins suffice?

Thanks!

What are you planning on putting on the shelves?

Honestly 18mm ply is pretty strong. We've got 18mm ply shelves, roughly a metre long used a bookshelves. They're only supported on the ends and they aren't sagging at all. However they're done in such a way that the ends can't buckle so that'll provide some extra support. Consider that decent flooring is 18mm ply, although admittedly with the extra support of joists. Definitely don't use MDF though.

ThisIsWeird has already made some good suggestions on how to provide extra support should you be concerned. Some beading along the back wall supporting the shelf along with a pretty piece of wood under the shelf at the front would give you an extremely strong shelf. 

paperclap

Well, if dado joints don't add anything to reduce the sag of the shelf, then certainly no point in going to the effort of doing them (and buying a dedicated router bit!). Though some say they do add rigidity to the overall structure. But in my case, I don't think rigidity of the carcasses is needed – as the carcasses will be screwed together, and then screwed to the side and back walls!

I could add a quadrant along the back to provide some support. But, I wouldn't be able to add a length of wood along the front to provide extra support, as then the "flip-overable" shelves method wouldn't work!  though perhaps with that extra front wood, the need to flip the shelf over wouldn't be a thing.

There will be a minimum of two shelves per carcass, of which I never see moving (up and down to different locations). One at the very top (above a hanging rail), and then one about 1 third up from the bottom, to be a shelf / cover over some drawers.

These re the shelf supports I had planned on using:
https://www.ironmongerydirect.co.uk/product/metal-shelf-support-7mm-diameter-polished-nickel-pack-of-50-462545

With the additional support of these:
https://www.ironmongerydirect.co.uk/product/shelf-support-socket-polished-nickel-pack-of-50-699159

ThisIsWeird

Can you explain what you mean by 'dado joints'? I'd assumed you meant a thicker support moulding along the sides (and back) to take the shelves, and having the shelves glued to these, hence hopefully preventing the shelves from sagging by being 'held' at each side?

"I could add a quadrant along the back to provide some support. But, I wouldn't be able to add a length of wood along the front to provide extra support, as then the "flip-overable" shelves method wouldn't work!  though perhaps with that extra front wood, the need to flip the shelf over wouldn't be a thing." That's the idea - if you were going to fit a reinforcing batten along the front, especially if this were box-steel - then that should be all the anti-sag you should need. I'd still have a quad (supports) along the back, tho', to prevent the back of the shelf now sagging...

These shelf supports are plenty strong - you ain't going to shear them. The sockets for them are a very nice touch and worth doing.

What I would do, then, is to fit 2 supports either side, and - ooh - a couple along the back too. Then make the shelves flipable.

MDF would, I think, be the most likely to sag - it just does... Ply is much stronger as it has the fibres running in opposite directions. Chipboard is somewhere in between. MDF still appeals as it's so much easier to work with and get a better finish on - the other materials will add a lot of work to this. So, perhaps 22mm MDF and turn them every time you do your mattress

moneysaver1978

Unsure if this helps but we have an IKEA Karlby kitchen worktop measuring 246cm wide as a desk.

We fitted 2 metal plates plus 1 angle plate in the back (bonus: holding up wires and hiding them!) to ensure there's no sagging issues in the middle.

Maybe overkill for you with plywood or if you are concerned about the look (we cannot see the plates under the desk but might be different for you in a wardrobe).

paperclap

This is a dado joint. You route out a maximum of half of the side wall thickness (so, 9mm from an 18mm side panel of the carcass), then slot in your shelf, and fix with glue.

ThisIsWeird

Ah, thanks (learn summat new every day )
That would help, I'm sure, but not fully prevent MDF from sagging. Yes, the shelf would be secured and kept in tension, but almost certainly the tiny amount of 'stretch' required to allow it to sag would still occur. For, say, a 10mm central 'sag' (obviously not a realistic amount!), the shelf would only need to shrink 'widthways' by a fraction of a mm. Yes, being glued in place will help hold it, but I suspect that an MDF shelf will simply stretch to accommodate the sag. Or, it'll pull your unit sides inwards by that mm.
So, I suspect it'll only help, but not cure. And it's a heck of a lot of extra work.
But I know by now you are a glutton
However, if it does still sag, then you are stuffed...
I'd still go for reversible shelves. And make them sturdy - at least 25mm, but why not 36mm, or 44, or 50?! It'll look good too...

paperclap

If I can get my hands on a thicker material, then absolutely. Certainly not opposed to thicker shelves at all!

Just a thought. What if a central (probably 18mm, because why not?) partition were added to each section? The horizontal shelves could / would be at least 25mm thick.

Red circles – Dowels and screws*. No glue.
Purple circle – Dowels and screws*. Maybe glue?
Blue circle – Dowels. Maybe screws*?

*Perhaps pocket hole screws, just so I can screw from within the carcass, rather than from the outside.

Or maybe no dowels at all, but instead simple shelf pegs. Only screws. Maybe the simple addition of the vertical partitions (screwed in) would negate the sag altogether.