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  • Plywood – directional strength
  • aracer
    Free Member

    I have a bit of 5mm plywood. 5 layers to it. Outer two plies are extremely thin – they almost seem like a cosmetic top layer – they’re ~0.2mm thick whilst the inner 3 plies are ~1.5mm thick each. Not being an expert on plywood or having used it much I’ve no idea if this is normal?

    Anyway I want to lay it so the strongest direction is a particular way – I was thinking this would be along the grain of the outer plies, but that was before I looked at the construction. Will it actually be stronger along the grain of the next to outer plies (which are 7-8 times the thickness of the outer ones)? I took a little bit of it apart, and as expected the individual plies have good strength along the grain, but very weak across it – the complicating factor being that the outer plies are better quality wood.

    What’s the standard wisdom on directional strength of ply?

    slowoldgit
    Free Member

    ISTR reading a long time ago that plywood is weakest at 45 deg where neither ply has its full strength. I’d choose along the two inners.

    sweepy
    Free Member

    If you’re worried about strength, and its not in a decorative piece, a criss cross pattern of Superglue drawn on in thin lines improves the strength of plywood no end.

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    ernie_lynch
    Free Member

    they almost seem like a cosmetic top layer – they’re ~0.2mm thick whilst the inner 3 plies are ~1.5mm thick each

    It sounds like a veneer finish to me – eg oak or whatever.

    I don’t really understand your second paragraph, it all sounds too complicated to me, and I’m lazy and tired from a long bike ride on the South Downs 🙂 But I will tell you that 5mm plywood has no significant strength to speak of, so which direction is the strongest is immaterial. I would have thought. Plus the whole point of the layers in different directions is that it gives it strength whichever way you use it I think 😕

    br
    Free Member

    If you are worried about strength, use thicker plywood?

    aracer
    Free Member

    Thanks for the help chaps – though I’m not sure I’m any the wiser. Googling everybody seems to say along the grain of the outer sheets is stronger, but I’m not sure if standard ply has thicker outer sheets.

    ernie – I’m just asking whether it’s strongest along the grain of the (thin) outer ply, or along the grain of the (thicker) ply underneath that. Sorry for complicating things!

    I also appreciate it’s not strong in an absolute sense, but I’m assuming significantly stronger than the 3mm MDF I was using before. It’s for use as the top sheet on a jump ramp for my 5yo – generally well supported, but ~7cm gaps between the supports which the ply spans between. So not major structural stuff. I figured 5mm was thick enough (actually sold as 6mm and I was expecting 5.5 as that seems to be the standard) given that I do need to be able to bend it to shape.

    ernie_lynch
    Free Member

    but I’m not sure if standard ply has thicker outer sheets

    Well compared to, for example, oak veneered plywood, I would expect structural ply to have more than a 0.2mm thick external veneer – they minimise the thickness of a veneer when expensive woods are used.

    Personally I think you’re worrying too much, there really can’t be much difference in strength depending on which way round you use it (although slowoldgit’s comment is probably valid I reckon). So if it’s that critical then you need to use stronger/thicker ply and not ponce around changing it’s direction. IMHO

    slowoldgit
    Free Member

    Guessing here, the thin outer sheets are for looks (veneer) or for smoothness (painting etc).

    Steaming or hot wet cloths would help with the bending, might save some cracking?

    TheFunkyMonkey
    Free Member

    Why not just use two sheets if you’re that bothered

    aracer
    Free Member

    Personally I think you’re worrying too much, there really can’t be much difference in strength depending on which way round you use it

    That probably sums it up – I’m not that bothered but figured I might as well have the stronger direction the way round it matters. Thought that would obviously be along the ply of the outer layers, but then got all unsure when I looked at the thickness of the plies.

    Not really sure why the stuff I’ve got is the way it is – bought on ebay as “exterior grade plywood” so not sold as having any special surface finish (cheaper than in any DIY store – though happy for any hints as to where I should go to get wood supplies cheaper).

    ernie_lynch
    Free Member

    Well it sounds like you haven’t got a specific hardwood veneer then, so as slowoldgit suggests it’s been finished with a smooth veneer to allow for painting without too much filling. All plywood tends to be hardwood and the stuff which provides a better finish is obviously going to be preferred as the external veneer. Shuttering ply and ply for structural applications however tends to be softwood and the external veneer can be pretty crap with countless knot holes, gaps, etc. I don’t ever recall the direction of the plywood being specified because of it’s strength advantages – only obviously it’s good practice to run 8×4 sheets counter to the direction of joists, for example.

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