Another thread that starts – I know there are experts in lots of things on here. About 20yrs ago I built a cupboard into the corner of our Ensuite, had a large piece of Mirror glass cut for the door(@1500mm x 500mm), brought a sheet of 12mm ply from somewhere like B&Q or Wickes, cut it to size and glued the mirror to it with the special mirror adhesive. I painted the back, hung it on the new frame I’d built and it’s been fine ever since, except some very small areas around the edge where the damp environment has got under the mirroring. After several redecorations I’ve recently put a new suite in and retiled the whole room. Lady Nasher wanted the large Mirror door back in place, but with a new mirror because of the damp ingress, which is fair enough. So I ordered a new sheet of Mirror Glass and when it arrived brought a sheet of 8mm ply to reduce the weight. I cut it to size, glued the mirror on the front with the special adhesive, painted the back and hung it on the hinges. The following morning the £80 mirror was in 3 pieces because the ply had warped. The top corner of the non-hinge side was about an inch away from the frame where it had been touching the afternoon before. I was obviously annoyed, especially as we don’t have the heating on yet, and hadn’t yet used the ensuite because I was still working on it. So I ordered a new piece of glass and brought a sheet of 12mm ply from Wickes which I cut to size and laid flat on the floor in the spare room. I planned to hang the door and mount the mirror with it in place so the adhesive would take up any small discrepancy due to warping. Within a couple of days the 12mm ply laying flat on the floor was propeller shaped by @1/2 inch on two opposite corners. It’s not damp at all in our spare room, and we don’t have the heating on yet, so I’m putting it down to cheap Wickes plywood. So, what spec ply should I be looking for to ensure it doesn’t warp after cutting to @ 1500 x 500mm? I'm sure the old favourite of generic 'Marine Ply' doesn't exist any more. Thanks Nasher
Weather and Boil Proof (WBP) ply is the stuff most people think is marine ply. You could try an elements board type thing, that’s what you should use when boarding up bathrooms etc. It’s lightweight and impervious to the elements. Failing that, cheap old shuttering ply if you won’t see it, if it can hold wet concrete without warping it should be good enough surely
Only problem with waterproof MDF is no one will cut it for you so you’re stuck with with a full board
I would not glue it but use corner supports at the bottom and a central support mid top edge and allow the mirror to float. Hang the door for a week in the room before fixing mirror. Should take up any movement. See if you can find old used ply, more layers the better. If you can cross batten the back of the door better still.
Perhaps the rules have changed then for dicing & slicing these days, but i do know it was banned in Germany twenty plus years ago.
Thanks all Petsmith, I've actually been recommended FSC Birch S+/BB Grade by a local supplier, so you've confirmed that, thanks.
You could lacquer the ply/whatever when you get it (i.e. flat), thus locking in/out moisture that'd potentially warp it. Then mount the mirror as before.
Wouldn't be better/easier to just fit a 10mm thick mirror on suitable hinges without plywood backing? https://www.wholesaleglasscompany.c...CB8wIVO4BQBh170QEFEAQYAyABEgJHQfD_BwE#SID=263 Should you decide to go down the ply route. In order to balance the moisture, paint both sides and obviously the edges of the ply in 3 layers of suitable paint.
hang the mirror with invisible fittings on some shallow battening that is fixed to the door with adhesive or screws or direct to the door depending on door construction. you dont need to glue the glass to ply on the door. floating option will eliminate the very problem you are trying to solve
