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I've got nine mirror tiles (30cm square, 4mm thick), and I want to fix them to a 90x90cm board to make a mirror to hang on the wall.

What kind of board would be best – MDF, particle board, plywood, something else?

I'm thinking probably plywood, because I'm going to need to screw something into it to hang it up, and I've heard that plywood's crossing grains are good for holding screws tightly. Also I know MDF can swell, and I'm scared that would break the mirror tiles. But to be honest I have little experience so I want advice on this.

I'm planning to set the mirror tiles flush to the edge of the board – so just the edges of the board will be visible from the side. I'll probably paint the edge grey or black.

Bonus question – would regular 'mirror adhesive' like this work OK? And is there anything else I should watch out for?

  • I imagine that you would need a flat surface, maybe a scrupulously clean floor, to arrange the tiles on and then stick the wood to them. That way the mirror tiles will be coplanar even if the adhesive and the substrate (wood) are not even. You may need to experiment with ways to keep the tiles arranged as you want them (e.g. with an even gap between). – Andrew Morton Feb 1 '18 at 19:20
  • @AndrewMorton thanks, I hadn’t even considered the importance of making sure the tiles are perfectly coplanar. Tiles face down on a smooth surface sounds like a smart approach – callum Feb 4 '18 at 11:07
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You definitely want to use mirror adhesive that is approved by the maker of the mirror tiles. The reason is, some adhesives will attack the backing on the mirrors and end up showing through the mirror. Follow the adhesive instructions carefully and it will work as designed. I have seen mirror tile installed with general purpose liquid nails with a blob in each corner and the install looked fine, for about a three days or so. Then four brownish blobs started to appear on each tile, one in each corner. The blobs eventually turned greyish-brown and the mirror over the blobs was gone, it was just semi-transparent glass at that point. The homeowner who did it was quite distressed but had covered an entire wall of her living room with them and decided to live with it instead of getting into the problem of taking them off the wall and what damage was going to be to the plaster wall.

Any of those products you mention will work fine a substrate. They each have different pros and cons though. Is this mirror you are making going to be in a damp or humid location? If so, MDF and especially particle board are prone to absorbing moisture and will swell up and eventually begin to crumble. Plywood can handle humidity better but it can still delaminate. There are plywood products made for damp and wet locations, such marine plywood. It is pricy.

How are you going to mount it? MDF is the heaviest of the three and depending of the thickness can be substantially heavier. Plywood will tend to warp more, especially those with fewer laminations and lower grades of plys like common CDX (a "C" side, a "D" side and e"X"terior rated).

OSB (orientated strand board), which you didn't mention, will warp less than plywood because of its smaller "strands" of wood. It will work better than MDF and partial board in damp locations but worse than plywood. Tends to be cheaper than plywood and is used a lot in the US now since it is made from fast growing trees and a lot of adhesive.

Plywood, and OSB hold screws better than MDF and particle board. All of these products will expand with enough moisture. They are all dimensionally stable if they stay dry. Particle board tends to off-gas a lot of formaldehyde. Not so good for the IAQ (indoor air quality) and could interfere with some adhesives. OSB, Plywood and MDF all off-gas as well, plywood the least and can be obtained with non-formaldehyde adhesives.

Another option is MDF with a veneer but the veneer is so thin and other than the cosmetic appeal of the veneer, it may as well be MDF since it has all the same characteristics.

If it where me doing it, I would choose a cabinet grade plywood with a lot of plys to resist warping. Since it will be covered with mirror, the plywood finish isn't that important and imported baltic birch plywood can often be found for reasonably good prices. I would also make a frame of birch or poplar and biscuit and glue it to the plywood edges to hide the plys. Or, if it was on the cheap, getting edge banding with heat sensitive glue already on it and ironing it onto the edges will hide the plys just as well and can be done quickly and onsite since it hardly requires any tools to do. You can get edge banding at Rockler or Woodcraft in the US. I would seal all of it prior to installing the mirrors to reduce the absorption of moisture.

  • I'm planning to mount it to the wall with an 18-inch interlocking cleat with five screw holes to screw it into the board (this). Based on your advice, I want to use 12mm ply. I'm hoping five 10mm screws should be enough to hold it. But if not, I could instead sink five bolts into the front face of the board (before fixing the mirror tiles on) so I can use nuts to fix the cleat bar onto the back. And then put wood filler over the bolts to get a flat surface, and then glue the tiles over the top. What do you think? – callum Feb 2 '18 at 16:53
  • There are are two primary considerations with fasteners, pull-out resistance and shear strength. The pull-out resistance is a measure of the force it takes to pull the fastener out of the material it is embedded in and taken perpendicular to the surface and assumes the fastener is also perpendicular. The coarseness of the thread, the material properties, and the amount of embedment are the main factors. Use the largest wire-size screw with the coarsest thread that will fit in the holes of the cleat. Use a screw that is as long as possible, in this case, that will not poke through the wood. – C Knight Feb 2 '18 at 20:44
  • The shear strength is resistance to a force the would cut the fastener perpendicular to its length. The material the fastener is made of and its diameter (wire-size) are the main factors. Use a steel screw, zinc-coated is fine and largest diameter that will fit through the hole in the cleat. The entire weight of the mirror acts with gravity to try and shear the fasteners. The total shear force is divided over the number of fasteners. The more more you have the force acting on each of them. So, use as many as you have holes in the cleat. – C Knight Feb 2 '18 at 20:53
  • We could do some math to figure precisely the forces but it is rarely needed for things like this as long as you use the appropriate fasteners and fill all the holes in the cleats. – C Knight Feb 2 '18 at 20:56
  • Ok. There’s five holes so it should be OK. Thanks for all the clear advice. I’ll post a picture when I’ve done it – callum Feb 4 '18 at 11:30

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