Laying subflooring on concrete for vinyl tiles

I wanted to replace the carpet in my bathroom with vinyl tiles. It is on the ground floor, and underneath is concrete--no wonder the carpet seemed to crappy. I have a sheet of plywood that is finished on one side that I can cut to fit the floor, but I would have to subdivide it to cover the dimensions. I have some questions:
1. How should I secure this to the concrete? That is, should I use both an adhesive and screws? Use nothing? 2. Should the edges of the cut boards be perfectly snug? Should they be perfectly snug with the walls?
If you're curious about the size of the boards I'd use, it's a 4'x8' sheet to cover a 5'x6' area. I was going to split it into 4 pieces; 3 would be 2x3 and one would be 2x4. I should be able to cover all the area with it. Would these smaller pieces be more trouble?
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wrote:

Why do you want to put a subfloor over concrete? Unless the concrete is in bad shape, it should be a great surface to glue the tile to.
JK
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The short answer is that the tile instructions explicitly stated to use a plywood subfloor, and *not* to use concrete. The side contacting the tiles has to be smooth too.
The longer answer from what I read is that concrete is too porous for the adhesive, and the tiles will start to separate within a few weeks. That and plywood gives a slightly softer contact then concrete.
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Instructions aside, if you have moisture problems at or below grade, you will have serious problems with the plywood over time. I'm assuming you've checked out to eliminate any potential problems there.
It is also possible there is a better material for flooring over concrete. I've not done any tiling for years so I don't have any recommendations with today's materials.
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wrote:

Go ahead and put down whatever piece size is handiest. Make sure the edges are treated (cheap poly). Use a good construction adhesive. Once everything is in place get all the edges level with a random orbit sander, then fill any gaps, low spots with good old Bondo. After it sets up, finish the substrate nice and level and you should be good to go. Doesn't seem logical to go tight to the wall. Wood in a bathroom is likely to expand a bit. You might ask you tile supplier whether bare plywood or sealed is preferred. Ina bath, the latter might work out better. HTH
Joe
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