Floor tiling

I have just finished the en-suite and SWMBO would like the floor tiled. I have tiled many a wall but never a floor and would like a bit of advice/opinions please.
The house is a modern build (1992) and the floor is made from those large boards that are a type of chipboard, not the normal Conti type but made up from large pieces of wood chippings, and has a slight Orangey tint to, it sorry don't know the correct term for them. The guys in the tile warehouse say I cannot tile to this as it will "suck out the moisture from the adhesive and the tiles will eventually pop off!"
Apart from the obvious of making sure the floor is 'solid' what preparation do I need to do on this type of floor? Can I not just seal it with a PVA/water mix (what ratio)? The tile guys say to cover in plywood and tile onto that. The thinnest ply I have seen is 4mm. If I go this way then the floor would end up approx 3/4" higher than the bedroom, the door opens into the bedroom so this doesn't cause a problem with that, but what is the best way of dealing with the 'step' from an aesthetics point of view?
Many thanks
John
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Normally the recommended thickness of ply is 18mm, screwed down at 200mm centres, you may be able to get away with 9mm if your floor is already solid, a good way to check is fill a glass of water to the brim and put in into the centre of the floor, then walk / jump around a bit, if you don't loose any water then you should be okay with 9mm (but don't go any less). I would recommend using a 'trade' tiling adhesive such as Ardex Ardu-flex 7001 timber system (approx. 30 / bag - 5sqm at 3mm thick) and its associated primer rather than one from your local tile depot.
As for the step, your local trade floor specialist will have an array of cover plates of different thickness' / colours etc to cover the join between the tile & carpet. If you didn't want so abrupt step then you could build a small wooden ramp say 6in wide under the carpet 1st. As this sort of thing is common is shops, when your next shopping have a look a bit more at the flooring in shops, also places like B&Q / Wickes also have transitions between tiles and showroom floor (albeit not quite perhaps as high) on some of their kitchen / bathroom displays, have a look to get ideas.
Jon
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