thanks for the advice. there's a bit more info below. I'd be grateful for some more advice as I don't want the tiles to crack! and I've already got the BAL single part flexible adhesive.
The chipboard sheets are glued along their halving joints and nailed down on to battens @ 14" centers (with insulation between) over a damp proofed solid concrete base. ( new construction 1991).
the floor is very solid and there's no sign off any movements or squeaks. There is definitely no movement between the joints in the chipboard.
why is an additional layer of ply wood necessary?
thanks
Chipboard itself is quite an unstable material for tiling onto. Adhesive manufacturers recommend overlaying with plywood for 2 reason, 1. To impart more stiffness to suspended floors to reduce deflaction as much as possible and 2. if chipboard does get wet, it can "blow" i.e it can swell up and push off the tiles.
All timber and timber based substrates are are prone to "movement" because of differing moisture content of the environment meaning the timber will expand and contract. All materials expand and contract to some extent but it is when materials "move" at different rates that causes the problem. Your tiles would expand/contract at a different rate to the chipboard, hence your problem.
However, your problem is not unsurmountable as you can, I I have done successfully, tiled straight onto chipboard if you use appropriate materials, AND THE FLOOR IS SOLID! Please note the emphisis there.
There are a number of alternatives. Your BAL Single Part Flexible is not part of it unfortunately. If you really want to tile straght onto your chipboard floor, this is what I would do.
Firstly i would go over the floor and screw the boards down to the joists at no more than 300mm centres with corrosion resistant screws (nails work loose too easily) leave it nailed as well. Then I would prime the chipboard with a product called Ardex P51 primer. Once that is dry I would use an adhesive called Ardex-flex 7001 to fix the tiles.
If you are unable to get hold of the Ardex products you would need to seek out something like BAL fastflex adhesive. This is a 2 part latex based adhesive but it is very expensive and messy to use but it will certainly do the job.
There are other alternatives as well but these include laying a "decoupling" mat of some sort and again will entail additional cost.
