I can see why is chipboard not suitable for tiling in some circumstances.
However my chipboard floor is solid, no deflections....therefore I see no need for adding ply for extra stiffness.
the chipboard is bone dry, been down for 17 years and is not subject to any wetting or moisture.
please explain in more detail why you foresee cracking or debonding before Christmas.
What you appear to be not understanding is that the atmosphere around your floor will vary constantly with temprature and moisture content of the air. Even though your floor is "bone dry" the atmosphere containes moisture and you have heard of this referred to as humidity. Different weather conditions WILL affect your floor even though it is dry. This is the reason you have ventilation under a suspended floor and also the reason why you have insulation boards between your joists.
Ceramic tiles (or natural products) do not have any flexibility and WILL expand and contract at a different rate to your floor. Yes your floor WILL expand and contract with differing atmospheric conditions. Even your chipboard surface will move at a different rate to the wooden joists they are attached to as they are different materials. These movements are inperceptable to the eye but they ARE there.
You say that any movement would only be taken up within 1 tile width, if that tile width goes along the entire length of your floor you have 1 big long crack to either look at for years or a lot of re-tiling to do.
The deflection issue you mentioned has ways of being overcome as mentioned in my previous posts without overlaying with ply. The real problem you have is the fact that you are going to use an ADHESIVE THAT IS NOT DESIGNED TO FIX TO TIMBER.
We here on the forum are only trying to stop you from having big problems in the future and wasting a lot of money and time. Bear in mind that if one of the professional guys on here came to do the job for you, using exactly the same materials and procedures you intend to do, when it cracked you would be striaght on the phone to get him back to fix it at his own cost. We are only advising you what we would do to do the job correctly and reduce potential problems to a minimum.
Your chipboard floor is solid. That is fine. Tile straight onto it by all means but please, please, use the correct adhesive. BAL Single part Flexible is not suitable for this job.
As an aside, it will be much less hassle to go and change your 10 bags of adhesive than to retile 35 or so square metres of floor. ( I assume that is the area looking at the number of bags you have).
