Ceramic tiles on a chipboard floor

The fitter that said straight on to chipboard is a numpty.A good tiler would be a better option than a kitchen fitter.Fitters arent generally tilers and it often shows:) Tell them to stick to units and worktops.

Most if not all professional tilers would never tile directly onto a chipboard floor. There are a couple of adhesives out there that state they are designed for use directly on chipboard, depending on the condition and thickness of the chipboard and providing that it is screwed, not nailed, in accordance with the manufactureres instructions. However, professionals generally take the view that, regardless of what the manufacturer claims, the tiler will be held responsible if and when the tiles start to fail. Generally, a minimum of 6mm WBP plywood or a tile backer board such as Hardy should be used. I always use a minimum of 1/2 inch WBP plywood to overlay the chipboard screwed at 300mm centres. In your case, since the existing floor is water damaged, it is useless, has lost it's strength where damaged and should be replaced. If the entire floor is to be replaced then I would use 3/4 inch WBP ply instead of chipboard. In my experience and as the kitchen is subject to heavy traffic, it would be best to overboard this with another layer of 12mm WBP ply before priming and tiling with a suitable flexible tile adhesive.

laying tiles on to a wooden sub-floor, in my opinion is never ideal, but laying them onto a ply base first would make more sense - having said that, a waterproof chipboard would be fine.Make sure you use a proprietary flexible adhesive. Regards J Chappell Joinery/Building

Plywood is definitely better , if you are taking up old floor , then no point using Chipboard

Tiling onto chipboard is a no go for me, 12mm ply screwed down every 6 inches in a grid fashion and tile onto that, some chipboards merely fall apart in time due to the reaction with adhesive.