Preparing a kitchen floor for tiling over joists - best way?

Hi there,

I've currently got a series of joists in my kitchen as the floor is being redone after cowboy builders laid 12mm ply over joists with natural slate over the top whilst we were away for a few days. I don't need to tell you the outcome.

We really want to get it right this time and are being very cautious with the best approach. We intend to tile with porcelain - I will look into flexible adhesives and grout elsewhere but I know BAL are recommended. I've also installed some kingspan insulation.

The joists are 6 x 3 inch approx with 400ish centres - its an old building.

Im looking at the following options:

1. 25mm WBP ply - tile directly onto it after a suitable primer or matting. Just concerned with the lack of T&G for the surfaces not on the joists.
2. 18mm chipboard with t&g that I can glue along the joints. Overboard with 6mm hardbacker or plywood.
3. 18mm OSB 3 t&g with the hardibacker or ply overboard

Can anyone advise please? The last thing I want is all the tiles coming loose again!

Thanks for your help

Option 2, but you are going to need additional noggins between the joist before you start. British Standards suggest the initial floor layer needs to be screwed down at 300mm. If your joists are 400, regardless of the layers you put over it, there will still be too much flex. So your noggins will need to be placed dead centre of the 400 but can be placed at 300mm in the other direction.

Lay the chipboard, glue, screw it then lay the 6mm backer down (the Hardiebacker is marked to tell you where to put the screws).

The guidelines for fitting porcelain onto a wooden floor is that there should be no more deflection than +/- 1mm over a 360mm distance. Difficult to measure, but the noggins will almost certainly help. The Backerboard is added for final rigidity and to make for a suitable tiling surface.

However you're not ready to tile yet. Its a new floor and it going to take alot of time to settle. It also needs to get used to expansion and contraction from temperature change. At this point it is inevitable there will be movement. So on top of the backer board, you will also need a de-coupling membrane such as BAL Rapidmat or Schluter Ditra Matting which will eliminate almost all lateral movement. This can be stuck down using the same adhesive you will use to stick the tiles. Which if you are sticking with BAL, should be their Singlepart Flexible.

If you do not use the matting, the subfloor will expand and take the tile with it. The decoupling will take the brunt of any movement and leave the tiles in peace.

Also, if you are is greater than 40 sq.mtrs, you will also need expansion joints, but that's a whole new story.

Hope this helps and doesn't put you off. You only want to do it once, so do it right first time. Its and expensive thing to go wrong

Paul, thank you that's really useful. What I should have mentioned and forgot after the kingspan reference was the fact we are adding a wet underfloor heating system too. The joists do have noggins but they are underneath the insulation - 3 x 2 I think and lots of them. So the top of the joist area that you see is insulation only and the underfloor heating pipes. Adding noggins to the top would mean interrupting the pipe flow or drilling the pipes through a hole. As it stands I don't think I'll be doing this as I've added the insulation and noggins already.
As a result of reading your post regarding movement, I'm now having reservations due to heat issue creating further movement and the fact I cant add noggins underneath the board edge.
Ideally I would love tiles but after wasting a small fortune first time around with cowboy builders I cant afford to make the same mistake. Would it be safer with a 22mm T&G chipboard and and engineered wooden floor on top instead? I'd rather play it safe this time? Or should I upgrade to 22mm chipboard and still add the tile backer and membrane on top. Thanks again