Gentleman

I’m tiling the ensuite floor so I’m going to rip the rubbish chip board up.

I understand if I was just to go with WBP ply, the minimum thickness should be at least 18mm, with many suggesting 21mm.

But I was thinking of laying 15mm WBP and over boarding that with a backer board for it’s insulating properties as I’m going with underfloor heating.

Apart from the extra cost am I wrong ?

Will 15mm WBP and 6mm backer board be as strong as 21mm WBP ??

ta

Surely it’s not a question of strength, but a question of movement?

Will there be more flex in the floor with two sheets rather than one, which will lead to the adhesive and grout failing?

Hmmm … thats not a bad point… a quality flexiable adhesive will be used though

Can’t you tile on existing floor if its in good nick?
Tiles, adhesive and underfloor heating on a backing board shouldn’t let any water through should it?
How wet’s it going to get?

The easy answer to your question is no, the 15mm and thermal backer won’t be stiff enough.

Is the “rubbish chipboard” the floor itself? If so, leave it be unless it’s falling to bits and rotten. Use 6mm or better still, 12mm Hardiebacker or No More Ply, glued down over the top of it with a flexible tile adhesive and screwed at 400 centres. this will be a much stiffer floor than ripping it out and putting other layers down.

If the chipboard is sound, you could also look at a decoupling layer such as Shluter Ditramat, again glued down with flexible tile adhesive. This will allow movement in the substrate without cracking tiles above. Both can be used with underfloor heating as long as flexible adhesive and grout are used.

I’d not bother with the thin thermal layers as, IMO, they don’t offer much performance for the added cost, and addicting additional layers is adding more potential failure points by my way of thinking.

(tiler for 20 years if that’s worth anything)

If you want something solid rip the chipboard up and use hardifloor either 19mm or 22mm depending on joist centres. Its rock solid and can be tiled straight onto. Helping keep the floor levels in the house the same. Have been using it on most of our bathroom floors now and its been great.

The chip board is definitely coming out.

Blazing thanks for info on the thermal layering… it was experience like yours l was after … so should l go straight in with 21mm WBP

If I’m reading this right, you’re screwing ply straight down to joists? then I’d probably want 25mm and make sure the joints and edges are fully supported. Otherwise, 18mm with a 6mm cement board glued and screwed as 18mm on it’s own won’t be stiff enough.

That Hardie floor is a much better solution though as it saves 2 processes.

Yeah thought that hardifloor was going to be my answer… but (as far as l can tell) it’s a good conductor of heat and is laid OVER ufh…?

Blazing … yep screwing straight onto joists. Sorry to labour the point and to question your experience but will the 3mm of difference between the 15mm and 18mm (if overboarded with 6mm Decker board) make that much difference… 600mm porc tiles btw

TBH, yes I think it probably would. Std floorboards or chip is 22mm, which the recommended method for tiling is to overlay that with 6mm Cement board/Ditramat or minimum 15mm WPB ply, pref 18mm.

We’re actually talking about the difference between 15mm (your suggestion) and 22mm (normal floor thickness) which is pretty much 33% thinner than recommended. You MIGHT get away with 18mm WPB screwed directly to joists with good edge and joint support, over plyed with cement board or a decoupling layer.

For tiling you’re looking for the stiffest floor you can muster, no point in corner cutting hoping it will work if you have the opportunity to do it to spec from the start. IMO of course.

Whats the opinion on using 18mm moisture chipboard flooring and then overboard in with 9mm ply, glued and screwed down.
What’s the benefit of using the cement board on the floor over ply?

Eveners?

9mm ply not thick enough, no point. use 6mm cement board instead or a decoupler like ditramat, same cost, less floor height, conforms to guidelines.

Cement board to stiffen the floor up. 18mm floor on it’s own is not stiff enough to take tiles.

I generally on a chipboard or ply floor use Ditramat, on floorboards use cement board.

Thanks for you help Blazin

Top man