Relaying lifted tiles on suspended floor (water UFH)

[Rosewood]

Hi,

We have been in our house a few years but noticed as soon as we moved in that the tiles were loose, with grout cracking. Over the years we've had to replace tiles that crack. We now want to relay the entire floor with porcelain tiles.

We have a suspended wooden floor and as far as i can tell there is 18mm Ply screwed into the joists with the tiles laid on that. I believe there is a dry screed between the joists.

Previously when we've lifted the tiles they were not stuck down at all, with the adhesive stuck to the floor but not the tile. I'm not sure why this happened but this has happened throughout the whole ground floor.

We did get the hall re-laid a couple of years ago and the tiler had to remove all the adhesive from the ply floor with an angle grinder (v messy!). He then laid another layer of ply (not sure what thickness but probably 12-18mm) and then laid the tiles. This floor has been rock solid and these tiles look great.

My concern with doing this again in our (large) kitchen diner, is that this room is only just warm enough in the winter here in Scotland as we have a lot of heat loss (big double glazed windows) and if we lay another thick layer of ply I do not think we'll have enough heat transmission into the room.

So after that long spiel, my question is how would you best prepare the floor before tiling to maximise rigidity but also maximise heat transmission? Would it be ok to lay tiles directly onto the cleaned up ply (unlikely this will be totally smooth due to old adhesive) or would you lay another layer, and for that layer would you use ply or something like Hardiebacker? Also would be great if you can recommend adhesive and grout?

Thanks

 

[Goldtrowel]

Hi,
it will be fine laying new tiles over existing ply, assuming it is well fixed, remove as much existing adhesive (or all if poss) if not flat you could use a rubber based leveling compound, to get flat without extra depth, and ensure that its primed and correct adhesive used!

 

[Spare Tool]

Hi,
it will be fine laying new tiles over existing ply, assuming it is well fixed, remove as much existing adhesive (or all if poss) if not flat you could use a rubber based leveling compound, to get flat without extra depth, and ensure that its primed and correct adhesive used!

The floors already failed once...wouldn't be tiling straight over that in a hurry, hardi and quite possibly a crack mat over that would be my choice

 

[Perry]

Hi Rosewood are you certain its ply and screwed to the joists?

 

[Rosewood]

Hi pjc. When the tiles were replaced in our hall previously we did not lift the ply to see, but it is definitely 18mm ply screwed to something!

 

[Perry]

I have only seen 18mm ply once in 35 years laid on to joists in a large area in a house, and the chipy tongue and grooved the sheets himself. Tiling large areas of ply even if it has no deflection, lateral movement can shear the adhesive.

 

[Rosewood]

Thanks - in which case i'm probably wrong about what the ply is fixed too! We're not planning on lifting the tiles until later in the year so I can't give you a clear answer.

So far I have a sugestion that I can tile directly on to this (once cleaned up) or that I should put down a layer of Hardi. What do people think? Is Hardiebacker better or worse than ply at conducting heat?

 

[Andy Allen]

You really need to know what your dealing with.

Some floors are 'floating floors' which is kingspan and 18mm t/g chipboard over the top but not fixed..... Just resting on the insulation.

This could explain the failure in the first place...

 

[Perry]

Until you are sure what you are dealing with its imposible to give any sound advice.

 

[Dumbo]

Better .wood does act as an insulator to a certain degree .

 

[Rosewood]

Ok thanks - I understand. If I can find out any more about whats under the ply then I'll come back to you.

 

[Rosewood]

I spoke too soon! I've just looked at the plans for the house and assuming that they were followed this is what they say about the floor construction:

"147*47mm treated timber floor joists at 400mm centres; 9mm stirling board on joists with 50*38mm timber battens on top of the joists, underfloor heating pipes with semi-dry cement mix over the same laid between joists; 18mm T&G chipboard moisture resistant chipboard flooring"

Hopefully that makes sense to some of you! I can see a small section of floor untiled and the floor is definitely ply not chipboard, so it does seem that the above was not followed exactly, but I think it reasonable to assume this is the floor construction.

Thanks

 

[Rosewood]

Hi. Any final thoughts given the above floor construction?

To overlay or not?
If so with ply or hardiebacker or something else - to minimise resistance to heat transfer from UFH?

Thanks