*Advice needed on under tile surface NEWBIE*

[Tile_novice]

Hello all,

First and foremost I would like to thank all those that have contributed to these forums for providing their wisdom.

To give you some background, Im currently doing some DIY in the house (took on a bit more then I could chew!).

Our floor was constructed on the old wooden 9 x 2 floor joists which had considerable movement leading to our tiles cracking over time so I decided to do a proper job and remove the floor boards, put boards under joists, followed by 50mm insulation, wet underfloor heating then a final 50mm layer of screed. I was hopeful this would make the floor more structurally sound causing less movement in the floor, ultimately eliminating the cracking.

So now screed is done and drying, I have been researching on the options going forward - I originally was going to do T&G floor, followed by concrete backing board then the tiles. I was advised against the T&G as the surface would not adhere well to the backing board and it wouldn't be as structurally sound as plywood.

So now my plan at present would be 2.4m x 1.2m 18mm plywood (have contemplated 25mm to make the floor more solid but thought it may be a bit overkill?) then the concrete backer board, then the tiles (marble).

Any advice on the above would be really really appreciated.

Thank you for taking the time to read this

 

[Flintstone]

Hello, I would forget putting any ply wood or any timber over the top of the underfloor heating!

 

[Chalker]

Use fermacell gypsum flooring. Done lots over that type of floor with no problems

 

[Tile_novice]

Hello, I would forget putting any ply wood or any timber over the top of the underfloor heating!

I see, is it because of future expansion and curving of the wood, or another factor?
Thanks in advance

 

[Tile_novice]

Use fermacell gypsum flooring. Done lots over that type of floor with no problems

Can that be used straight on the screed layer? The main reason for us using a wood layer on top of the screed was for stability, as well as lifting the floor height to match the existing kitchen (higher then the Hall/front room area I’m doing). Considering the long joist length underneath what thickness gypsum (fibreboard I’m assuming?) would you recommend using?
Thanks very much for your time

 

[Chalker]

It’s 25mm thick and as long as your floor will not flex, it will not move.
Will also help the underfloor heating, as is has mass.
For tiling, after fitting the floor. You can tile straight onto it. Sweep, vacuum and then use a gypsum based adhesive.

 

[Tile_novice]

It’s 25mm thick and as long as your floor will not flex, it will not move.
Will also help the underfloor heating, as is has mass.
For tiling, after fitting the floor. You can tile straight onto it. Sweep, vacuum and then use a gypsum based adhesive.

Many thanks for that - my only concern is the potential movement of the floor underneath (as joists are long) but I'm assuming the 50mm screed between would have significantly strengthened the floor.
I'm going to research the gypsum floor you have recommended thanks for your help

 

[Dumbo]

Many thanks for that - my only concern is the potential movement of the floor underneath (as joists are long) but I'm assuming the 50mm screed between would have significantly strengthened the floor.
I'm going to research the gypsum floor you have recommended thanks for your help

I'm not sure about that . Your joists weren't designed to carry an extra 85 kilos per square metre static load .

 

[Tile_novice]

I'm not sure about that . Your joists weren't designed to carry an extra 85 kilos per square metre static load .

That was also a concern of mine - I initially was going to double up the joists to strengthen it because these joist lengths aren't used in modern day construction. My biggest fear was the floor completely collapsing, however with the screed done in 1 pour I assumed (maybe naively) that the load would be spread uniformly. As you said the additional static weight is substantial. You are triggering my anxious nature lol!