Adequate Sub floor Buildup

[rckdev]

Hi all,

New to this Forum. Got 10 years experience of building, do quite a few bathrooms and would like to know others peoples opinions on subfloors for tiling.

My question is regarding sub floor buildup. If a wooden joisted floor has little deflection and is covered with solid timber boards, I will usually put plenty of screws through boards into joists, cover over with ply screwed in grid formation into boards (as little as 4mm thick), glue ditra over the ply and then tile with flexi adhesive.

In a perfect world, I'd love to take up the entire floor back to joists and put down 18mm ply, counter board it with 6mm ply, glue ditra mat and then tile but obviously that's overkill and no-one will pay the overkill price. What do everyone else consider a reasonable joisted subfloor buildup for compromise between piece of mind and cost?

Thanks in advance

Ross

 

[White Room]

BS is 15mm for overply, 4 & 6mm ply is a no no, glue and screw 6mm hardie backer board are my favorite but they don't strenght to the floor.

 

[Mike]

as with whitebeam, i overboard with a minimum 12 mm ply then 6mm cement board fixed with spf adhesive and screwed at 150 centres. this is not overkill just minimum requirements to prevent failure

 

[Daz]

As per WB,

I prefer to add noggins & then replace the whole floor with 18mm ply. I will overboard with 6mm cement boards (glued & screwed) when the floor has no deflection.

Ditra is a decoupling membrane - it will only help with lateral movement not deflection / bounce.

Daz

 

[rckdev]

Thanks for the reply, the reason I go for the 4mm and 6mm and sound existing floors is purely to give the ditra a uniform surface to adhere to. Being a carpenter to trade, it was my thinking that the plywood was to prohibit deflection and increase strength. If the floor does not easily give way under foot then why replace a strong floor with one that is only similarly stong? Is it the expansion difference between wpb and solid wood and if so, so does the ditra and flexi adhesive not allow the tile and subfloor to move independently?

Any further info much appreciated as I like to do things because I understand why and not just to blindly follow.

 

[rckdev]

who said ditra mat solved bounce?? cause it wasn't me!

 

[Daz]

who said ditra mat solved bounce?? cause it wasn't me!

Nobody said that, I was just ensuring that anyone reading the thread doesn't get the wrong impression.
I rarely use decoupling membranes, it is only for UFH and natural stone that I would specify.

For reference, 6mm cement boards will be much more stable than 6mm ply on a deflection free floor. There is a possibility that 4 & 6mm ply will quite easily delaminate so it is not recommended.

Daz

 

[rckdev]

Never considered delamination as a factor, thanks for your opinion, I have a floor to tile next week, the joists are rock solid 10 x 2's spanning 1900mm built into brick, I will defo consider trying the cement board if the decoupler isnt really required as even though it's more expensive than ply, without the ditra, the prep should be cheaper. On a final note, the shower tray I've picked has 3 upstands, 2 of which will be chased into solid walls, the third will be a new partition, was thinking of building the partion 12mm back from the tray, cladding with m/r plasterboard and then using the 6mm cement board down to the upstand, does this sound like an ok idea to you guys?