Tanking floor - ply, liquid membrane, tile backer? - TilersForums.com

[johnplum]

What's wrong with this following system that I can't help considering?

- I'm starting with recently laid solid and level 18mm wbp over suspended timber floor joists which comprises a bathroom floor. This ply replaced floor boards in bad condition. Area about 4 sq m
- A shower tray former if 900 x 800, rises 8mm above ply.

- the idea is to tape ply joints and apply liquid tanking membrane over the whole ply area and also up rendered shower area walls, before following by laying down on the floor 6mm Hardiebacker board with thinset bed of 3mm to fill voids, and seal with taped joints.

- finish floor with bal SPF (6mm) and 10mm granite tiles.

Usually the liquid membrane is applied over ply before direct tiling, or over tile backer - though Hardiebacker have said, I read, they don't recommend liquid tanking over their board.

I could have simply used 25mm ply and tiled directly.
The idea I had is that as SPF is only water resistant, and as ply may go mouldy etc, the tanking of a small bathroom floor under the tile backer is ideal.

Critical comments and advice welcome!

 

[Chalker]

Why not use 6mm insulation boards and tank over the top, then tile ?

 

[Dave]

Why not over board the floor with a cement faced poly core board like marmox ?

Tape joints and perimeter floor to wall transitions and fixing points with correct tanking tape , no issues then .

then you can either liquid tank walls or use membrane tanking . Following same suit as floor for joints .

 

[johnplum]

Yes I was considering that option, of a waterproof insulation board with cement facing.

I have the liquid membrane, enough of it. In a way this is constructing my own waterproof cement-faced board.
I bought the Hardiebacker, but could return it, I was told; hence I'm asking the question.
Would this idea not be comparably as good in terms of water resistance? AN interesting comparison, methinks at this time.

I'm not absolutely confident in extruded polystyrene (XPS) degrading faster over a score + of years. XPS may be better for walls than floors, with the impact of feet.
I haven't been able to see the Marmox board myself (I would have to try harder). I see its a comparable price to Hardiebacker.

 

[Dave]

Yes I was considering that option, of a waterproof insulation board with cement facing.

I have the liquid membrane, enough of it. In a way this is constructing my own waterproof cement-faced board.
I bought the Hardiebacker, but could return it, I was told; hence I'm asking the question.
Would this idea not be comparably as good in terms of water resistance? AN interesting comparison, methinks at this time.

I'm not absolutely confident in extruded polystyrene (XPS) degrading faster over a score + of years. Hardiebacker may be better for walls than floors, with the impact of feet.
I haven't been able to see the Marmox board myself (I would have to try harder). I see its a comparable price to Hardiebacker.

if you tank the floor first then hardibacker , the fixings at basically every 200mm will puncture that water tight membrane. Water could then permeate at those fixing points and seen as hardi do not recommend tanking on top , how would you over come that .?

the other alternative as you have the hardi is to use a sheet tanking membrane over the hardi adhered with SPF adhesive. Again taping joints et .

 

[johnplum]

@David: Good point about the screw fixings to the tile backer. SO a sheet membrane is a good idea Screw fixings are a weak point for water penetration through any tile backer or insulation board - bitumen sheet would be superior for waterproofing.

:detective: This raises the question , why use a tile backer on a floor anyway over ply?
- a sheet membrane would also lose any advantage of tiling to a backer board, which is the reason given by Hardiebacker, for not tanking with liquid butyl etc. Perhaps they're wrong - that stuff is very sticky, and you're supposed to tile over it., in which case,

:2guns:Perhaps there is no good case for using tile backer board over ply flooring when tanking?

If so, I could simply screed the floor with a mix of my choice, apply liquid tanking and then tile over SPF.

 

[Ajax123]

Not going to comment on the structure but if you do go the liquid membrane route consider a two coat system and broadcast sand into the second coat whilst wet in order to make sure it has a good mechanical key.

 

[johnplum]

@Ajax that's good advice, and a useful reminder, as I juggle with ideas... It's easy to overlook that, as I've not seen it mentioned on company 'how to's' It's what I do with bitumen and render, with sharp sand.

Speaking of which, I'm considering applying bitumen as a sealer to the ply, whatever I use to raise the ply floor level - screed or tile backer.

Comments on structural consideration still welcome.

Should I screed or use tile backer and, if so, should I apply liquid membrane to the tile backer, or is sheet better, or apply nothing, relying on liquid bitumen on ply?


Bearing in mind the shower area has a waterproof tray former, in a small bathroom.

 

[johnplum]

Reflecting a bit more on the choice, I think Hardiebacker could be wrong to advise not to tank their cement board, adding that their board soaks up any moisture without deforming. Major waterproofing companies (e.g. DUnlop, Everbuild…) produce their liquid membrane specifically for tiling over, and list compressed fibre-cement sheet flooring as a suitable substrate.

A tanking membrane makes cement board redundant for wet floors, and a good screed is sufficient. Also, means you don't have to use extra fixings, tape, and adhesive bed.
Nomoreply state that their product is to go over ply flooring in bathrooms and save it from becoming damp: But, that's not considering tanking with liquid or membrane.

 

[Dave]

@David: Good point about the screw fixings to the tile backer. SO a sheet membrane is a good idea Screw fixings are a weak point for water penetration through any tile backer or insulation board - bitumen sheet would be superior for waterproofing.

:detective: This raises the question , why use a tile backer on a floor anyway over ply?
- a sheet membrane would also lose any advantage of tiling to a backer board, which is the reason given by Hardiebacker, for not tanking with liquid butyl etc. Perhaps they're wrong - that stuff is very sticky, and you're supposed to tile over it., in which case,

:2guns:Perhaps there is no good case for using tile backer board over ply flooring when tanking?

If so, I could simply screed the floor with a mix of my choice, apply liquid tanking and then tile over SPF.

i agree , apart from 18mm ply IMHO isn't sufficient regarding strength , my choice would be 25mm min , tanked with a sheet membrane .

but if you have already installed 18mm as said in your OP, then advice was given on that scenario as I see it .

others might differ and you take from a thread which way you want to proceed.