Floating floor replacement in a downstairs toilet.

[bobdabuilda]

I intend laying ceramic tiles on the downstairs toilet floor but on lifting the vinyl floor tiles, I have found that the 18mm chipboard floor underneath, is floating on a layer of 20mm polystyrene and blue polythene membrane. Underneath this the floor is concrete. The house is a bryant built property from 1996.

Now would the concrete floor that's there already, have an integral dpc below it or is the blue membrane the only dpc used? Could I use a deep base levelling compound directly onto the existing concrete floor to make up the 38mm that I need to raise the floor before tiling or do I have to put some screed down on top of the existing blue membrane before putting down the self levelling compound?

 

[bobdabuilda]

A picture is worth a thousand words so here is the current set up.

Untitled

• bobdabuilda
• 7 Mar 2011
• 1

My dilemma now is to either :

1) Hope there is a dpm below the current concrete floor (getting rid of the blue dpm and polystyrene insulator)and pour on a self levelling compound to make up the 38mm deficit- Problem with this would be a potentially damp floor and cold. I don't have the room to leave the dpm and use a 50mm recommended screed.

or

2) Keep existing floor and put a 6mm ply on top of the chipboard and then tile using flexible adhesive/grout - Just worried about the amount of movement of the floating floor given that I don't have the room to use 15mm ply to overboard the chipboard.

 

[Richard C]

You’d do better getting Mods to move this post to tilling as I only spotted it by accident. 6mm ply overboard is completely for a tile base, the minimum you should use is 12mm WBP (BS is actually 15mm).

Be warned that tiling onto a floating floor (as opposed to a suspended floor) especially one with a timber base is very risky; you need to consider preparation & special tiling materials if you want to avoid tile failure. A great deal depends on the rigidity & compressibility of the insulation used & that doesn’t look good to me. I have no knowledge of your particular floor construction & weather there is a DPM within the screed but you can get screeds that can be laid less than 50mm but don’t use an SLC. A better solution may be to use an insulated backer board such as this;
http://www.insulationexpress.co.uk/Tile-Backer-Board/Marmox-Waterproof-Insulation-Boards.htm
Depending on what you end up with may also need an uncoupling membrane between the tile base & the tiles. I would advise you need to do rather more work to make the floor rigid or stick to vinyl which may be more cost effective for a downstairs loo.

If you do decide to go with the tiles I would advise you do a lot more research as to appropriate materials & the preparation needed or it could all end in tears & unnecessary expense.

 

[bobdabuilda]

The ceramic tiles have been bought already and must be fitted or my life won't be worth living from her indoors! Why do you advise a 38mm screed as opposed to using a self levelling compound? Would a suspended floor using 20mm tanalised timber batten mounts and 18mm marine ply over the current insulation be a better option?

 

[Richard C]

The ceramic tiles have been bought already and must be fitted or my life won't be worth living from her indoors! Why do you advise a 38mm screed as opposed to using a self leveling compound?

Not all SLC’s can be laid to that sort of thickness & those that can are expensive. A bonded, granolithic or resin screed will be cheaper & can be laid much thinner than the 50mm recommended for an unbonded sand/cement screed.

Would a suspended floor using 20mm tanalised timber batten mounts and 18mm marine ply over the current insulation be a better option?

I can’t see how a suspended floor using 20mm battens will have sufficient rigidity & how are you going to fix the battens to prevent the floor moving? The problem with installing anything over the current insulation & vapour barrier is that it will still be a floating floor &, judging by what I see in the picture, subject to movement & flexing. Any movement or flex will result in tile failure, the main reason why floating floors are not best suited to tiling. I think you will stand a much better chance replacing it with an appropriate thickness of Marmox (or similar) insulation board, an uncoupling membrane & tiling over that. You may also need a latex based adhesive (expensive) to accommodate movement but even that will fail if you exceed the maximum permissible flex.

 

[bobdabuilda]

I'd fix the battens to the floor using tapcon screws and seeing as the room is only 2.2m by 0.8m I think there'd be plenty of rigidity in the floor using 18mm ply on top of the existing insulation.

How would you fix marmox board to the subfloor, is it using the latex adhesive to which you refer? What exactly is an uncoupling membrane is it just a vapour barrier membrane?

I would prefer to fill the floor with a deep base slc like the ones Wickes do which are suitable upto a 50mm thickness apparently.

 

[bobdabuilda]

Ok having looked at the various options I have now decided to go with the 40mm marmox boards on a latex adhesive over the concrete subfloor. Reading about the uncoupling membranes they say only for use on floors of more than 5m squared but mine is only 1.76 so would I be ok with just flexi adhesive and then tiling on that? Is there an alternative to the hugely expensive marmox waterproofing tape? Can you recommend any particular brand of adhesive and cost please?

 

[Richard C]

The waterproofing tape is used when constructing wet rooms or tanking cellars. If you have a damp membrane in the screed (& looking at the photo I thinks it’s unlikely you won’t have) & all you need use is reinforcing tape.

 

[bobdabuilda]

Thanks Richard would bal fastflex adhesive be ok or is there a cheaper alternative that you'd recommend? Also would sandwiching 2 lots of 20mm mamrox board be more rigid than using one lot of 40mm?

 

[Richard C]

On a floor with known rigidity, either Rapidset Flexible or Single Part Flexible would be perfectly OK. By bonding the Hardy insulation board directly to the screed, effectively it will no longer be a floating floor so I would think either would be OK in your case as well but it’s difficult for me to say for sure unless I see it & the last thing I want is for you to come back & say my advice didn't work; if you go to Fastflex then it will be more than enough. Your call really but, thankfully, it’s only a small room so the additional expense won’t be too great.

Laminating 2 boards of, say, ply or tile backer board ½ the width will never be as strong as a manufactured single board the full width so I wouldn’t go the sandwich route.

 

[bobdabuilda]

Great stuff, I'll let you know how I get on and thanks again for the advice.

 

[bobdabuilda]

After taking up the rest of the floor it now appears that the floor is of bison beam construction with no screed over the top!! Aaargh! Is it still ok to do the above on this? Would you recommend an acrylic primer before going ahead and if so is there one you'd recommend?

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• bobdabuilda
• 10 Mar 2011
• 1

 

[bobdabuilda]

Richard?????

 

[tictic]

dpm and then a sand/cement screed mate...

most of them block beam(dense/hollow block)should have them,thats the way i do them.

 

[bobdabuilda]

dpm and then a sand/cement screed mate...

most of them block beam(dense/hollow block)should have them,thats the way i do them.

Are you saying that the dpm is usually under the blocks already? I'm guessing that there is, as the floor is bone dry.