A few complications - Ply or cement board?

[0Floz0]

Hi everyone

Appreciate it’s been asked a few times but my question is slightly different as I’ve got a few complications.

Looking to install an en suite in a loft conversion. This will have full tiled walls and a tiled floor. First problem is low ceilings so height is an issue. The thinner the better for the floor.

Current floor is tongue and groove chipboard 18mm. I know not to tile onto this. Problem is what to replace it with. I’m going to rip it up as I can’t go too thick on the floor. So was going to ply onto the joists. How thin on the ply can you go? I was tempted to put cement board on top of this if that’s a good way of doing it. Would you screw this in or just use adhesive? Or would thicker ply be better than using cement board?

3 of the walls are plastered and painted already. The remaining wall is a partition I need to put up. For the partition I assume just a timber frame and then cement board straight onto it or would it need ply first?

For the other 3 walls I assume tiling is fine onto the plaster (assuming it’s flat) and just put some liquid tanking wherever there will be water present? If the walls aren’t level, what’s the best recommendation? Just try and level it with the tile adhesive? It’ll have 600x300 tiles on it.

Thanks all

 

[robinbanks]

All of my family's bathrooms are tiled on T&G chipboard, with either Schluter Ditra, Durabase CI or screwed ply overboarding as decoupling layers. Other approaches such as cement board, Marmox etc would also work

timber frame and then cement board

That's fine

tiling is fine onto the plaster

Tiling onto plaster is ok as long as it's sound but you did say it's painted.. I wouldn't recommend tiling onto a painted surface

walls aren’t level

How out of level? Some of it you can take out with dot n dab approach but if they're like a motocross course, fill the hollows first as if you were plastering

 

[0Floz0]

All of my family's bathrooms are tiled on T&G chipboard, with either Schluter Ditra, Durabase CI or screwed ply overboarding as decoupling layers. Other approaches such as cement board, Marmox etc would also work


That's fine


Tiling onto plaster is ok as long as it's sound but you did say it's painted.. I wouldn't recommend tiling onto a painted surface


How out of level? Some of it you can take out with dot n dab approach but if they're like a motocross course, fill the hollows first as if you were plastering

Can the painted surface be sanded back and then used? Trying to keep costs to a minimum but appreciate some trade offs won’t be worth it and I’ll have to suck it up.

I’d say the biggest deviation is about 5mm. Of the three existing walls, one of the walls is pretty decent but one wall bends in about 5mm and the other bends out about 5mm.

Wickes have got 1200x600 cement boards for £10 so looking to stock up whilst the offer is on. Got another 10% off that with trade and then 6% off that using a discounted gift voucher. So works out at £8.50 ish per board.

 

[tell80]

You can sand the paint to roughen it up but if its soundly fixed then you could use SBR primer instead of sanding.
To take up a few mm's out of plumb then comb the wall with adhesive and butter the tile, and you should be able to straighten the surface out.
Constantly use your level both ways.
Be cautious at inside corners.

 

[0Floz0]

Hi all

Follow up question. I ripped the plasterboard out and have gone with the cement board route. The floor is 18mm ply and 6mm backer board is going on top. Before I screw the ply down, do I need to prime the underneath and edges? I’ve bought some Ka bond Sbr but struggling with the ratio. Is it 1 Sbr : 4 water or 1:1? I’ve read some sites saying I should prime the top side so it takes the adhesive better for the backer board. But that means sealing the whole thing. Is that an issue? What’s the normal route for this?

Thanks

 

[robinbanks]

do I need to prime the underneath and edges?

No

Is it 1 Sbr : 4 water or 1:1?

A primer for suction control would be more like 1:4 though some makers vary; check their instructions..
1:1 is for when you're using it to waterproof screeds etc and you don't add any water when using it as a bonding slurry to stick the superstrate to the substrate

But that means sealing the whole thing. Is that an issue?

Why does it?

What’s the normal route for this?

Prime, apply adhesive, lay

 

[0Floz0]

No

A primer for suction control would be more like 1:4 though some makers vary; check their instructions..
1:1 is for when you're using it to waterproof screeds etc and you don't add any water when using it as a bonding slurry to stick the superstrate to the substrate


Why does it?

Prime, apply adhesive, lay

So I only need to prime the face that’s taking the adhesive? I’m a tad confused as loads of sites are saying to prime the edges of the ply and underneath. Is that not necessary because the cement board is going on top? I’ve got the Sbr so if it’s belt and braces to seal the whole board I don’t mind doing it as it’s only a small area.

 

[robinbanks]

If water is getting to the board, you've bigger problems than a bit of primer will solve

 

[0Floz0]

If water is getting to the board, you've bigger problems than a bit of primer will solve

True. Is there anything wrong sealing all of the board? Both sides and all edges?

 

[cdbe]

You seal it to take adhesive, not to make it waterproof. The normal use for cement board on floors is over uneven floor boards, where you would prime both surfaces and fix with flexible adhesive and screws. I don't know what laying cement boards on top of new 18mm ply achieves.

 

[Deleted member 174758]

The thinner the better for the floor.

Current floor is tongue and groove chipboard 18mm. I know not to tile onto this. Problem is what to replace it with. I’m going to rip it up as I can’t go too thick on the floor. So was going to ply onto the joists. How thin on the ply can you go? I was tempted to put cement board on top of this if that’s a good way of doing it. Would you screw this in or just use adhesive? Or would thicker ply be better than using cement board?

From a carpentry perspective, 19mm T&G chipboard really is as thin as you can practically go, even if you switch to plywood. You maybe need to realise that at one time dwellings had 1-1/8in (29mm) thick floorboards, then T&G softwood came in and the thicknes went down to 25mm, then 22mm and finally 19mm. The softwood was then replaced by plywood or chipboard, but the floor thickness hasn't reduced because even with 18mm onto 400mm/16in centres you can sometimes feel the floor flexing.

Affixing cement fibreboard actually helps stiffen the floor to an extent, but you need a mechanical fixing (ring shank nails or screws - buy yourself a Trend drill/countersink and spare pilot bit if you intend to screw)

We install a lot of cement fibre board in bathrooms and toilets these days where the walls are being tiled (often 12mm, installed in lieu of plasterboard) - not only is it waterproof, but it is a heat and sound insulator. The tilers seem to like it as well, possibly because it us flat

QUOTE="Jamesn88, post: 5474603, member: 292003"]3 of the walls are plastered and painted already. The remaining wall is a partition I need to put up. For the partition I assume just a timber frame and then cement board straight onto it or would it need ply first?[/quote]
You only need plywood where you intend to fix stuff to the wall, such as sinks, WCs, showers, radiayors, towel rails, etc. You'd be surprised at how often these are forgotten - and the extra work that takes to correct

 

[0Floz0]

From a carpentry perspective, 19mm T&G chipboard really is as thin as you can practically go, even if you switch to plywood. You maybe need to realise that at one time dwellings had 1-1/8in (29mm) thick floorboards, then T&G softwood came in and the thicknes went down to 25mm, then 22mm and finally 19mm. The softwood was then replaced by plywood or chipboard, but the floor thickness hasn't reduced because even with 18mm onto 400mm/16in centres you can sometimes feel the floor flexing.

Affixing cement fibreboard actually helps stiffen the floor to an extent, but you need a mechanical fixing (ring shank nails or screws - buy yourself a Trend drill/countersink and spare pilot bit if you intend to screw)

We install a lot of cement fibre board in bathrooms and toilets these days where the walls are being tiled (often 12mm, installed in lieu of plasterboard) - not only is it waterproof, but it is a heat and sound insulator. The tilers seem to like it as well, possibly because it us flat

QUOTE="Jamesn88, post: 5474603, member: 292003"]3 of the walls are plastered and painted already. The remaining wall is a partition I need to put up. For the partition I assume just a timber frame and then cement board straight onto it or would it need ply first?

You only need plywood where you intend to fix stuff to the wall, such as sinks, WCs, showers, radiayors, towel rails, etc. You'd be surprised at how often these are forgotten - and the extra work that takes to correct
[/QUOTE]
That’s interesting. I’m using 18mm ply. I’ve got a load of noggins in the floor to help prevent any flex but having another 6mm of cement board on top should make it pretty solid.

In terms of walls. The only thing being fixed into it is the shower enclosure. I’m assuming 10mm cement board will be strong enough to take that. I’ll probably put a stud where the fixing is going to be anyway just for belt and braces.

I’ve got a dewalt driver where you can set the depth so screwing it all down should be ok. It’s also got washers to screw into it. Not sure if these need to be pushed into the board so it’s flush with the rest of the board?

Thanks

 

[robinbanks]

having another 6mm of cement board on top should make it pretty solid

It's not really the case that adding multiple thin layers to build up a floor means flex is reduced. 3 of 6mm panels would be far more bouncy than a single 18mm sheet

 

[Deleted member 174758]

I’ve got a dewalt driver where you can set the depth so screwing it all down should be ok. It’s also got washers to screw into it. Not sure if these need to be pushed into the board so it’s flush with the rest of the board?

I just drill and countersink the cement fibreboard and screw the boards onto the studs. I'm often installing 12mm CB on walls, because the walls are to be plaster skimmed at the top (onto 12mm plasterboard)

 

[Deleted member 174758]

It's not really the case that adding multiple thin layers to build up a floor means flex is reduced. 3 of 6mm panels would be far more bouncy than a single 18mm sheet

Not the same material, but I was amazed at how stiff a plywood diaphragm floor, made from two skins of 9mm plywood, could be when I installed a load of it a few years back (far stiffer than a single 18mm sheet would be). The downside to that was the need to use about 300 ring nails per installed sheet and use a very particular nailing pattern. When we fixed 12mm cement fibreboard over the top (screwed at 300mm centres) it seemed to not make much difference one way or the other. Maybe something to ask the S/Es on here?

But I reckon that adding a sheet of cement board to the top of 18mm plywood could potentially increase the stiffness, but from what I've read it would need a similar nailing or screwing pattern to that used for ply overboarding, i.e 325 ring nails per sheet on 100mm centres or about 153 screws on 150mm centres.

Of course the joints would need to overlap and installation would be a bear without a gas or compressed air nail gun (I wouldn't even consider screws TBH)