OSB / Plasterboard bathroom walls

[MortarThePoint]

I was hoping for some tips on using OSB under plasterboard in bathroom walls. How far off the screed should the bottom of the OSB sit? Particularly next to the shower tray should I raise it clear of where the top of the shower tray will be and use either smaller OSB offcuts on the studs as packers or perhaps use 11mm thick timber there? Is it worth treating the bottom of the OSB with anything?

 

How far off the screed is it normal to raise moisture resistant plasterboard? Note both of these are distance off screed not off top of tile.

 

I plan to tank the plasterboard when done. Is that considered a good approach or will it create difficulties for tiling or plastering?

[Nickfromwales]

18 minutes ago, MortarThePoint said:

I was hoping for some tips on using OSB under plasterboard in bathroom walls. How far off the screed should the bottom of the OSB sit? Particularly next to the shower tray should I raise it clear of where the top of the shower tray will be and use either smaller OSB offcuts on the studs as packers or perhaps use 11mm thick timber there? Is it worth treating the bottom of the OSB with anything?

 

How far off the screed is it normal to raise moisture resistant plasterboard? Note both of these are distance off screed not off top of tile.

 

I plan to tank the plasterboard when done. Is that considered a good approach or will it create difficulties for tiling or plastering?

OSB to about 15mm off the screed. Same for PB. Tanking should be done after plastering, not before.

The tray gets fitted ( bonded ) back to the surrounding walls with a decent amount of clear CT1, and then no water can ever get to either the PB or the OSB. Cementitious tile adhesive gets used too fill the 15mm gap whilst tiling the floor, which is pretty impervious to moisture.

The aim is to keep the water where it’s supposed to be, so the quality / integrity of the tiling / wall covering is key to success.

[MortarThePoint]

21 minutes ago, Nickfromwales said:

OSB to about 15mm off the screed. Same for PB. Tanking should be done after plastering, not before.

The tray gets fitted ( bonded ) back to the surrounding walls with a decent amount of clear CT1, and then no water can ever get to either the PB or the OSB. Cementitious tile adhesive gets used too fill the 15mm gap whilst tiling the floor, which is pretty impervious to moisture.

The aim is to keep the water where it’s supposed to be, so the quality / integrity of the tiling / wall covering is key to success.

 

I've sat the 11mm OSB on offcuts of 11mm OSB so am a bit shy of the 15mm. I haven't fitted the plasterboard yet though so could try cutting 4mm off though could be tricky.

[Nickfromwales]

3 minutes ago, MortarThePoint said:

 

I've sat the 11mm OSB on offcuts of 11mm OSB so am a bit shy of the 15mm. I haven't fitted the plasterboard yet though so could try cutting 4mm off though could be tricky.

In absolute honesty, no need. Most use an off cut of the wall / board material, so you’re on the money  

[tonyshouse]

OSB is not dimensionally stable, it can expand if it gets damp. 
 

why use plasterboard at all, that doesn’t like getting damp either. 

[Nickfromwales]

2 minutes ago, tonyshouse said:

OSB is not dimensionally stable, it can expand if it gets damp. 
 

why use plasterboard at all, that doesn’t like getting damp either. 

Because they will all be tanked.

[tonyshouse]

I like fail safe 

[Nickfromwales]

Just now, tonyshouse said:

I like fail safe 

Agreed, which is why I always tank / make bombproof every single bathroom job  I do not like cement board, as it doesn’t comply to the uneven nature of stud walls etc. Last time I used it, to an architects spec, it was bonded with Sikaflex as well as screwed.

Never had a bathroom fail, and I learned fast back in the day through insurance work. A good way of knowing what ‘not to do’ was to take note of what had been done ( which failed, sometimes catastrophically ) and not repeat it.

[nod]

In actual fact It doesn’t really matter what you put on If the water gets past the tiles then the tanking Your screwed 

I’ve been tiling for forty years now 

The tile adhesives and grouts are almost problem free and the tanking systems are fool proof 

But as Nick says they need to be applied correctly 

[MortarThePoint]

Thanks this is all really helpful. So sounds like the winning recipe is:

1. Studs at 400mm c/c (70mm C-studs in my case)
2. OSB (in my case 11mm) onto studs, held 15mm off the floor screed
3. 12.5mm moisture resistant plasterboard onto OSB, held 15mm off the floor screed
4. Plaster skim (Thistle Multifinish or is there a better choice for bathrooms?)
5. Tanking solution (any recommendations?)
6. CT1 shower tray against the plastered wall and tile above as needed
7. Elsewhere, use cementitious tile adhesive to fill the 15mm gap when tiling the floor

Edited May 8, 2022 by MortarThePoint

[MortarThePoint]

BG give the guidance below when it comes to wall tiling. I presume an 11mm sheet of OSB counts as equivalent to a 12.5mm plasterboard and so it's OK to use 12.5mm over it?

 

 

[Faz]

Before tanking you need to make sure you use the tanking tape to the corners and around the tray.  I have recessed shower valves so taped around them too.

 

I used the Everbuild system and it seemed pretty good.

[Jonny]

Just to add, anywhere you are tiling doesn’t require skim. It actually reduces the weight carrying capacity of the plasterboard. 

[MortarThePoint]

3 hours ago, Jonny said:

Just to add, anywhere you are tiling doesn’t require skim. It actually reduces the weight carrying capacity of the plasterboard. 

 

We aren't considering any particularly fancy tiles so weight probably not a concern.

 

Do you think it reduces the bond strength of the tiles to plaster though? I'm just thinking it is easier to ask he plasterer to do a whole room that mark out where he shouldn't plaster.

[Jonny]

Personally I’ve never had any problems with tile adhesive bonding over skim or straight onto board. Like you, I’ve never gone for anything too fancy/heavy! 
Having said that, marking out a shower area not to tile will save somewhere around 6m2 ok skim, and depending how your paying your skimmer, every little helps! 

[PeterW]

You don’t need to skim MR plasterboard but you do need to tank corners and where it joins floors etc.