What type of render for tiling onto in a shower situation?

[mattshaw]

Im building a shower cubicle in a u shape. The right hand wall is down to red brick and is 20mm out of plumb. I am going to be tiling the shower using a slate weighing 32kg pm2. I do not have space to dot and dab aquapanel or moistureboard so I figure rendering the wall is my only option. My questions are:

1) will a render have strong enough adhesion to the brick to support the tiles?

2) if so what type of render do I use?, sharp or building sand, ratio? And do I need any additives?

3) do I just damp down the brick and apply or do I need to seal the brick? I've heard pva is a bad idea because it would react with any moisture.

4) please could you give me any tips on application and achieving a level finish.

Thanks

 

[stevethespreader]

use soft sand you can buy it in wickes prebagged use a scratch coat of 4-1 and float coat of 5-1 with a bit of Plasticizer in both coats, s&c is best used in bathrooms because of its ability to breath and regulate the moisture better than lightweight render, if your wall is 20mil out of plumb use the screed method get a spirit level and holding it against the wall so the bubble is level place a dab of render at the top and a dab of render at the bottom you will notice that they will be different thicknesses because of the wall being out of alignment, then fill in the gap between each dab of render with more render,you will now have a straight screed to help you rule off and get the rest of the wall straight

 

[joe-90]

I'd use plastering sand if I were you. It's like courser than soft sand and is easier to use.

 

[stevethespreader]

I'd use plastering sand if I were you. It's like courser than soft sand and is easier to use.

disagree here john soft sand is the easiest to use because of the size of the grains. plastering sand (graded sharp sand) is harder to use but has better strengh qualities for outside render

 

[mattshaw]

Brilliant advice, thankyou. My only concern is the adhesion between render and brick. My plasterer friend suggested attaching a sheet of metal mesh to the wall with fixings and rendering through that to ensure it doesn't comeaway from the wall, I suppose it might also help with getting a level. Do you think this is necessary?

 

[stevethespreader]

Brilliant advice, thankyou. My only concern is the adhesion between render and brick. My plasterer friend suggested attaching a sheet of metal mesh to the wall with fixings and rendering through that to ensure it doesn't comeaway from the wall, I suppose it might also help with getting a level. Do you think this is necessary?

no its not necessary in this type of application good luck

 

[Richard C]

Nothing to add to the advice already given on rendering but can I ask what your using for the other 2 walls?

Edit See new post below.

 

[Richard C]

Just seen you other post in tiling; by MR board are you referring to plasterboard? If so, you are right on the tile weight limit for plasterboard, does the 32 kg/sqm weight you quote above include adhesive/grout? This could add another 4 kg/sqm which will put you way over! The other problem is that you should be tanking plasterboard in wet area, even MR. You would be far better off using a cement tile backer board which could take it up to 40/50 kg/sqm & wont disintegrate if it gets wet.

With regard to re-rendering the out of plumb wall; I’ve also just noticed that you have existing plaster & although a sand/cement render base will take up to 50 kg/msq, if you put it over existing plaster the bond strength between the two will only be as good as the current base & will be restricted by the existing Gypsum plaster to around 32 kg/sqm. Again, you right on the tile weight limit & that’s for plaster/render correctly applied & in good condition; . You may be better stripping the plaster/render off, which will allow you to build out plumb with a decent cement backer board.

 

[mattshaw]

Thanks for your advice. Even after stripping away the plaster I do not have enough space to use a plasterboard on the bare brick as I have a 20mm discrepancy at the bottom near the tray, then the wall is leaning inwards and at 2m high it is to nothi g, so essentially I need to apply a wedge of render 2m x 20mm x 1mm.

I assume I need to wire brush the brick wall and then wash. Leave to dry then dampen prior to rendering.

The other two walks are moisture board. No I haven't allowed for weight of adhesive and don't know wether to risk it or re board with aquapanel. I have bought a Dunlop tanking kit which I am going to apply prior to tiling, but this is at least four weeks away if i am going to be waiting for the render to cure.

Any mote advice greatfully appreciated.

 

[Richard C]

I need to apply a wedge of render 2m x 20mm x 1mm.

Most render/plaster will add up to an overall thickness of around 12-15mm, have you already removed it? You’re never going to be able to waste down to 1mm thickness with render; the minimum you’re going to need is 4-5mm or it won’t have any binding strength at all.

For the other walls & with tiles of that weight, I strongly advise you not to risk plasterboard; I’d go with Auapanel. In addition, if you have a recessed shower valve, the PB will have a water risk from behind if there is a problem with the valve unions or pipe work & no amount of tanking on the front side will help with that.

Plan your tiling layout & make sure that no board joints will coincide with grout lines. Your also going to need a quality, flexible cement based powder adhesive & grout for those tiles, the cheap DIY stuff will never cope.

 

[mattshaw]

Gone for a aqualisa digi shower with controls sited outside of the cubicle and showerhead is above the area I'm tiling so hopefully no water ingress from there. Also using bal rapidset flexible on advice of this forum.

With regard to the render if there's no chance of feathering it out at the top of the wall then what do I do? How much depth can you achieve with adhesive, not much I expect with 40cm square tiles!

Thanks for your advice.

 

[Richard C]

Move the tray over to give you more space to pack & board out would be the best & most obvious solution even if it means dismantling what you already have; is there no way you can accommodate this?

Powder adhesive can be used up to 12mm to accommodate locally discrepancies but you can’t space out with adhesive to 20mm or over such a large area; you would never get it flat & it would be cost you a fortune. I would try to accommodate a cement board in there somehow, it will be a lot less hassle than trying to render the wall level & with tiles that size you’re going to need a nice flat base or the tile finish levels are going to be all over the place & look awful.

Only other alternative is to live with it; with large format tiles it wont be so noticeable if you plan the layout.

 

[mattshaw]

Going to do the rendering, I've bought some building sand, cement and some Everbuild Evermix 3 in 1 waterproofer/admix. My only doubt now is going on to the brick wall, I have chipped off the plaster and I'm going to wash the wall down. Would you recommend I PVA the wall or not - obviously I'm aware of the properties of PVA and moisture! I will be tiling directly onto the render and the render will be in a wedge shape approx 20mm to 5mm in depth.

 

[stevethespreader]

Going to do the rendering, I've bought some building sand, cement and some Everbuild Evermix 3 in 1 waterproofer/admix. My only doubt now is going on to the brick wall, I have chipped off the plaster and I'm going to wash the wall down. Would you recommend I PVA the wall or not - obviously I'm aware of the properties of PVA and moisture! I will be tiling directly onto the render and the render will be in a wedge shape approx 20mm to 5mm in depth.

no dont pva the wall just damp it down, but if i was you i would give the wall a skim first followed by a coat of pva before you tile it

 

[mattshaw]

A skim of plaster? I've been advised not to use PVA at all by the tiling forum as it reacts with any moisture it may encounter when the tiles and grout get's wet. I've also been advised that render is a more sound surface and holds more weight than a skim? Anyone anything to add???