Bathroom reno - mortar bed

Hi everybody, new member here. Great knowledge base here. Ive been reading a lot of posts and everybody seems really helpful!

I'm renovating a bathroom and I'm up to the stage of laying the mortar bed and rendering walls. It will be a tiled roman bath. Used to have a fibreglass tub in there but it was a bit on the weathered side so we are going the tile option. After reading a lot of related topics on the site I'm sort of understanding the process, but maybe I'm missing some things? So a few questions...

1. I have to key the existing concrete footing in order for the mortar to bond. Is acid the easiest way to do this? Or will bondcrete suffice? Part of this footing is newly poured and smooth.

2. When rendering the floor do I divide it into 4 planes with gradual slopes converging on the drain?

3. What mix should I be using? A few people have mentioned river sand or 'sharp' sand, will good old brickies sand do? And what ratio? 3:1? Bondcrete?

4. Once rendered, should I prime it all? Got somebody coming in to apply the membrane.

That's it for now. Any advice or answers would be appreciated!

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Originally Posted by nevalast

Hi neverlast - answers in red

1. I have to key the existing concrete footing in order for the mortar to bond. Is acid the easiest way to do this? Or will bondcrete suffice? Part of this footing is newly poured and smooth.
Doesn't hurt to give the concrete an acid wash to etch and clean it. The best keying method I've heard is just to spray the floor as you go and sprinkle neat cement powder over it.

2. When rendering the floor do I divide it into 4 planes with gradual slopes converging on the drain?
This will depend on the floor plan, tile size etc.

3. What mix should I be using? A few people have mentioned river sand or 'sharp' sand, will good old brickies sand do? And what ratio? 3:1? Bondcrete?
Just use plain water, too hard to work with additives. Don't use brickies, pit sand -too weak. Use river sand or medium bedding sand

4. Once rendered, should I prime it all? Got somebody coming in to apply the membrane.
Generally the membrane is put down under the bed. You don't need to prime before tiling

That's it for now. Any advice or answers would be appreciated!

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Cheers
Michael

Hi Michael,

I note u said the waterproofing is done to the concrete slab before screeding. Wondered what is the logic behind that. I would have thought the membrane would be used to keep the screed dry as well?

cheers

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Originally Posted by juan

Hi Michael,

I note u said the waterproofing is done to the concrete slab before screeding. Wondered what is the logic behind that. I would have thought the membrane would be used to keep the screed dry as well?

cheers

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Yeah I'm sure I read elsewhere that the membrane is the last thing to go on before tiles. What's the reason for putting it on before the mortar?

Thanks Michael for all you answers too by the way. Making sure I get the right sand tomorrow. Gonna render the walls first. Got a few chisel marks in them, and a few areas where the bath was need to be built up.

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Originally Posted by mic-d

Generally the membrane is put down under the bed. You don't need to prime before tiling

Cheers
Michael

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Michael,
I am going to be here soon, doing a Tiled Shower, Toilet and Laundry.
Would you; or someone else, please expand on this.
If the Membrane material is on the Concrete Slab and Below the Screed material.
What is the use of etching the Slab?

Please, one step at a time, you are speaking to a Ludite.

Ta.

I'm no expert, however I was reading the manual for the Davco K10 waterproofing membrane that I used in my ensuite, and it said that you should lay your mortar bed first, and then put the membrane on top. The reasoning they based this on was that the mortar bed won't stick as well to the membrane as it would to other substrates, such as cement sheet or concrete. Therefore, I followed their advice. It made sense to me also, from the fact that my waterproof membrane is now able to direct water down the drain, due to the slope it is on from the mortar bed.

I dont think you will get a consensus on why the membrane goes under the mortar - or why it should go on the top of the ramping scree/mortar.

Generally the membrane goes under the mortar - but it depends on the membrane. In the case of K10 - ordinary scree wont stick to it unless you use additives in the mortar mix.

I have seen the membrane done both ways , under and over - Myself has put it both places :wink:.

Tilers have told me all sorts of reasons for doing it either way - but sometimes it just depends on the shedule of the waterproofer LOL

Quote:

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Originally Posted by juan

Hi Michael,

I note u said the waterproofing is done to the concrete slab before screeding. Wondered what is the logic behind that. I would have thought the membrane would be used to keep the screed dry as well?

cheers

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Sorry Juan, didn't see this question. It is possible to do it either way as doogie said. I just use a waterproofer who does it underneath the mortar. The only grates I have seen are designed for this method, where the flange is w/p into the floor and a telescopic floor grate with weepholes fits into this. Any water that may get through the tiles and mortar drains though the weepholes. I don't know what the grate looks like for applying the membrane under the tiles but it shouldn't be the same type - and you need to get that right.

Cheers
Michael