Levelling slight bows in marmox

[sh]

Hi
I have just had a shower room boarded out with dotted & dabbed 12.5mm Marmox as the chased back walls were very uneven.
The plasterer has done a pretty good job but some of the boards have a bow across the width (600mm).
I will be doing the tiling and the tiles we have chosen are 820mm X 410mm, so my question is, to get a super flat finish across the whole wall can I fill the bowing of the boards with tile adhesive first prior to tiling.
If so, do I let it dry first ?

Thanks in advance

 

[PrenticeBoyofDerry]

How much have they bowed? Normally with large format tiles, you will have a lot more tolerance with uneven walls.

 

[sh]

They all have a 'dip' in the middle to a maximum of approx 5mm

 

[Richard C]

Sorry PBD, the larger the tiles the less tolerance you will have not more!

You can level with cement adhesive in fact it’s the best way to do it, don’t use plaster or conventional render. Use a Rapidset flexible adhesive & you can tile over the same day. Powder cement adhesive only for fixing that size tile.

They are large, heavy tiles you should not just rely on dot & dab to hold the boards, you also need mechanical fixings or the whole lot may end up on the floor.

Tiling isn’t rocket science but there are many things to catch you out particularly if you intend tiling a suspended timber floor. I would advise you read the Tiling Sticky & Forum Archive posts before doing any more work or buying materials, it could prevent you making disastrous & potentially expensive mistakes. It’s also important to use only quality trade tilling materials of the correct type for your tiles & tile base; cheapo own brand & DIY stuff is mostly crap.

 

[sh]

Thanks for that.
I had chosen the Marmox originally based on the weight of the tiles + addy and posted prior to the Marmox going up and this coming weekend I intend to mechanically fix as well (9 per board 1250 x 600).
The adhesive I got the plasterer to use was :
http://emctilestrade.co.uk/products...esives&cat=Fixing Materials&code=EMC00757&t=d

I guess that will be ok to fill the bows ?

Many Thanks

 

[PrenticeBoyofDerry]

Sorry PBD, the larger the tiles the less tolerance you will have not more!

Sorry RC but I don't agree, the smaller the tile the flatter the surface must be, you have more tolerance with uneven walls with larger tiles surely!

 

[Richard C]

Sorry RC but I don't agree, the smaller the tile the flatter the surface must be, you have more tolerance with uneven walls with larger tiles surely!

The smaller the tile the easier it is to accommodate bowing or unevenness as the points of contact with the tile base are less & closer together. Larger tiles will bridge more high spots or, in the case of bowing, have larger/smaller gaps under the tile edges (depending on which way the base bows) resulting in uneven contact with the adhesive bed you’ve just trowelled on. The larger the tile the less likely it is to have even contact with a given thickness of adhesive bed, that’s why we use thick solid bed & large format trowels. These have deeper notches &, hence, provide a thicker adhesive bed so that all the tile is in contact with the adhesive which is critical on floors. Another method is to back butter the tiles but unless you’re experienced at doing this, it can be tricky to get them level; far easier to level the wall with powder adhesive before you start.

I know from experience but here are a some links for you to read;
http://www.leafcutterdesign.co.uk/info_fixing_large_format_tiles.php
http://www.stoneworld.com/articles/bigger-tile-bigger-challenges-working-with-large-format-tile
http://www.laticrete.com/portals/0/tds/tds193.pdf

A search will throw up many more.

 

[PrenticeBoyofDerry]

But surely on small format tiles the undulations of the wall will be more apparent.
For example with mosaics.
I am not taking inches out or floor levels.

 

[Richard C]

The adhesive I got the plasterer to use was :
http://emctilestrade.co.uk/products...esives&cat=Fixing Materials&code=EMC00757&t=d

I guess that will be ok to fill the bows ?

It is a flexible cement powder addy so should be OK but it’s not one I know or normally recommend; personally I’ve never used it. I use BAL exclusively & you should be able to get a 20 kg bag of that for around the same price without too much agro. Setting time is a little slow compared to rapid set so you’ll have to leave it longer after leveling before you apply the tile adhesive bed but don’t let it dry out or you will have to prime or it’ll suck adhesive bed dry & the tiles won’t stick; very important with tiles that size/weight. Use a thick bed trowel to ensure you get >100%% coverage under the tile; personally I would use a Rapidset on tiles that size as it will grab & hold much quicker.

 

[Richard C]

But surely on small format tiles the undulations of the wall will be more apparent.
For example with mosaics.

Mosaics are not a fair comparison as the tiles are so small & you can always adjust the adhesive bed to suit.

Don’t want/ain’t got time to get into a drawn out debate about it m8 it’s fact. If you don’t believe me do some research on it, I’ve already provided you with 3 links that categorically back up what I’ve said but you will find many other sources.

 

[sh]

Thanks for the reply Richard.
The only concern I have is that if I use a rapid set I wont be able to keep up with it when laying the tiles (thats why I chose what I did)
If I filled the bowing and let it dry first what would I have to prime it with ?

Thanks

 

[PrenticeBoyofDerry]

You have a large file format, so ideally you need to use rapid set.
Just don't mix to much up at a time, your coverage per tile will be quite large, you have 45 mins in the pot with the adhesive and 2 hours before it sets, in good conditions.

 

[Richard C]

Rapidset (flexy in your case) is best for fixing large tiles as it will grab & hold the weight better, less chance of the tiles moving around. Although the pot life is only around 45 minutes, as PBD said, it shouldn’t be a problem as long as you only mix up a quantity you can use in around half an hour or so. This gives enough time to clean your bucket & tools before mixing the next batch which is crucial as any residue will affect (reduce) the set time of the next mix.

It may seem very daunting & you will be slow at first but as long as you plan the layout, apply the addy correctly with the right size trowel you should be fine. Only lay on enough addy for, say, 2/3 tiles at first but your speed should increase rapidly as you become more confident. Also being plasterer, I prefer to work from a hawk rather than direct from the bucket & never transfer any excess adhesive you’ve cleaned off the tile base back into the bucket (it shortens the pot life), use it straight away on the next bit.

Another option is to use a quickset render to level but if you let the adhesive/render skim dry out, damp it down with water using an old kitchen spray to reduce the suction a little but don't overdo it or you will affect initial tile adhesion. Ideally you should lay the tiles on as soon as the skim has set but not dry.

 

[sh]

Thats great Richard, Thanks for such a comprehensive reply.

What are you thoughts in response to my question concerning using a primer ?

Thanks

 

[Richard C]

What are you thoughts in response to my question concerning using a primer ?

if you let the adhesive/render skim dry out, damp it down with water using an old kitchen spray to reduce the suction a little but don't overdo it or you will affect initial tile adhesion. Ideally you should lay the tiles on as soon as the skim has set but not dry.