Tiling straight onto slab

[lizzie]

Hi all

 

I have a standard mbc poured slab with steel, no floor build up ufh in slab.

 

Its 180sq m single storey and I have 600x600 porcelain tiles to run the whole of the floor. I am being told by some tilers that I need a lot of floor prep and its very costly.

 

My slab is up and down variance of up to 20mm in places but that can be sorted with a self leveller.  

 

I am told the slab should be 'sanded' prior to any work commencing - this is to remove any calcium carbonate that may be present. This sanding is a dirty and expensive process and is done with a floor sander of the type you would use to sand wooden floors with an appropriate sanding disc. I have to hire the specified machine for their use. It will be a three day job to sand the floor.

 

 Slab has been down three weeks so far and there is no evidence of any calcium carbonate on the surface so far so no visible reason for this to be suggested, when asked why we need it they say they do all new slabs this way. We are another five weeks off starting tiling so maybe it will appear. I have never heard of this before, do you know if this is something recommended for slabs?

 

The process continues after sanding with the self leveller going down then a primer then a decoupling mat then adhesive and finally tiles.

 

All this ends up at over £70 sq m plus tiles, adhesive and grout.  My floor will end up costing circa £140sqm all in.....I had not budgeted that sort of prep cost. Thank goodness I got the tiles in the sale!

 

Does anyone know if I actually need all that or is a leveller, primer, adhesive and tile with a silicone expansion joint at doors enough? Two tilers have suggested the expensive method and two the simpler method. I don't know which method is what I actually need.

 

Thank you for any help.

[Jeremy Harris]

We have an MBC slab.  The only prep done to it by our tiler, before laying around 45m² of large format travertine, was to sweep and vacuum the concrete and apply a very thin PVA sealing coat to it.  Ours didn't need any levelling, as it was within a few mm of being dead flat, and we have the travertine running flush through four doorways from room to room.    We have no expansion joints, and they are not needed, as the slab has steel reinforcement and so won't crack.  The slight difference in the coefficient of thermal expansion between the concrete and the travertine is accommodated by using flexible tile adhesive.   The flooring has been down a couple of years and is still as perfect as the day it was laid.

 

In terms of cost, I think we paid around £20/m² for labour, and around the same again for the travertine.  Add in the cost of adhesive and sealant and the total cost came to around £45 to £50/m².

 

Not sure where the calcium carbonate is going to come from with a normal cured RC35 concrete slab mix, either.   

Edited July 25, 2017 by JSHarris

[Nickfromwales]

Are these two heros confused as to whether or not you have a liquid screed . 

All that needs doing to a fresh concrete slab, if finished reasonably well, is a good douse with a flexible tile primer, watered down 50/50, and left to dry. You'll go through a lot so a disposable mop and bucket is the easiest route or a roller. Put plenty on. 

Silicone expansion gaps at the door thresholds for a slab of that size laid in one straight run wouldn't hurt, but as its a low temp slab it really shouldn't be an issue vs a higher temp retro-fitted system.

id go with the two who want to lay direct, and, fwiw, I don't use Ditra mat unless it's a wood floor or an unknown substrate, eg extreme circumstances. 

Make sure any self levelling is done ONLY where the low spots are. I hate it when people level the whole floor, making the high points even higher. Waste of time, materials and cost. Ask these questions of the two more sensible candidates and see what their response is. 

[lizzie]

29 minutes ago, JSHarris said:

We have an MBC slab.  The only prep done to it by our tiler, before laying around 45m² of large format travertine, was to sweep and vacuum the concrete and apply a very thin PVA sealing coat to it.  Ours didn't need any levelling, as it was within a few mm of being dead flat, and we have the travertine running flush through four doorways from room to room.    We have no expansion joints, and they are not needed, as the slab has steel reinforcement and so won't crack.  The slight difference in the coefficient of thermal expansion between the concrete and the travertine is accommodated by using flexible tile adhesive.   The flooring has been down a couple of years and is still as perfect as the day it was laid.

 

In terms of cost, I think we paid around £20/m² for labour, and around the same again for the travertine.  Add in the cost of adhesive and sealant and the total cost came to around £45 to £50/m².

 

Not sure where the calcium carbonate is going to come from with a normal cured RC35 concrete slab mix, either.   

Thank you so much. Sounds like its a try on then.

[lizzie]

8 minutes ago, Nickfromwales said:

Are these two heros confused as to whether the or not you have a liquid screed . 

All that needs doing to a fresh concrete slab, if finished reasonably well, is a good douse with a flexible tile primer, watered down 50/50, and left to dry. You'll go through a lot so a disposable mop and bucket is the easiest route or a roller. Put plenty on. 

Silicone expansion gaps at the door thresholds for a slab of that size laid in one straight run wouldn't hurt, but as its a low temp slab it really shouldn't be an issue vs a higher temp retro-fitted system.

id go with the two who want to lay direct, and, fwiw, I don't use Ditra mat unless it's a wood floor or an unknown substrate, eg extreme circumstances. 

Make sure any self levelling is done ONLY where the low spots are. I hate it when people level the whole floor, making the high points even higher. Waste of time, materials and cost. Ask these questions of the two more sensible candidates and see what their response is. 

thank you I will ask as you suggest

[Alex C]

My German tiler was super fussy about everything and worked to a tolerance of about 0.5 mm, but even he tiled straight to the mbc slab after putting down a primer (not pva). If you can get hold of a laser level go over the whole floor with a stick with a mark on it and see for yourself exactly how out of level it is. Mine was within 5mm except for where it dipped by 2 doors, and that was easy to bring back to level with more adhesive. we have a large open plan ground floor with over 100m2 of 1000 x 500 mm tiles, didn't use decoupling matt and didn't have expansion joints, just a good quality flexible adhesive. Finished result is absolutely perfect with zero deviation between tiles. With large tiles you need to make sure they are a good quality and flat as some have a bow in them, and make sure they are fully bonded not just on dabs of glue. In my part of the world good tilers cost £40 per m2 for labour only. As a side note, if you have very large open plan areas on a floor of that size, your 600x600 tiles will look quite busy.

[IanR]

Hoovered and then primed on mine as well, no other surface prep required. Due to the UFH my tiler insisted on Ditra matting "to avoid grout cracking". I personally didn't think it was necessary but I've only tiled bathrooms so it wasn't much extra and I didn't want the the fact I would have gone against his advice to be used if there were any problems in the future.

 

I'd avoid sanding/grinding. My experience is that few trades have worked directly on to a power-floated C35 slab and all underestimate how hard the surface is. It will likely be a much bigger job than they expect, and from what I can see, unnecessary.

 

More importantly though:

 

4 hours ago, lizzie said:

My slab is up and down variance of up to 20mm in places but that can be sorted with a self leveller.  

 

Eeeekkk... Hopefully MBC are putting that right for you.

 

Can you use self-levelling compounds for that thickness? You may need a cementitious product. 

 

Is it just a few high spots that could be ground down, or are there hollows that didn't get enough concrete poured. That also questions how much coverage you have over your UFH, be careful if you need to drill or screw into the slab.

 

Edited July 25, 2017 by IanR

[lizzie]

32 minutes ago, Alex C said:

My German tiler was super fussy about everything and worked to a tolerance of about 0.5 mm, but even he tiled straight to the mbc slab after putting down a primer (not pva). If you can get hold of a laser level go over the whole floor with a stick with a mark on it and see for yourself exactly how out of level it is. Mine was within 5mm except for where it dipped by 2 doors, and that was easy to bring back to level with more adhesive. we have a large open plan ground floor with over 100m2 of 1000 x 500 mm tiles, didn't use decoupling matt and didn't have expansion joints, just a good quality flexible adhesive. Finished result is absolutely perfect with zero deviation between tiles. With large tiles you need to make sure they are a good quality and flat as some have a bow in them, and make sure they are fully bonded not just on dabs of glue. In my part of the world good tilers cost £40 per m2 for labour only. As a side note, if you have very large open plan areas on a floor of that size, your 600x600 tiles will look quite busy.

Thanks Alex. Tiles are top quality Porcelanosa so hopefully no issues there. 600 was a deliberate choice for size I am happy they will look ok in our situations. I am in the Midlands and don't really know what the going rate labour only is...quotes are circa £40-50

[lizzie]

20 minutes ago, IanR said:

Hoovered and then primed on mine as well, no other surface prep required. Due to the UH my tiler insisted on Ditra matting "to avoid grout cracking". I personally didn't think it was necessary but I've only tiled bathrooms so it wasn't much extra and I didn't want the the fact I would have gone against his advice to be used if there were any problems in the future.

 

I'd avoid sanding/grinding. My experience is that few trades have worked directly on to a power-floated C35 slab an all underestimate how hard the surface is. It will likely be a much bigger job than they expect, and from what I can see, unnecessary.

 

More importantly though:

 

 

Eeeekkk... Hopefully MBC are putting that right for you.

 

Can you use self-levelling compounds for that thickness? You may need a cementitious product. 

 

Is it just a few high spots that could be ground down, or are there hollows that didn't get enough concrete poured. That also questions how much coverage you have over your UFH, be careful if you need to drill or screw into the slab.

 

I dont think MBC are coming back have to leave that to project manager. We have one area around an internal stud wall which is really bad that is the deep one. Then we have some dips and peaks here and there. I am told those are within tolerance of 10mm. I dont know about self leveller and if  is that is enough or if it will need something different tiler will have to tell me I guess. Not a happy bunny as you might imagine

[IanR]

I'd park it back with MBC to resolve.

 

A 10mm tolerance is +/- 5mm about the nominal height. So from the highest to the lowest should not be more than 10mm. That's a reasonable tolerance to have achieved, if they've not done so then they need to correct it.

 

If the slab is thinner than the SE specified thickness (incl. tolerance) allows for then I'd be concerned where you will stand with any Warranty if a problem occurs in the future.

[lizzie]

1 minute ago, IanR said:

I'd park it back with MBC to resolve.

 

A 10mm tolerance is +/- 5mm about the nominal height. So from the highest to the lowest should not be more than 10mm. That's a reasonable tolerance to have achieved, if they've not done so then they need to correct it.

 

If the slab is thinner than the SE specified thickness (incl. tolerance) allows for then I'd be concerned where you will stand with any Warranty if a problem occurs in the future.

Thank you for the advice I will investigate. I think just that one area falls outside the 10mm tolerance