Can you tile on a floor without a concrete slab?

[NThake]

I've just pulled up the tiles to my laundry (1975 Warwick, WA house) and the laundry tiles essentially sat on a layer of screed and under that is bare sand (no concrete slab). The screed is in fairly good condition and not cracking or crumbling. I need to take back a small amount to make it level for a flush transition to the wood floor, so my question is can I leave this screed layer, patch up deeper holes with new screed (using. bonding agent) and use a self levelling compound to finish before waterproofing and tiling?

Or does the whole lot have to come up and pour a new slab?

[EricL]

Hello @NThake

Welcome to the Bunnings Workshop community. It's wonderful to have you join us, and thank you for sharing your question about your laundry and bathroom floor.

When you say sand, do you mean sandy? From the picture, it looks to be like a solid stone surface. If it was sand you would sink into it. I suspect that it's some form of stone. If it was a soft sand base the tiles would have cracked and the foundation of your walls would have started to move. Amazingly it has stayed still since 1975.

It could have been a form of filler to increase the height of the floor. I suggest testing a corner of the floor and see if you can reach a cement base. If you do hit a cement base, we can safely say that it is a floor filler. But due to its sandy and dusty nature, I recommend taking it off. It's not an ideal surface to screed or cement. 

Let me call on our experienced members @TedBear, @tom_builds, @ProjectPete, @diy_hausdesigns and @Brad for their opinion on what kind of floor base you have.

Please keep us updated with your progress, we look forward to identifying what kind of floor base you have.

Eric

[MitchellMc]

Hi @NThake,

This could be a slab in terrible condition, and it appears like very little cement has been used to bind the material. There is a possibility that sand has been screeded over, but that wouldn't be considered an acceptable building practice.

Whether this is a disintegrated slab or sand, the surface, in my opinion, isn't suitable for tiling over, and you'll need to investigate what's going on here. If the property is not built on sand, then I can't see any reason why sand would be used to fill the area. It's common practice in many countries to still tile directly over soil or sand. However, the issues that arise from such methods are likely more accepted there.

I believe @EricL's suggestion of testing an area would be the best approach. If there is a solid concrete slab underneath, removing this material and starting from scratch would be the best approach. 

I'll be looking forward to hearing more about your project and providing assistance.

Mitchell

[NThake]

Sorry I thought I was quite clear in my original post. The layer you can see is screed at 60mm thick. Below this is sand. No concrete pad. 

the same applies in the bathroom. 

[MitchellMc]

Hi @NThake,

There might be some confusion as screed is usually a thin layer of cement over an existing surface, predominantly used for levelling purposes. I believe @EricL and I were both thinking this was a thin 5mm layer of screed, and what were we seeing was the sand beneath. If this were the case, it would certainly be cause for concern. Your screed looks exceptionally sandy and has some significant damage to the surface, which threw us.

There is less concern now that you've confirmed that the screed is 60mm thick and more akin to a slab. However, 60mm is relatively thin for a slab, so I'd advise caution whilst taking it back that small amount. If you prime the deeper holes with a bonding agent before filling them, I can't see any reason why you couldn't then use a self-leveller over this area.

Mitchell

[Vis-á-vis]

The answer to your question is technically yes given someone else has already done it and it appears to have remained in place for over 40 years. On the other hand, screeds are not a structural material so you'd never get away with a screed as a floor if building today.

Are you sure there is no steel reinforcing in that floor? It seems almost inconceivable that it could have stayed in one piece all this time with just sand and cement! I guess the sandy underlay would have helped with any movement.

The primary purpose of a screed is to create falls. No such thing as a 5mm screed. Standards state that the minimum thickness for a bonded screed is 15mm and for bonded is 40mm -m that is at the lowest point in the room. If you have a floor waste then you'll need minimum 1:100 falls so if the waste is 3M away from the furthest point in the room then you'll need to allow for a 30mm drop in screed height, so to get to the 15mm minimum thickness at the waste then your screed should be 45mm at the edges.

If it were me I'd be pulling it up and pouring concrete. You'd need to go down about 150mm minimum, lay gravel, a vapour barrier and then reinforced concrete of at last 50mm but ideally, no less than 85mm and perhaps go 50% thicker at the edges (edge beam) coming in 100-150mm from the walls. Of course, you'll need to pull out all of your fixtures and fittings, architrave etc, take down your plaster and put formwork in, allow for expansion joints etc. If you have floor wastes then you'll need a screed on top so allow an extra 40mm depth to allow for the screed. If you don't have floor wastes but will have a shower waste, then you'll need to dig the shower area deeper and have a 40mm setdown in the shower area for the later installation of a screed to get your falls to the waste, alternatively, the slab can be the same in the shower area if you want to install a poly marble type base with falls built-in.

Concreting will be a fair bit of work.

If you are convinced that the existing screed is strong enough for what you want to do then you should level it with a high strength leveller or mortar (not screed, as screeds do not like feather edges). You'll definitely need a laser or a dumpy. Map out your floor and work out where all the high and low spots are. Take off any areas that are higher than you'd like and work out how deep your lowest points are. Then check the manufacturer's technical data for minimum and maximum thicknesses for their products.

Good luck.

[MitchellMc]

Welcome to the Bunnings Workshop community @Vis-á-vis. It's sensational to have you join us, and many thanks for contributing to the discussion.

I trust @NThake appreciates you taking the time to provide such an in-depth response to their questions. It sounds like you are very knowledgeable about bathroom and building construction, and I'm sure our members will benefit greatly from your knowledge. In return, you'll find plenty of inspiration for your projects.

I'll be looking forward to seeing what else you contribute to the community and hearing all about your projects and plans around the house and garden.

Mitchell
 

[NThake]

Solid response thankyou @Vis-á-vis !!!

No I haven't located structural framework yet, I've semi given in to this one and have a tiler coming out to quote tomorrow so I'll see what he says! The level I have to knock off at the threshold I imagine will be too great to cut into the screed to get the falls you mentioned the other side of the room so I fear that it will be a new pad!! Thanks again