Any reason not to not bother sand & cement screed, and just self level instead?

[VDubDan]

Building up a solid concrete floor for a fancy brick-base greenhouse project - the finished floor is going to be reclaimed tile.

When I built my extension I did the typical concrete and then 75mm of sand/cement screed and then ended up self levelling anyway because I'm not very good at screeding as it turns out! At the time I thought the screed was because I'd gone insulation over slab, but it seems for concrete floors the spec/drawings I can find always have a sand/cement screed on top.

Is there any reason that I can't simply build my concrete to a little below FFL, and then just self level for the tiling? Or is there is an issue with that?

 

[Deluks]

I did my kitchen floor with concrete and tiled on top. Got a mate to help out, a professional brickie, I left the floor levelling to him with a steel flooring trowel. I wanted to check it and do a final tamp but he assured me that it was fine and that it's better not to mess about with it while it's going off.

When I came to lay the tiles the concrete was about 25mm out over 3 metres. This made laying the tiles a bit of a ball ache but I built it up with tile adhesive and checked each tile with straight edge and level.

Moral of the story, bricklayers are often overconfident!

Short answer, yes you can.

...and for the size of a greenhouse you should be able to get it very flat and level. You can just pour the slab as normal, do it in layers and use a quantity of sharp in the final layer, and trowel it. Effectively this means you are pouring a very thin screed of sorts onto your concrete at the same time, but it will be easier to smooth as the bigger aggregate will be just below the surface and won't cause any dragging.
Make sure you have plenty of tile adhesive and you won't need to mess about with SLC.

 

[Ivor Windybottom]

Self-levelling screed is a godsend, chuck it on, swish it about a bit, job done. You should be OK to go straight from concrete provided it's reasonably smooth and level. Prime with SBR first.

Just check the maximum depth, some are only a few mm. It varies between brands.

As you're tiling over you don't need to worry about the one issue, air bubbles. Just leave them, they'll pop and you'll have a little hole that will fill with tile adhesive.

 

[^woody^]

You won't get concrete "a little below" FFL, to allow for self-levelling as the dimensions involved are mm, and you won't work to that tolerance.

Level the concrete with a board at the correct level, then lightly float it smooth and that will be good enough for tiles. Don't have the concrete too wet as then puddles will mask the surface for levelling

But yes, if it does need leveling you can use self-leveller or if it's just a few spots, neat cement slurry.

 

[VDubDan]

Thanks all - so on that note, my tiles are nominal 25mm (though reclaimed/rustic style), how far off FFL should I aim for?

 

[noseall]

Is there any reason that I can't simply build my concrete to a little below FFL, and then just self level for the tiling? Or is there is an issue with that?

No problem.

 

[cdbe]

I concreted my 20m² patio and tiled straight onto it. Just set up your screeds accurately.

 

[VDubDan]

@cdbe Wow! What were you using to screed?

 

[cdbe]

Cut up paving slabs.

 

[phatboy]

A lot depends how big the slab will be. I set ply vertically with a laser level, and stacked a load of 6" blocks to the outside. Having used the laser, I knew the ply boxing was within 2mm accuracy.

Filled the box I'd made with concrete, and used a length of 4x2 wider than box to level it out, sliding it back and forth as we moved over it. Did 3 passes and walked away, only coming back to spray a bit of water on before bed,