I want to lay a screed over an existing sloping concrete slab (interior, no DPM in existing slab but plastic sheet test shows no damp whatsoever).
The problem: half the room has a new concrete slab complete with insulation and DPM. I've necessarily had to lay this new slab so it rises 25mm over its 3M length in order to floor over a new 100mm drainage pipe running in it (invert of existing connecting drain is very shallow).
I now want to screed over the adjacent existing slab to matching levels i.e. a finished level which rises 25mm over 3M. The screed thickness will be 45mm thick to zero.
What should I lay down to form this sloping screed from zero to say 10mm? I intended to use a cement:fine sand mix with a good gob of PVA in it bonded to the existing slab with PVA. Should I alternatively buy some SBR bonding additive and use it throughout the new screed, given that there is no DPM?
Any thoughts or better suggestions out there?
The floor is to receive glazed floor tiling so a perfect finish down to zero is not essential.
If the existing slab is clean concrete - wet it, prime it with a cement slurry and lay a strong cement + sharp sand screed.
If the existing concrete is dirty you will have to use some self-levelling compound which includes latex.
Forget the PVA.
I'd originally discounted self levelling compound as I thought it would "self level" and I need to finish to a slope. I've never used the stuff. Can I lay it to a slope? And I'd have to buy it and I'm tight. Anyhow, I've removed ceramic floor tiles from the concrete so there are only the slightest remnants of the tile cement on it's surface. I'm guessing this is a "clean" surface suitable to receive the cement slurry then cement:sharp sand screed. I'll forget the PVA. Thanks for the advice. I like the cheaper options, I've got sharp sand and cement to hand. By the way, are there any particular disadvantages of using PVA in a screed or other applications? Cheers.
if feathering your screed out to nothing i would be inclined (no pun intended!) to use an SBR additive as it will give the screed extra strength both with the bond to the base and with its own inherent strength. the problem with laying very thin screed is that it has the propensity to break up.
and in my experience self levelling doesn't so much self level as self smooth. it can be laid on a slight gradient and hold itself depending on the slurriness of the mix and the ratio of the gradient. it is probably not best suited in your case however - and as you say it is actually quite expensive.
Whilst you can use various latex based screeds to form a slope you CAN NOT use them on a sub floor without an adequate DPM - unless you encapsulate a surface DPM in between 2 screeds. have a look at the ardex website for a e.g.
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