Thin slab/screed on top of existing slab

22 Aug 2010
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United Kingdom
Hi guys,

I have a very old garage/shed on top of what must be a very old concrete slab. The slab appears to have been poured in 2 parts and I think is not very level. Either way it is solid.

I now want to rebuild the structure as a timber frame workshop. I'd really like to extend it on one side very slightly and get top level. I'd like to avoid having to break the old slab up but wondering whether it will be okay to pour a 15 to 20 mm slab or screed on top of the existing one?

I guess with the thinness it won't be a concrete mix with large stones. Is their any advice from the experts? Mix? Additives for strength? Possibly a mesh?

The floor will be timber joists or floating on top of the slab. So it won't be exposed.

Many thanks
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If you are putting wood down then you can use self levelling compound. Its not strong enough for a finished floor but fine with something over it. If you really want to use concrete screed then you can at that thickness but it will need to be bonded. Clean the floor then prime it with a cement slurry with sbr in it. Put the screed down while this is wet and it should stick. Screed should have some sbr in it to strengthen it.
Thanks Nick, I appreciate your advice.

Can I just check, but when you say concrete screed, are you referring to a dry ish mix floor screed, or a wetter mix of conrete (perhaps cement and sharp sand only?) Guess if I go wetter, then I'd need to contain it with boards.

I just wonder if a traditional floor screed will be okay in an outside environment.

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