Can I lay new concrete floor over old?

[Brett75]

Evening all,

Refurbing/converting old bungalow into large chalet style house

Ripped floor out this week, and found a very poor screed over the concrete raft the bungalow sits on.

Have got loads of ceiling height, so wondered if I can stabalise the existing screed, then lay a dpm (currently none installed),insulation then a new screed with underfloor heating running through the screed. Am I correct in thinking you can just lay a dpm over existing? Also, what thickness of insulation/screed would you recommend?

Really appreciate any advise You can give

Brett

 

[RedHerring2]

If the existing screed is very poor, wouldn't it be better to rip up the old before laying a new screed?

 

[tim00]

I agree with RH, it would be best to remove existing screed. This will allow you to see the condition of the slab and prevent you from raising the FFL. Personally, i dont like to lay a membrane DPM directly below a screed. There are various paint-on and custom DPM installations out there that might go directly below the screed (eg. Schluter) depending on your pocket.
Always strive to finish your proposed FFL level with the original FFL or you can build in umpteen knock-on difficulties.