damp proof screed or levelling compound? Help, advice, information, answers and tips on all types of flooring from laminate and carpet to timber and vinyl

[crabby]

I have a concrete floor which needs to be levelled, requiring a new screed or levelling compound from 25mm down to nothing over approx 2.5m. When removing old plastic tiles from the floor the old , thin screed broke up, so i removed it down to what looks like a layer of old bitumen paint, most of which came away with the screed.The concrete subfloor appears dry but considering its age (60 years plus) i am doubtful. So my question is : can you buy a levelling compound that will seal, damp-proof and handle these differences in thickness without breaking up or cracking, or do I have to use seperate primers, sealers etc. If so what products can anyone recommend?? Any help greatly appreciated!