Originally Posted by
PeeBee
Mark, I have executed around 100 slab upgrades over the years so can pass along some learnings. If the slab is 50mm, which it looks like its less from the photos, you are on a hiding to none - it will crack and fall apart. There isn't enough depth for any reo, and you need it for any cement base product. The interface will be a distinct problem as the slab will dry out quicker on the concrete side as the concrete tries to dry. Your surface needs to be scabbled and very coarse. If this was a floor I was doing, it would get a scabble, or at worst a rough grind, then around the edges I would have some low level anchors punched into the floor like ramset nails so you get a mechanical key, then the surface would be primed with a penetrant sealer, then the floor would be coarse filled with a synthetic aggregate that has a primary layer of coarse material to bring the floor up to say 3-5mm below finished height. From there a primer is applied after a night of curing, a second layer of material of finer aggregate is applied, could be trowelled or could be a pour on self levelling/semi levelling applied. This will give you a brilliantly flat base to work with. I know you said its going to have a floating floor, but if you prep it right, the job will be magnificent. This is the same prep for a commercial vinyl floor, tile floor, synthetic floor etc.
I have been thru this recently at home where we laid Karndean plastic boards over the top of tiles. The tradesman was spectacular in every sense. He ground the tiles first to break the polish/surface, then to get all the high spots off, then he did a primary floor level with self levelling grout, followed by another full day of grinding and re-levelling, then, and only when he was happy the floor was level and free from any grout lines - these will show up on direct layed floors, he started the plank laying.
He did a massive living/kitchen/laundry and 2 bathroom/ ensuites, and 3 billion cut-ins, took him 8 days. It is a perfect job in every way, but the foundation is the base floor. I am going to get him back for upstairs and thats a timber floor and creaky and un-level. It wont be cheap but it will be perfect. Spend the money on the base and you will reap the results 10 fold. If you want this guys details I can dig them out, highly recommended. Hope this helps!