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Plenty of levelling compounds that will do a thickness greater than 20mm. I forget the threshold but you may need to mix an aggregate to help make up the total thickness. Give Ardex tech support a call and they will give you the 411 oj the ideal product.
Personally I would replace your floorboards with the 18mm ply/substrate to ensure a better fix down to the structure but you do you.
EstelleGetty-
Thanks - will give them a ring in the morning but realistically needs to be something I can walk in and get somewhere tomorrow for reasons of kiddo being away for a day or two with grandparents.
What's the reason for taking away the boards? I'd need twice the ply to come up to the level of the wonky concrete then. Doable but expensive unless there's a really solid reason to sack off the boards?
I'd still have a lip at one end of the concrete (it's just shy of 30mm at its tallest (a thin peak about 40cm long) but I figure I might be able to sand/grind it down?
CYOA
What's the max depth I can fill with some kind of self levelling compound?
We've got some wonky concrete which is angled and higher than some floorboards in another part of the room. If been looking at ways to bring the boards up but what if....
We put some thick (16mm and above) ply/hardboard down over the floorboards. Then self level the wonky concrete up to the thick ply, then board over the whole thing.
I've seen some that do 10mm in 3 hours but is there anything that'd do 20mm. Can you do two sets of 10mm? I don't want to grind down the concrete or dig it out because it looks like a wet sandcastle underneath it.