Self-adhesive Underlay OR tongue and groove glue. or neither?

Hi, thanks in advance for your help and advice,

I am planning to lay solid oak floor boards in our hallway and lounge (floorboards= 175mm wide and 1200mm long, 16mm thick), and I need to lay them as a floating floor.

The pack information says "this flooring can only be floated using a self adhesive underlay". I originally bought the self adhesive underlay as instructed, but was concerned it wasn't thick enough as my subfloor (thin layer of old style thermoplastic tiles on top of concrete) has a few minor irregularities in it. I therefore swapped the self adhesive underlay for thicker fibreboard underlay. I have dry laid this with boards on top, and it does a really good job of absorbing any of the sub-floor irregularities, so I'm happy with this.

But here is my question: Now that I can't use self adhesive underlay, how do I secure the boards? Do I :
a) use self adhesive underlay on top of the fibreboard underlay, so that I get the irregularities absorption and adhesive option?
b) Use tongue and groove glue instead (but I'm concerned by this if the pack didn't mention it as an option and said "only" use self adhesive underlay)
c) Not use any adhesive at all, and just dry lay the boards on top of the fibreboard? I suppose this would also allow free movement for expansion?

Advice gratefully appreciated- happy to provide further info if necessary.

Many thanks,
Pete

You shouldn't lay solid flooring floating in the traditional sense and definitely not if the flooring is pre lacquered as the glue won't bond properly with the wood and as for just pushing it together don't even think about that.
I don't see any reason why you couldn't lay Elastilon over another underlayment but to be honest if the irregularities are so small they have been taken out by fibre boards I doubt they would have been noticeable anyway.
For a solid subfloor my personal preference is always to glue the floor down, I have used Elastilon in the past but you always get that hollow sound with a floating floor.
If you haven't used Elastilon before make sure that you have your flooring butted up properly before removing the backing it sticks like 5h1t to a blanket, instantly and is almost impossible to pull up without ruining the backing and relies on the backing being contiguous, always make sure your joins end up halfway under a board.

Thanks Chappers, appreciate the advice,

I'm not keen to glue the boards down directly to the subfloor as I'd be glueing directly to the layer of thermoplastic tiles, which I don't think is advisable. And taking up the thermoplastic tiles would open up a can of asbestos worms I think.

So it's good to know using both underlays shouldn't be a problem.

Does anyone else have experience of laying self adhesive underlay on top of another underlay?

Thanks,
Pete