Solid oak or engineered boards with veneer?

[Sparkly5]

Hi. I am looking at laying an oak floor. I like the idea of solid as if there is a flood, I would think it would warp less.
The room is just under 6.5m by 5m and is a new extension adjoining the kitchen with an open plan feel due to a large gap being made in the wall. There is a sink and dishwasher in the kitchen and the floors are about the same height though the kitchen floor does run out by about 5mm over a metre or so at the threshold.

Do you have any advice on
1. Solid or engineered?
2. How to deal with the run out of the kitchen floor at the threshold ( which is tiled of which we have a limited number of spares and which has wet underfloor heating).
3. Likelihood of expansion and how to deal with it?
4. Wood thickness, 15mm any better or worse than 22mm?
5. Chipboard or plywood or what under the oak flooring onto concrete underneath this new build?
6. Do you need to leave expansion gaps?

I have loads more questions but will leave those for later!

Any other advice also appreciated.
Thanks

[Job and Knock]

I assure you that if there is a flood solid oak flooring might well rip itself off the subfloor regardless of how it has been installed - I've seen this happen and it wasn't pretty. If you are going directly onto concrete you absolutely must get the internal MC (moisture content) of the concrete down to under 4% (check using something like a Tramex flooring meter BTW - conventional building or woodworking moisture meters are absolutely useless as they are so wildly inaccurate) and that's before you put a chemical DPM onto the floor (something like 2 to 3 coats of Mapei ESM). Your sub-floor (i.e. the concrete) will also need to be very flat to accommodate timber flooring. It might well be better to install an 12 to 18mm plywood inter layer with 12 to 18mm gaps between sheets to take out any major discrepancies (this still requires the MC check and DPM). Normally when I've nailed oak flooring onto a plywood inter layer using a flooring nailer it's been one or two penny washers introduced every metre between planks to give a bit of expansion room every 3 to 4 rows (these need to be removed once you are a further 6 to 8 rows on as otherwise expansion will lock them in permanently), but every manufacturer of solid oak flooring has different instructions and you are well advised to follow their installation instructions to the letter. Personally I wouldn't lay solid wood flooring over underfloor heating as that is another source of issues IMHO