Laying new floor

[danr86]

I'm going to be laying Engineered flooring in my downstairs. The floorboards aren't the greatest in some places so to level it out i'm going to lay 6mm plywood with foam underlay on top.
I was only going to do this in the hallway rather than in the lounge as well so i'll need to fit a threshold strip in the doorway.
Where should I finish the plywood:
a) In line with the frame in the hall; or
b) In line with the frame in the lounge; or
c) Somewhere in the middle of the frame.

I'm going to be laying the flooring long ways down the hall way so the boards are parallel with the doorway.

What sort of nails should I use to secure the plywood to the floorboards?
Also do I need to apply water to one side of the plywood before laying it or is that only for hardboard?

There is also a small section under the stairs which is concrete so i was going to put down some plastic to act as a dpm between the concrete and the plywood - what should I used to fix the ply / dpm in this area?

Thanks in advance

 

[danr86]

Any thoughts?

 

[dazlight]

Use 20mm ring nails to fix the plywood. Every 6" and around the perimeter every 4".
Don't wet the ply.
On the concrete area toy should use screed really rather then ply. If you ply it with a dpm sheet under in you can't screw it down.
You would be better drill an screw the ply to it then put the Dpm sheet on top of the ply.