Solid wood flooring, Plywood/Fibreboard and underlay

[Aziz1]

Hi, a couple of simple questions.

I have a pine subfloor which runs in the same direction as the solid wood flooring I am laying down. So, I'm told I need to use 5-6mm plywood or hardboard over it to make the floor as level as possible.

1 - Can I use underlay such as the fancy foil stuff (e.g. High Performance Underlay, Cushion Acoustic Gold Underlay) if I'm secret nailing/screwing the solid wood flooring down?
2 - If not, what type of underlay is suitable?
3 - Does underlay go above or below the plywood/hardboard?

Also, are "hardboard" and "hardboard underlay" 2 different things or one and the same?

 

[WoodYouLike]

So if I understand correctly you plan to install your floor secret nailed?

If so, install the plywood (stapled) to the existing floorboards and then nail the new boards on to this.
No underlayment (which would impress by the power and the angle of the nails used turning it useless)

 

[Aziz1]

Hi yes, I will secret screw with tongue tite.

Is it ok to nail the plywood down? And does plywood need the same 10mm expansion gaps around the edges of the room as the solid flooring?

 

[WoodYouLike]

Is it ok to nail the plywood down? And does plywood need the same 10mm expansion gaps around the edges of the room as the solid flooring?

Screw the ply every 25 cm, leave a slight gap in between the sheets and around 10mm around the perimeter of the whole floor

 

[Aziz1]

Excellent, thanks for the advice.