Prepare wooden subfloor before installing hardwood planks

I am about to lay oak solid planks on top of existing wooden subfloor. As long as I can see it - wooden planks over wooden joists.

The room is ground floor and I want to make an adequate protection.

I read a lot that it is recommended to lay 15 pound asphalt felt under the new flooring.

But I cant find it in the UK. It seems to be a US material.

Where to buy asphalt felt in the UK?

What would you lay?

Any advice?

Are your existing floorboards level and in good nick? Do you plan to install the new boards in the same direction as the old ones?

If not and you want to install the new floor floating, a foam underlayment (without DPM) is sufficient, adding a DPM can cause condensation underneath and cause moist problems - even rotting joists - because you restrict the normal air flow underneath.
If you secret nail the new onto the old no underlayment is needed.

If you want to go in the same direction as the existing boards, it is always best to install (diagonal) hardboards sheets (or plywood if old boards are wobbly) first and then the foam underlayment

thanks. i am going to lay it at right angle to the existing floorboards.

i dont think there are problems with underfloor. but you never know. and i cant tell unless the job has started and the carpet is removed.

and i dont want it floating to be honest. not sure why. just doesnt seem right.

but the original question was about of having a builder paper underneath.. it is not exactly damp proof membrane. just additional protection.

do you recon it is not necessary if the existing floor wont show signs of rot or excessive humidity?

what would be the humidity figure in this case?

If you go in the same direction it is always best to lay a thin sheet floor first to prevent joints of the new floor being installed straight over a joint in the existing floor - draft.

Building paper can work, but I'm always at a loss how it can protect if it is then punctured by staples (to secure the hardboard sheets) and/or nails (secure plywood sheets and in your case no doubt the new floor itself).