laying real wood floor over floorboards

just a few questions for any experts out there that can help me. I am a reasonably proficient diyer - i have bought real oak flooring - 15mm thickness and have existing floorboards which i need to lay the floor over - the floorboards are reasonably good although i need to lay the flooring in the same direction as floorboards therefore will need some sort of plywood to cover boards and then secret nail wood over top - is this the best option and what thickness plywood would i need ( dont want to go too thick as negotiating doorsteps etc - if flooring goes above doorstep leading into lounge - how do i negotiate this ??

wood u like has helped me so far but i have read so many different ways to do it just want to make sure i get it right - preperation is the key i assume....

ty

Hi Jason,
strictly speaking you should lay a minimum 6mm ply, sometimes it can 12mm or more but it realy depends on the subfloor you have. Often original floorboards can be cupped or vice versa producing an uneven fix if you are nailing.
I have, in the past succesfully secret nailed solid oak to original floorboards in the same direction, at the customers discretion without fixing ply first. 6 or 12mm can cause other problems if you are only covering one room for instance, especially in doorways.
A simpler way to put it is: Ply isn't essential but recommended if your floorboards are uneven.
There are door bars and profiles for most situations and if they are solid oak can be planed, re-shaped etc to fit.
Plus, if you lay in the same direction you will find the joists using 40 or 50mm secret nails. Hope this helps...

why don't you pull up the old floorboards?

15mm thick not recommended as structual floor - 18mm minimum.
If the boards are pretty flat use 6mm ply.

but I mean you could pull the old boards up and lay a ply backing, which will be flat and wee-fixed, then new boards onto that.

I don't know how 15mm oak compares to 18mm cracked old softwood anyway...