Oak t&g flooring installation

I've just taken delivery of a load of 135x19mm T&G oak floor planks for the two downstairs reception rooms, and whilst I finish some other jobs and wait for it to acclimatise I'm going through the detailed planning for it's installation.
It's a Victorian terrace, so the floor downstairs is traditional joists over a ventilated cavity, and of course all the services (gas, heating, electric) are routed under the joists. A couple of the joists are knackered - previous damp problems caused mild rot and a subsequent infestation of wood-boring weavils - so they will be replaced with new, treated joists, so I'll need to get most of the existing boards up to carry out that repair.
The people that delivered it expressed surprise that I wasn't going to install it straight over the old boards, but I really don't want to raise the existing floor level by another 19mm, and on top of that I'm really concerned about access to the underfloor should the necessity arise in the future.
My plan, therefore, has been modified from my original intention of screwing & plugging the new floor straight onto the joists (this seems like a ridiculous amount of work and complete overkill) and instead installing a WBP ply subfloor & secret nailing to that, but creating a couple of removable sections with the boards secret nailed to the ply, but screwing and plugging those sections through to the joists instead of fixing them permanently.
This seems to have a couple of advantages, namely
1) it creates a nice, flat subfloor for easy laying of the new floor; 2) it means that the boards will be somewhat protected from the humidity variations in the ventilated cavity and hopefully prevent cupping and warping; 3) the boards aren;t end matched (t&g each end) as I'd expected, so I won't have to ensure that joins are above the joists.
However, there are a couple of questions about this...
- is 9mm ply thick enough to nail into, and is it enough to provide strength for any joins that occur between joists (they're 14" centres) - which ply to use? Champion stock Far Eastern WBP, Shuttering Ply WBP, BBA Approved Construction Ply WBP, Marine WBP, at widely varying cost - when secret nailing, is it necessary to pre-drill? I'm concerned about nails splitting the tongue. Or will a (hired) nailer prevent this?
If 9mm isn't up to it, then I could just about accomodate 12mm, but 18mm would be way too thick & I'd have to rethink (again).
Oh, whilst I think about it, I'll want a finish to darken the floor. Wax? Oil? Any suggestions welcomed...
cheers Richard
-- Richard Sampson
email me at richard at olifant d-ot co do-t uk
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<snip>
Thanks, Andy. Inline comments.....
>

strength
Nails holding is kind of my primary concern. If I can manage to get away with 12mm instead it may be better.

BBA
ta, that's reassuring.

Can't quite get my head around this.
I'm quite familiar with dimensional change when it comes to moisture change, and if it were being installed as a floating floor I'd install it as I would a laminate and leave an expansion gap around the edge.
However, it isn't - each plank will be nailed to the subfloor, which being ply will be relatively dimensionally stable.
If the floor were to expand then the centre might stay put, and each board away from the centre would move laterally by a linearly increasing amount. Therefore the edge boards may move by anything up to 1cm perhaps. This obviously can't happen without them ripping the nails out.
Am I missing something, or are you saying that the boards just shouldn't be cramped up too tightly together? The question appeared a week or so ago about leaving an expansion gap, and I questioned that then, and also when I asked a related question a long while ago it was suggested thatif anything the boards might shrink, opening up gaps between them slightly (though being T&G it won't matter too much).
I certainly need to understand this particular aspect of the board's behaviour before I install if I am to avoid a costly mistake!

Wax?
I'll take you up on the offer, thanks. I may be after something a little darker though - more akin to the colour of church pews (a lot of church woodwork was made out of oak, wasn't it? it's been a long time...)

Ah, already clocked that one from looking at the pile! Yes, laying the boards out loose on top before installing seems to be quite a wise move!

many thanks Richard
ps. was there much response to the suggestion of a meet in London?
-- Richard Sampson
email me at richard at olifant d-ot co do-t uk
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RichardS wrote:

Who supplied it? It may be very dry, in which case you don't need to let it "acclimatise".
[ ... ]

I don't like the sound of this *at all*, I'm afraid.

14" centres? You will lose minimal material going straight on to joists. It's only a 12" gap. 9mm ply is *nowhere near* enough to nail into on its own, you will find lifting problems & have to re-do.
IIWY I'd lift the old floorboards (may be saleable), fix any joist/sleeper/dpm/infestation/service problems, and go straight on the joists. Where you may need access (is the void deep enough to enter and work?) screw the boards down, removing the under- tongue of the last.

Depends on the timber and the nail (use square-ended as in floor brads, not as in wire nails).

I just wouldn't do it. If you really want a vapour barrier, use pitch paper or even polythene DPM. You might give some consideration to insulation between the joists, too, such as jablite.

I wouldn't use varnish of any sort, oil first/wax after should be fine. It will need a polish every litle while, but it's easier to do this than re-varnish!
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