At last, a question I think I can answer.
I've done this a lot, preparing for either carpet or vinyl.
About 10/7 years ago I boarded out the 3 rooms upstairs using hardboard. Bad idea, it follows the curvature of the boards, you need heaps of those gripper nail thingies. It cuts badly, and makes heaps of dust when you saw it. Horrible and not very effective.
Downstairs 2/3 years I did the living room and the dining room. I used 9mm WBP ply. The advantage of 9 mm ply is that it's thick enough to take a number 10 countersunk screw. It will also bridge pretty much any uneveness. And using screws means it's completely removable.
Screws are a great idea. I actually used a mixture:
2 inch 10s (ok, ok, 50mm x 5) to go right through the ply, through the floor boards and into the joists. So as long as you've drilled good pilot and clearance holes and countersunk them, they pull the ply against the joists. i.e. very very firm. Where I was just going through to the floor boards I used inch and a quarter 10s. Whatever that is in metric. For number 10 screws you need a 3mm pilot hole, a 5 (or maybe 6)mm clearance hole.
Before you start that you need to strip your existing floor covering, lift every nail and tack you can, bury any you can't and refix/replace any floorboards the plumbers/electricians have carefully cut and replaced (hah hah!). Also, depending which floor it is remember that plumbing/heating/electricity may be just under the boards. Water (either supply or CH) will probably be notched into the top of joists, unless it's a ground floor with underfloor access. Electricity SHOULD be drilled through joists at least 50 mm down. But the electrician who did our rewire didn't always bother. If there was a convenient notch cut out for pipes he would lay cable in there, sharing with pipework. Risking me and my family's safety. I don't suppose forum rules allow me to say the name he trades under in Leeds, West Yorkshire.
So before you started driving any fastenings check whats underneath. Of course any boards that have been cut, or don't look like they are original are a pretty sure sign there's something underneath.
If the original boards are in really rough condition just buy a few new ones. They don't cost much. If two floor boards meet at a joist I usually try to make sure both of them are SITTING on a joist, but screw (preferably) a bit of 3 by 2 or something to the side of a joist to actually screw the board down to.
Then lay your 9 mm ply. If you've got a lot of alcoves then you're going to have to learn how to scribe. Try to make sure that your 9mm boards don't meet where 2 floor boards meet. And take the ply right into the door rebate. As that's where carpet fitters seem to put the joiny threshold thing (whatever it's called).
Any large differences in level (in my case between old concrete hearth and floor boards) I evened out by packing up the ply. Which took bit of figuring but I managed it. How? IIRC like this - Lay the board unfixed. Drill a small hole. Using the bit you've just drilled mark the height of the board. Stand on the board so you can feel it's right down to the floor boards. Mark THAT on the bit. The difference between is how much space you've got under the board, and how much you need to pack it at that point. Or any other way you can think of. With any luck you won't have any gaps like that.
Also remember that once you've started one room like that, you've kindof committed yourself to doing the whole level like that. Perhaps.
I wouldn't bother with glue or nails. Glue just sounds like a mess. Nails will loosen if there is any movement. Leave a small gap (1mm? ) between the sheets of ply to allow for expansion.
Of course the way I do it is very time consuming. But it's rock solid. There's absolutely no movement atall, no squeaks, and flat as a pancake, so vinyl and carpet won't wear. The carpet fitting was gobsmacked - 'this is the best floor I've ever seen'.
Don't know how you fit wood floor so take that into consideration. You'll have a few screw heads if you do it my way.
Cheers, Mike
