Leveling floor before fitting cork tiles

I'm planning to fit cork flooring in the living room of my flat, which currently just has floorboards resting on joists, that stop a few mm above the concrete slab which forms the downstairs flat's ceiling.

The problem I have is that some of the boards are quite bowed upwards in the middle and in other places some boards are sloping down towards one edge or the other. I understand I need to fit 6mm plywood on top to provide a clean, smooth surface to stick the cork tiles to but even with that I'm concerned that the unevenness in the underlying floorboards will make itself visible in the tiles.

For the boards that are bowed upwards in the middle, could I just sand these down to make them level or would it make more sense to replace them? That would mean cutting the tongue and groove between those boards and the ones either side but maybe that won't matter as I'm planning on screwing all the boards down to stop any squeaking anyway (only some squeak at the moment but I figured I might as well screw them all down, seeing as they won't be accessible once I've laid the tiles).

I'm not sure whether I'm going to use these basic cork tiles http://www.corkfloor.co.uk/floor/GTBASIC.aspx or something lacquered on top like these http://www.corkfloor.co.uk/floor/GTLSNOW.aspx . The latter might be easier to clean and protected from being dented by heels or furniture but the former can be varnished to seal it which I imagine stops liquids spills getting between the tiles, whereas I'm not sure that's an option with the lacquered tiles. Both types should have the same sound insulation and cushioning properties but I wonder whether the lacquered tiles will have the same warmth underfoot that the unlacquered tiles have?

Another option might be floating cork floor like this http://www.corkfloor.co.uk/floor/GNCSTD.aspx which I believe doesn't require the subfloor to be quite as even but the instructions still say that only unevenness of 2mm over 1m is acceptable http://www.corkfloor.co.uk/laying-guide.aspx and it's not clear how thick the actual cork is in those products.

The floorboards in my hall are somewhat worse than the living room, so I'm planning to remove the worst parts and replace them with 18mm plywood sections. I could lay cork tiles there as well but to save money I might go with foam-backed vinyl. I think I'll need to lay 6mm plywood over the floorboards whether I go with cork or vinyl, so as long as I find a 4mm thick vinyl it will all be level with the living room.

 

I was concerned that 6mm ply would be too thin to hide the bumps. There's already 6mm ply with very thin vinyl (probably 1mm) on top in the kitchen and bathroom and that has some very noticeable bumps in it.

I can see that even 12mm (or 18mm) laid on top of the floorboards could still end up uneven between boards, depending on where the underlying bowed boards sit in relation to the edges of the plywood sheets.

To be honest my first thought was to just take all the boards up and lay large squares of 18mm ply, as I figured that a larger piece of wood will distribute any weight placed on it over a wider area/more joists and thus reduce the risk of squeaking/creaking. The main reason I though of laying ply over the floorboards is time and cost, as taking up all the boards and laying 18mm ply in their place will take longer than just screwing ply on top of the boards (maybe not that much longer though), plus there's the time to remove all the nails and cut the boards down to fit in the car to dispose of. In bulk 1220x2440mm sheets of 5.5mm/6mm ply will cost me £14 vs £29.50 for 18mm, so it will cost around £130 rather than £62 to do the 13m2 living room. I'm planning to replace the skirting anyway, so that isn't a factor.

If I do replace the lot, is there much likelihood of the joists themselves being bowed, with that being why the floorboards are bowed, or there being differences in height from one joist to the next, so that I could still end up with the new 18mm ply being uneven?

As for the hall, if I'm laying 4mm foam-backed vinyl might that even itself out if I stuck it directly to the uneven floorboards?

 

I see that I can get OSB 3 18mm board for £16, so that would only cost about £69 to do the living room. Could I then stick the cork tiles directly to that or would I still need a 6mm plywood layer on top of the OSB 3?

I've drawn a rough diagram of my living room

I was thinking of cutting separate strips for the borders, where they'll partly be under the new skirting, as indicated by the coloured sections, then filling the rest of the space so that if necessary, squares of OSB near the edges can be lifted without having to remove the skirting. The nearest joist to the window wall is about 8cm in, hence why the red section is rather narrow and on the opposite side (yellow) it's about 25cm. For the green and orange sections I've just shown the width of the current floorboards that are there at the moment but I guess they don't have to stay the same size. The green section doesn't seem to have any nails in it so I'm not sure what's holding that in place, unless it's nailed to the joists under the skirting.

As the floorboards are exposed in the hall I can take the OSB out into there to the nearest joist if necessary. There's also a joist 5cm in from the right-hand wall (not shown on the diagram) so I should be OK to cut the boards that cross into the kitchen at that joist and screw the two halves to it, or a batten attached to the side of it if necessary.

 

When I replace the floorboards with 18mm ply/OSB in as large cuts as I can manage, should I leave an 8mm expansion gap between the boards or just around the perimeter?

I'll think I'll need to fit noggins to the joists where the boards meet as they're only 45mm wide, which is probably too narrow to allow me to screw down two adjacent boards. The joists aren't attached to the walls, so presumably they're secured to the concrete slab underneath. Would it be worth just replacing the joists with wider ones? If it wouldn't cost that much to buy new joists I'm thinking that might be quicker/less fiddly and avoid any risk of the noggins shifting relative to the joists over time and creating unevenness in the subfloor.

 

Thanks for the warning but I'm planning to put a 6mm ply layer on top of the OSB to stick the tiles to anyway, as I figured it would make life easier in future if I want to replace the tiles with something else if I can just remove the ply and tiles, rather than have to try and scrape the tiles off the OSB, or even have to replace the OSB if I can't scrape them off.

 

It looks like 12mm Multi-Pro costs around £60 per 2.4x1.2m sheet, so about 3 times the costs of OSB. I don't know what SLC is but 18mm OSB to replace the floorboards with 6mm ply on top seems like the best option.

 

I had someone take a look at the floor and he suggested fitting 6mm ply and SLC over that. He quoted me about £440, £200 for labour, £120 for the ply and £120 for the SLC.

I think I'm going to just remove the existing floorboards, joists and insulation, lay 25mm rockwool, cut holes in the rockwool to fit 19mm rubber isolators at 400mm spacings and lay two layers of 18mm OSB on top, leaving an 8mm perimeter gap which I'll fill with acoustic caulk. Doing all that won't cost much more than £440 and will considerably improve the soundproofing.

I may still need to lay 3mm of SLC over the concrete slab first, depending on how uneven it is but I guess it won't be as important to get a perfect finish as it would be if it was on top of the floor, so maybe I'll be able to manage that myself. Is it fairly easy to lay SLC and what tools would I need?

For removing the floorboards, is the quickest/easiest method to cut along the joists using a mini circular saw with a titanium blade (in case I hit a nail), set to the depth of the floorboards (18mm)? I'll probably also have to cut the floorboards and joists down to fit in a car boot, although I might need to hire a van to collect the OSB so I could possibly use that to take the old wood to the dump on the same day if I plan things well.