Advice needed for subfloor not leveled.

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Hi folks,

I'm preparing the kitchen floor for tiling. The floor is not level (been like this since I bought the house) and want to address the floor before tiling but don't want to go overboard unnecessarily.

I took the spirit level and drilled some screws on each end to determine how much it had sunk by.

The max gap is 40mm - see below where the floor has sunk
View media item 107103
The gap in the middle is around 20mm.
View media item 107102
So in effect the current situation is,. Left to right it's 0mm > 20mm > 40mm.

I plan to lift the existing T&G boards. I have the following options:
1- Use 18mm ply, seal the sides and bottom bit with a primer/sealer.
2- use 18mm OSB, seal the sides and bottom but
3- use 8mm plywood over existing floorboards

Please can someone point me at the direction.
a) what option is the best? (From above)
B) how do I level the floor? Do I lift the joists?

Any pointers will be greatly appreciated
 
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The best way to "lift" the joists is to plant 2in (50mm) nominally thick timbers onto the sides of them with the tops of these new timbers level. I'd look to do the job with 4 x 2s (around 95 x 44 finished) screwed (5.0 × 100 or 6.0 x 100 screws) at 100 to 150mm centres.

As a sub floor OSB probably needs to be given a layer of SLC (self levelling compound). My own preference would be to use 18mm hardwood plywood glued down and screwed at 150mm centres (4.0 x 50mm) or 18mm tongue and groove (T&G) P5 chipboard flooring (2400 x 1200mm) glued and screwed down at 150mm centres as either will generally take the floor as is without any need for further levelling. In all cases I'd glue the sub-floor to the tops of the new "joists" with a D4 flooring adhesive (as well as the grooves in any T&G).

If you go the other way and lay 8mm plywood, you will still need a lot of SLC built up over 2 or 3 layers to get the sub floor level as many SLCs only allow 8 to 12mm of build-up per layer
 
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Thanks JobAndKnock sounds a fair plan.. I will use 18mm plywood over the new leveled 4x2 joists. Can I get away with screwing them? I was under the impression they had to be bolted?

So as per your advice, the plan is:
1- life the old T&G boards
2- use 4x2 to level the floor (C24s or C16s?)
3- glue and screw 18mm plywood at 150mm centers
4- prime the plywood
5- tile straight onto the plywood with flexible adhesive.

Have I missed something or should do as best practice.

Please advise
 
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