BigWallop wrote:


Thanks for that suggestion Tim, but 18mm would be a huge step up from the
rest of the floor level (18mm + adhesive + tile), and would also mean
cutting a lump off the bottom of a door.

I've got a couple of bedroom floors with steps like that due to engineered
wood over insulation panels on concrete. It's actually fine - but I put oak
thresholds in and planed a chamfer on them to avoid having a trip hazard.

Trimming the door is easy - take it off and run a circular saw, a jigsaw or
a handsaw over it

I'm still worried that even with
some type of ply or screed boards laid to reinforce the part of the floor
being tiled, even though my thought was to fix 6mm boards on top of the
floorboards, that the joists themselves will bend enough over their length
to pop the tiles back off.

I think, given how you have described your floor, 18mm ply would be just on
the edge of "will probably be OK". Any less (or none) and I'd give up and
stick vinyl down!

Can you replace the floorboards with ply? Ply glued and screwed to the
joists is a *lot* stiffer than boards so you buy more stiffness for your
thickness. As you are tiling the floor, there's not much to be lost by
gluing the ply too. But if that worries you, use screws every 10cm or so
into every joist (not just round the edges of the ply).

Have done a bit research on adhesives, being normal cement based, resin
reinforced and the full flexible stuff, and they all state that the worst
situation they can withstand is up to 10mm deflection on the floor
structure. I'm sure that these old joists are moving as much as that when
we all gather in the room to chill out etc. I think it's time to gather
the muster, and the time, to go ahead and explore the floor fully to make
an informed decision on what to do.

Once more into the breach, dear friends. LOL

You'd definately want the best flexible stuff - something made by BAL or
Mapei. Some are "ready to mix" flexible, some are "regular" to which to buy
and mix with a flexible additive liquid.

Don't forget, you'll want "flexible" grout too - I used Mapei Keracolor with
Fugolastic additive when tiling over flexible substrates.

Cheers,

Tim

--
Tim Watts