Uneven floorboards. Will tiles come loose?

[bureaucrazy]

Bathroom floor needs tiling.

The tiles are 30cm glazed porcelain 9mm thick.

No more ply tile backer boards will be used.

Mapei Keraflex maxi adhesive.

The floorboards have warped over the years so they are not flat anymore. They are curved upwards along the edges so they look concave running along the length. This might be from moisture. Also, some replacement boards have been fitted which are about 1mm thicker than the originals. However, the top of each replacement is about the same height as the top of the curved edges of the other boards.

Overall, the floor is level despite the distortions of each board.



I'm worried that the no more ply will not be uniformly supported by the wooden floor, e.g. gaps underneath where the floorboards are concave. Could this cause it to move/flex when stepped on and cause the adhesive to break? If the nomoreply is laid down on the floor it seems OK, not wobbly.

I was thinking to get the whole floor sanded down about 1-2mm with a coarse grit belt sander to make it all flat but this might do more harm than good if it is not sanded uniformly?

Another option is to use BAL fast flex so that even if the nomoreply bends under load, the tiles will stay stuck (although the grout may not?)

3rd, is to use plenty of nomoreply glue under the backer boards to bridge the gaps a bit. I don't want to spread tile adhesive under the nomoreply as it would raise the floor too much.

What's the best approach here from those of you with experience?

 

[Tile Shop]

Solid bed of tile adhesive under the NMP is the best option, then however many screws NMP suggest to secure them in place. The adhesive, if you say the floor is relatively level, will only need to be of a final bed depth of 3-4mm to bridge the 2mm high points (which you should try to also flatten with screws) which is very little overall height gain. I don't know how thick you think the NMP glue will be, but to do its job it would need to be the same depth as tile adhesive or it will be pointless, and it still won't give you full coverage, leaving potentially problematic voids. So you will be raising the floor height however you fix it down.

 

[Dumbo]

As above tile adhesive under the nmp to fill the gaps and screw as recommended. I dont know if nmp recommend tile adhesive under their product so if not switch to Hardie backer , and screw as recommended but also make sure. Floor boards are screwed down before you start . Don't worry about the small step up it is better that than the floor failing due to incorrect prep and think of the cost of redoing it , probably 5 times what you are spending now.

 

[bolts]

If it was me I’d re-board the floor first, then overboard with NMP as per the instructions with NMP glue and screws.
Last bathroom I did was just like this but maybe not quite as level. As it happens I needed the floor up anyway to access the pipework so it just made sense to replace it.

 

[bureaucrazy]

OK, sorry for long delay here.

@bolts: Do you mean take the floorboards up and put new ones or overboard with plywood before using the no more ply?

The problem with adding height is that anything much over about 22mm will mean the toilet will be too high to fit into the soil pipe. The tiles are 9mm thick plus the backer boards 6mm does not leave much for adhesive.

I called no more ply and they said their megastrength adhesive would work but I could put regular tile adhesive under the no more ply if I wanted to.

One question I have is, if I stick the backer boards down with cement based tile adhesive and then some years down the line want to rip it out and renovate, will it be possible to remove the old backer boards and how can I remove the old adhesive from the floorboards without destroying them?

 

[Flintstone]

No more ply is liable to curl up if to stick it down with tile adhesive, I have had this first had. Use Hardie or xps board and bed into tile adhesive as above

 

[Time's Ran Out]

Your either doing now to last the journey or not!

 

[Dumbo]

Height build up not an issue , you can use an offset pan connector . DO NOT worry about how difficult it is to take something apart in the future only worry that it will last that long .

 

[bolts]

I ripped up the old boards and re-boarded with ply. Then over-boarded with cement board and as been said you can use an offset toilet waste, which I did.
In this case I had to pull up some of the floorboards anyway, and also found that some parts of the floor weren’t supported properly.
I also added noggins onto some of the joists to reduce the chance of movement.