Foam Underlay beneath Hardie Backer Board

[It's me]

We are tiling our bathroom floor which has 70 year old floorboards exhibiting the usual wear, tear and irregularities. There is also a slight bow to the floor, albeit only noticeable with a long, straight edge. Replacing them with chipboard is not really viable because two of the original walls have been built directly off from the floor and the boards go under the walls.

I plan to put 12mm Hardie board down before tiling, but was wondering about putting some neoprene-type foam underlay beneath it to absorb some of the irregularities in the boards. Is this a sound idea or would it lead to the tiles cracking over time?

Regards,

James

 

[Swwils]

Ply over.

 

[It's me]

Thank you.

I was going to use ply, or hardboard, but as someone gave me a roll of the foam, I was wondering whether it could be used.

 

[daggermark]

There's a few ways to go about this IMO.

1. Cut out the original floor. This can be difficult when it disappears under walls (similar to my house), as you then have to install noggins and joist sisters to sit the new floor on. None of that is too difficult until you get to places where pipes run and you can't physically fit the noggin. On the plus side, the addition of multiple noggins will add rigidity to the floor and reduce deflection which will help the installation last longer. You can then put 18mm (or thicker) ply down, and have a rock solid basis for the tiles without any concerns about height transitions at doorways. Screw it down with floortite screws to minimise movement.

2. Overboard existing floor with 12mm ply. If you can do it in 1 sheet, all the better. Screw it down with floortite screws to minimise movement.

3. Cement board over existing floor. This is probably the most risky because you aren't adding much strength / resistance to deflection. In this scenario you lay the cement board with adhesive and screw it down every 6 inches with stainless steel screws.

I've seen all these options last 10+ years, even option 3 when there is still some level of deflection in the floor - flexible adhesive and grouts help. But it would be a last resort.