Bathroom floor build-up – advice please

[MissTinyMuscles]

Hi all,

We are refurbishing our bathroom (located on the first floor of a small Victorian terrace property)
Existing floor construction is 18mm floorboards on joists and our ideal finish is tiled floor with under floor heating. The build up that has been suggested to us is a 12mm tile backing board on top of the floor boards, then the heating mat and then the tiles (15mm thickness).
My issue with this is the added thickness of this construction in relation to the floor level outside the room, so I was thinking of replacing the floorboards (which I could use elsewhere in the house) with a thin rigid board fixed to the joists, for example 9mm marine ply with a maybe thinner tile backing board. Is it advisable or will it flex too much? Not sure about the loading strength either...
Any advice would be much appreciated

 

[tony1851]

If the joists are at 400 centres, you will be asking for trouble if you use a 9mm board due to the excessive deflection.

 

[catlad]

You could fix lots of extra noggins in before you put your 9mm ply down.

 

[MissTinyMuscles]

thanks Tony
That is my fear too - the deflexion of the ply under the rigid thin tile backing board... will probably crack / break the board (joists are @400 centres)

Hi Catlad
The extra noggins would definitely help but I might be creating quite a big job for myself (noggin every 200 - 300mm)

Can you think of any alternatives to ply that could work better?
Or any ideas on a different build up?

Thanks,

 

[catlad]

Well you could sister some 3x2" to the sides of your joists with plenty of 4" screws that would narrow your centres a bit more.

 

[Lower]

Put in plenty of noggins, fit 18mm ply and don't bother with the tile backer board.

 

[MissTinyMuscles]

Catlad - bridging the gap between joists will definitely improve the situation.

Lower - Don't you think there is a benefit to the tile backing board?

The recent consensus is to avoid ply as a tile substrate, although I have used it myself in the past on two of the walls of our existing bathroom and tiled over it without issues (5 years ago)

Now I've been doing a bit of research and came across a board that claims to be structural and ready for tiling amongst other finishes. It's called HardieFloor - structural interior flooring

Has anyone heard of it or used it?

 

[Lower]

With plenty of noggins and 18mm ply you'll have a very stable floor that you will be able to tile onto quite happily.

 

[JohnD]

My BIL is over from Australia and was telling me they use 20mm cement board as bathroom floor, screwed down and tiled. No ply, chip or floorboards needed. Have not seen it over here.

 

[MissTinyMuscles]

Hi JohnD

The product I'm looking at is a cement based product and I believe new to the UK market. I spoke to their UK representative last week and he has send me through some product info and most importantly floor loading info. (it seems to comply with the standard domestic loadings under building regulations).

The only issue for my application is that as it is a good conductor of heat, it is advertised as a great product to use on top of a wet underfloor heating system and in our case we have planned to use electric mat underfloor heating between the boarding and tiles. We can still do that, however we will be loosing heat downwards!

So we are still undecided:
-To use this new product and insulate underneath, between the joists to minimise heat loss
- Or just go for good old ply with thin tile backing board (for the electric mat)