1st Job: Under my tiles... (UFH, Hardibacker, Ply?)

[DanMarch]

Hi,

A little help if I may, I'm tiling my bathroom floor soon and whilst I've tiled before, this is my first floor-tiling and the first time I've used underfloor heating.

It's a second floor bathroom with wood floorboards, my understanding from reading around is this is the right way to get a good, stable, crack-free finish.

1) Ply on floorboards
2) Hardibacker on top
3) Heating wire taped to hardibacker
4) Flexible adhesive
5) Tiles + Flexible grout

Is that right? Or do I need an insulation layer there? I've no problem raising the floor height by adding an extra layer - the ceilings are high, I need some extra height to avoid having to put the loo on a plinth and a stubbed toe wakes people up in the morning

Thanks!

[Sir Ramic]

An insulation board would help the UFH work more effectively. Its common practice to use SLC over the UFH too.

[Bri]

You are almost right. The ply would be used as a replacement for the floor boards (22/25mm). Glue and screw the backer boards, then the heating, at this point you need to cover the heating wire before tiling with a self levelling compound. Then as you say flex adhesive and grout. If you are using a stone tile also consider a decoupling mat (this goes inbetween the SLC and tile) this will help if any lateral movement occurs.

[Bri]

An insulation board would help the UFH work more effectively. Its common practice to use SLC over the UFH too.

Overlap! I'm not copying!

[DanMarch]

Gents, thanks for detailed replies, overlap and all!

A couple of quick questions then. Do I have to replace the floorboards with ply or can I just lay the ply right over the top? I assume you normally replace the floorboards to avoid raising the floor and shrinking the room but I need to do that anyway or is there a structural reason for it?

I understand that as I'm laying on wood I'll need flexible adhesive and grout to allow for any minor movement in the building, I assume the same goes for the SLC. I've been selecting Bal products so far given good reviews in places like this, but they don't seem to have an SLC with "flex" in the title:

BAL Adhesives - BAL Levellers

Do they not do one? Can you recommend someone who does? Or am I wrong about needing "flex" for the SLC, in which case which Bal product would you go with here?

Oh, and if anyone's interested I was going to go with Superflex Wide Joint for the grout and Paid Set Flexible for the adhesive.

[Bri]

You don't need to replace the floor boards, the key thing is to have a sound, deflection free floor. If you do ply over the boards you can use 15mm (this is th BS minimum). Your SLC does need to be flexible. I use one called 'ultra one' but there are a couple of good ones on the market.

[DanMarch]

Bri, thanks again. That's Ultra-Floor : Products - Floor Levelling Compounds - Level IT one HDB right?

If so, would you not go Bal for the other products too?

[Bri]

Bri, thanks again. That's Ultra-Floor : Products - Floor Levelling Compounds - Level IT one HDB right?

If so, would you not go Bal for the other products too?

Yeah thats the one. I like BAL products, they are very good, the problem is the price. To put it simply, there are products out there that are just as good for a fraction of the cost. Mapei are highly rated, so if you have a Tile Giant near you, they stock all the Mapei products.

[DanMarch]

Bri, again thanks - you're advice is great.

One last question, if it wasn't a cost issue is there a Bal product you would use here? (You're quite right about the price, but I've found some OK prices online, and getting them all delivered together will save on some postage!)

If anyone else stumbles across this from Google, I finally went for:

1) Ply screwed to floorboards
2) Hardibacker glued & screwed on top
3) Insulation glued & screwed on top
4) Heating wire taped to insulation
5) SLC
6) Flexible adhesive
7) Tiles + Flexible grout