Replacement of 1st Floor Bath Floorboards with Tiles, Electric UFH

[Fallingditch]

Been doing a lot of reading - and it seems like its entirely acceptable to go with the following sandwich:

 

• wooden substrate (see below)
• Ardex AF200 matting bonding adhesive
• Dural CI FH (decoupling membrane = Ditra Heat), with embedded UFH loose cables eg from warmstar)
• tile adhesive
• 9mm ceramic tiles

 

'wooden substrate'

1970s two storey bathroom extension on an old Victorian house.  7" x 2" joists at 16" (400mm) centres.  The 20mm pine floorboards were were taken up. 20mm OSB sections were cut to go between the joists, being supported on battens, to give a solid, fairly level floor.

 

 

During the ordering of the above components, my tiling material supplier said "we cannot recommend using our products on an OSB substrate, as you have described".

 

I have been reading a lot of posts on this topic from long time posters such as @nod, @Nickfromwales, @PeterW and it was their experience and suggestions which convinced me that the sandwich ingredients listed above would give me a warm, solid, trouble free and tasty bathroom floor.

 

So this Is where I am: do I need to insert another layer in my sandwich?  and if so layer of what?  6mm tile backer board? ply?

 

Or can I assume that the combination or Ardex and Durabase will be sufficient to cover any depressions in my wooden substrate (or whatever its called) ?

 

 

[Bitpipe]

We had OSB deck, 6mm insulated tile backer board (screwed down), primer, WarmStar electric mats, 2 part latex levelling compound to top of wire then adhesive and tile. Total build up was 25mm as we had a shower tray of that depth that magically was flush with tiles.

 

Remember to put down a conduit for the thermostat which should go in same time as the mat.

 

Get a testing box for the mats (detects earth fault and N/L shorts) and a digital multimeter to take the readings necessary for warranty.

 

You can get decent kits with all the bits, usually with a cheap wall stat that you can replace with something nicer if you wish - I got the whole lot from WarmStar and fancier stats from UFHSS. Latex on eBay I think.

 

 

[nod]

The Ardex and Mat are a great surface to fix to 

As mentioned above 6 mil screwed down are good also 

The handy thing with the 6 mil insulated boards are that you can stick them down with tile adhesive to make a perfect level floor Then bang some screws in the following morning 

 

[Nickfromwales]

Yup. Never screw them down when the adhesive is wet. You can get the floors like glass with that technique. 
If going for UTH then consider 10mm for better local heat insulation. Faster heat up times can be achieved with a few minor tweaks / attention to detail.