What Tile Backer Over Dry Screed?

[al0104]

Hello all,
I'd really appreciate some expert advice about what type of backer boards I should use for my bathroom floors.

The original floor is 20mm ply glued & screwed to metal web joists. On top of that we've fixed 25 x 50mm battens (at 400mm centres) & in-between the battens we've got UFH heating pipes covered with a dry screed. A picture of the work in progress probably describes it better:

The finished floor now feels incredibly rigid. Would the best backer board for floor tiles be a 6mm cement board like Hardiebacker or NMP? Or would you recommend something else?
Thanks in advance, Alan

 

[Spare Tool]

For me I'd be looking at a thin coat of slc to get perfectly flat then ditra or ci mat.
Hardibacker or any other cement board will insulate the heat.
Have your floor installers not specified anything?

 

[al0104]

Thanks - I didn't realise Ditra worked over UFH - I thought it would block the heat more than cement board. I'll have a look at that.
The house is in a fairly remote part of Cornwall so we don't have too many specialist tilers locally. Mostly general builders whose eyes start glazing over when I tell them about our floors 😵

 

[Old Mod]

The house is in a fairly remote part of Cornwall so we don't have too many specialist tilers locally.

Oow! My wife would love to move to Cornwall,
I'd worry too much about getting my reputation back up to the level it is now!

No smart arse comments please! 😀
This is a serious half enquiry! 🙂

 

[J Sid]

which every method you choose, me I'd use Ditra. It will help the heat spread evenly over the floor as well as deal with any lateral movement.

my biggest concern would be making sure the all the moisture is removed before it is cover.

I have seen a similar floor cover when there was still a small amount of moisture left, when heating when on it sweated and the ply just rotted away.

 

[White Room]

I thought the screed should be minimum 65mm over those pipes....

 

[al0104]

Thanks for the replies. The screed has dried out completely as it's been down for almost a year now so we shouldn't have any issues with moisture.

The screed on the ground floor does have 65mm minimum cover, but the UFH on the 1st floor uses this low-profile screed instead of spreader plates. It's more efficient at conducting heat and more importantly, much cheaper. Also the weight of the screed was taken into account when the house was designed & the joists were up-rated according to structural calcs - hopefully that should mean we will get less deflection in the floor.

 

[Ajax123]

i've done this method numerous times in the past although of course I always use a flowing anhydrite screed between the joists. I think though that I would want a solid board between the screed and the tiles as each joist constitutes a joint and as the timber and the screed will shrink over time cracks will open up. Additionally the level of thermal expansion due to the heat cycling will cause potential movement issues. I think a 6mm hardibacker board would be my choice then ditra. If you stick the hardi down with a pourable adhesive as well as screw through to the joists you will get good intimate contact with the screed thus minimising the loss of efficiency. given the depth of screed you will want something to help diffuse the heat sideways anyway as the pipe size and screed depth will mean you could get hot spots along the pipes. That's, and of course I sell it, is the main reason I use anhydrite as it has better thermal properties.

 

[Ajax123]

oh and as for 65mm . that is correct if you are using the screed surface as your floor covering support but if you screw and glue hardi down you remove this requirement completely as the hardi and the timber do the work and the screed just acts as a support and heat transfer medium

 

[al0104]

Thanks very much for the advice. That is really helpful. If using hardibacker + ditra over this system, would there be any restriction on size of tiles we could use?