floor repair before backer board/tiles

[drives]

Hi all,

I'm redoing our old 70s fireplace surround at the moment. Having removed all of the old blocks and hearth tiles I'm going to build a timber frame, clad it in hardiebacker, and I have some tiles to finish it off. At floor level I have some matching 600 x 600 tiles which I will lay just around the area in front of the fireplace.

To do this, I started chiselling away at the old concrete that was under the hearth tiles, in between the joists. My intention was just to install the floorboards in this area, overboard with hardiebacker as I usually do, and tile away.

However, in front of the fireplace the joists don't extend to the external wall and it is a reasonably solid block of concrete underneath. Thus I was going to alter plans and leave this section as is and just make sure my hardiebacker is level. To this end I obviously need to build it up a bit underneath as you can see from the photo (the section with the black arrow).

Should I just use a bag of ready mix concrete and then just sit the hardiebacker on top of it, making sure it is at the level I need- then let it all set?

 

[23vc]

I’d dab it down on rapidset tile adhesive, you don’t want the course aggregate of concrete. Assuming no signs of any damp on the old structural hearth.

 

[drives]

I’d dab it down on rapidset tile adhesive, you don’t want the course aggregate of concrete. Assuming no signs of any damp on the old structural hearth.

thanks. I was going to do that but particularly the section towards the middle needs a fair bit of building up. Can I use tile adhesive do you think to build it up this much?

There's no signs of damp at all. I did wonder whether it was a good idea that the joists are effectively running through solid concrete but can't see any adverse effects in fairness on the ones I have exposed.

 

[23vc]

Yeah I’d still go for tile adhesive. Or a sharp sand mortar mix. Or probably get away with plasterboard dab adhesive tbh. I tend to use whatever I’ve got available as I’m tight. But tile adhesive probably safest in terms of keeping it stuck down…

Edit - assumes not a working fireplace. Probably need to be a bit more selective if it is

 

[23vc]

But deffo sbr (prime) the old concrete first whatever you use

 

[drives]

thank you. No not working. We may get a small bioethanol fireplace to place in the opening in due course but nothing that will really affect the integrity of the surround etc.