How to prepare this uneven concrete floor for tiles?

Question

We are planning to lay a tile hearth in our fireplace but unfortunately, the concrete subfloor is quite uneven, see pictures. We have had this fire opening widened, so the black damp proof course is is where the old internal brick course was. There is only a small 1 inch x 5 inch area is the very corner where there is a void.

How can I create a flat even surface ready to lay tiles that is suitable for use under a wood burner?

I had considered using a Hardie Backer (concrete board) to cover this entire area, but this would add too much height. So if there is another solution I would like to try something different.

Would I be able to level this out with concrete?

Answer 1

You'll want to research/google "self-leveling concrete". It's a special mixture of concrete and polymers that you mix up and pour onto a surface like you have and it levels itself off and dries as a smooth, level, and hard surface. Perfect for setting tile.

While it's bad form to recommend specific products here, you should be able to find a selection of brands and package sizes at your local BigBox store (and most hardware stores.) Essentially, you mix it up, pour it out, and wait for it to dry. They also sell it as a pre-mixed compound which you may find easier to work with and possibly available in smaller quantities.

You'll want to fill that void first.

Answer 2

For the inside of the fireplace you will use a "Stove and Fireplace Cement" or may be called a "High Temp Cement". You will need to read a few different products as some are more suitable for a skim coat and crack repair and others are more suitable for thick applications.

For the rest... Just a high quality modified (latex) thinset. If you are tiling outside the fireplace you should flatten/level your surface with a thin coat of the modified thinset. Give it a few hours to harden a bit, then lay your tiles (adding thinset to the back of them of course).