Fireplace brickwork right angles

[MasonC]

Hi, don’t know if anyone can help me unpick this.

We pulled out the gas fireplace that was previously installed and removed everything back to the original chimney breast brickwork. As we did that it became apparent that the person who installed it hacked the right angles off the fireplace brickwork.

We are planning on lining the recess with Hardie board, but I am worried that where the board meets at a right angle the void behind it will render the board unstable or prone to break. I think the best option is to pack it out, but I have no idea what to pack it put with… (apart from mortar but I am concerned it won’t key in enough and wouldn’t know the correct mix for what I want, and would I have to shutter it etc…)

Any help or suggestions would be great.

Lots of pictures attached. The Ply is just for demo purposes

[markc]

No problem with a Hardybacker over sailing this corner, even 6mm is very strong when covering voids.

if you want to buildout, clean surface, key with diluted pva, ordinary mortar mix.

if you leave a small hole at top of backer board this could be fixed in place and then push mortar through the hole and let it drop into the void. But really not needed.

[Dave Jones]

could foam it after board is in.