General Tile Discussion

Hello Friends- Newbie here and really loving all the Tile Coach content.

I'm just starting a demo and already have a question. I was about to rip out this green board to open the wall to the studs and planned on using Kerdi board/system. Then before pulling it I wondered if that was necessary or required?

Is it best to use the kerdi board directly on studs or is it better to apply the kerdi board over the green board if it's dry?

Really appreciate any insight, thanks!