Wanting to do some tiling in my bathroom (1970’s build) Have realised the current tiles have been placed on the original tiles. I think these are just porcelain and were originally yellow but have discoloured. However, you can see a fibrous material behind the original tiles and was wondering if this could be AIB? Behind the boards is a void where the pipe work is located and then my neighbours wall. Pics were taken from underneath unit. You can see toilet waste pipe

