Hi All. For a bathroom renovation we are removing the old tiles and retiling from floor to ceiling. The current tiles only go half way up the wall. However, after removing a few tiles from an exterior wall, some of the plaster sounds hollow when you tap it, and where some of the top layer of plaster has broken away, the plaster behind is little more than powder. The plaster above the current tile line seems hard and firm. I assume that I should remove all the plaster where it sounds hollow because it will not support the weight of the new tiles which are bigger and thicker? If so, could I remove all the plaster and replace it with plasterboard? If that is possible, how should I fix the plasterboard to the bricks? My house was built in the 1930's.