Originally Posted by
bob w
If it is masonite underlay it will be 5.5mm thick and the manufacturers instructions are strict when it comes to fixing, so yes there will be "lots" of staples or ring shanked nails. These sheets are only 1220x915mm by the way.
If your sheets are larger than this it will be standard or maybe tempered masonite and could well give heaps of heartache if you put additional layers over it. What may happen is by adding ceramic tiles and its underlay you form an impervious barrier that will cause the moisture that rises from the sub-floor to build up under this barrier and that is just where the masonite is. You will therefore have a "drummy floor" due to the masonite buckling. I know its a major job but I would be lifting the masonite and starting from floor level.
You stated it was glued down, this is also contrary to all fixing instructions that I know of and again would make me cautious.:(