I've just ordered some 18mm marine ply to lay on the top of the existing wooden floorboards in my 1930's house bathroom before I lay the tiles. The problem I've got is the floor height, when tiled it will be raised approx 30mm which I think is too high. I'm sure I read somewhere that I can rip up existing floorboards ,add some extra noggins and fit the plywood onto the joists thus loosing the floorboard thickness, is that correct.