Im connecting on to an old cast iron stack cause of the postion of the tee im goin connecting on to to get the correct hight 7.5 to connect on to the pan of the toilet. Were my flexi pan connector connects in to the 4inch pipe it falls in to the wall cavity. Is this ok or is it bad practice as you won't be able to get to the joint as it wil be plasterd up on inside and cemented up on the outside.