Hi I've just been asked to change a toilet over as the old one has been constantly blocking since the people moved into the house 18 months ago. House is only about 10 years old. I've changed te toilet over and the new one is doing the same. On further examination and removing a tile to get access into some boxing the toilet is on a durgo. Looking in the other bathroom so is that one ad the downstairs toilet goes straight into the floor. The toilet I have changed is the only one that has the backing up problem (this durgo is the highest). However when testing I found if I remove the cup in the shower tray waste that creates the trap the toilet is fine. So in theory if I change the trap on the whb to an air admittance one does that sound like it will solve the problem as the basin tees into the same waste pipe that the shower is in before dropping in to the floor. I'm going back tomorrow to solve (hopefully the problem) thanks