I have a single storey barn conversion. When the bath empties, water comes back up the shower waste, but not quite overflowing into the shower itself. As the bath finishes emptying it pulls the water out of the shower waste leaving only a just enough for the trap to be effective. Is this OK / normal. The shower waste has adequate fall. The bath waste runs behind the bath, then the shower waste is tee'd in, then there is another tee, bringing in the basin waste from the opposite direction and diverting the waste through the wall. Outside there is a standard elbow, about 50cm of pipe before another standard elbow and connector to the soil pipe. I feel that the bath waste doesn't have adequate flow to prevent this backing up. I am not convinced that changing the 2 elbows for swept elbows will make sufficient difference. The only other option that I can see is to run a separate waste from the bath and either drill another hole through the wall and take the pipe to another boss on the soil pipe. Or connect into the soil pipe immediately behind the wc. The second option would involve alot less work, but I am not sure that it is OK to connect so close to the wc.
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