Very Slow Draining Bath Tub that can back fill the toilet and vice verca

[Hugh Jaleak]

If it goes up to roof level, and is open to atmosphere, (might have a cage on the top but that's normal practice), then it'll be ventilating the sewer. I'd still want to see an AAV on the stack taking your WC (and theirs), as there is a distinct possibility bath/basin traps could be pulled under certain circumstances. Seems you're already experiencing the opposite, and whilst admittedly an AAV wouldn't help that, the whole installation is of very poor design.

 

[MeldrewsMate]

It looks like there's a third pipe joining your 4" soil pipe just before it exits the bathroom (a basin waste?).
Does this basin drain quickly, without affecting the bath or toilet?
Does the bath seem to work better (drain faster) if no water has been flushed or drained for several hours?

It appears the 4" soil pipe is substantially blocked between the point where the bath connects and where the 4" pipe joins your neighbour's.

 

[Tomellis921203]

It looks like there's a third pipe joining your 4" soil pipe just before it exits the bathroom (a basin waste?).
Does this basin drain quickly, without affecting the bath or toilet?
Does the bath seem to work better (drain faster) if no water has been flushed or drained for several hours?

It appears the 4" soil pipe is substantially blocked between the point where the bath connects and where the 4" pipe joins your neighbour's.

I don’t think there’s is a 3rd pipe joining as the basin in the bathroom joins the bath waste piping. The basin normally drains okay unless you try to drain it after also draining the bath as they drain from same pipe work. If the basin tap is ran for awhile it eventually causes the bath to backfill

The only other pipe I can see is the thin pipe for the toilet overflow but can see that goes out the other side of the wall as A thin spout you can see in the photo.

Yes the bath drains quicker first thing in the morning after no flushing or draining and then back ups again. Which makes me think there’s is a large blockage but with some room for water to exit just slowly.

I can hear the neighbours pipe work flush quickly if they flush the toilet so I assume the blockage is as you say at the top of our pipe before it joins/combines with the neighbours. Does that sound correct to you ?

 

[MeldrewsMate]

The only other pipe I can see is the thin pipe for the toilet overflow but can see that goes out the other side of the wall as A thin spout you can see in the photo.

Yes, that's what it was.

Yes the bath drains quicker first thing in the morning after no flushing or draining and then back ups again. Which makes me think there’s is a large blockage but with some room for water to exit just slowly.

That's because this assumption exactly fits the evidence /symptoms.

I can hear the neighbours pipe work flush quickly if they flush the toilet so I assume the blockage is as you say at the top of our pipe before it joins/combines with the neighbours. Does that sound correct to you ?

Yes.

 

[OvertonPlumber]

Cut the soil just above branch use a 110mm universal rubber joint then insert another branch to take your soilpipe renew the elbow and section through wall take the pipe up to above flood level on outside.
2-3 hour job for a decent plumber.