Bath trap.

A lot of baths and showers only have around 25mm of water seal due to space restrictions. It's not usually a problem because there is not usually a vertical pipe run after the trap which is often what often sucks them out. It's often possible to fits a deeper trap under the bath by cutting a hole in the floorboards under the bath if it's bothering you. Or/also the bath can be raised a bit in some cases (depends on design) It's also best if the (shallow) bath drain goes to the soil stack alone, ie not teed in with anything else (eg wash hand basin) which might cause a problem.

In message , harryagain writes

AS I said, I really need to kep this trap as it is combined with the pop up waste, etc. Hence the thought of putting in a normal running trap in the pipe run. And will be easier to do this whilst installing everything.

Well have bath, shower and handbasin, I think at least 2 of those will be combined. But I'll be running in 50mm waste pipe I think becuase of the length of run.

A larger pipe does not mean it is less prone to blockage

In message , harryagain writes

Not really expecting any blockages. AIUI, it's to do with the water being sucked out of the traps - max run for 40mmm pipe is 3m and 50mm it is 4m. shower run will probably be more than 3M so might as well do the lot in 50mm

If you fit

That's an idea for under the bath. (I'd forgotten abut those, so thanks for reminding me).

Not really concerned about the shower - plenty of space for a 75mm water seal trap underneath. and the arrangement of the pipe runs, means I think it unlikely that I'd get back flow. Will probably install resealing/anti-syphone traps anyway, just in case.

It is what we have. And was what I was going to post if no-one else had.