Waste pipe sizing and issues

Joined
8 Feb 2026
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United Kingdom
Hello,

I’m soon to connect my new shower tray waste pipe, the existing waste is 40mm and as per the attached picture takes bath, sink and shower. When letting the bath out it used to fill the shower a bit, I don’t want this happening with the new one. Do all the pipes need upping to 50mm? Could I just go to 50mm with the pipes in the picture (this is the only exposed area of pipes) leaving the 4m to the soil stack as 40mm or would that not improve things? Everything will be solvent weld.

Thanks in advance for any help.
 

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