Drainage design, help needed.

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KIt’s been a number of years since I have been on site, apart from a few days ago when I eventually remembered my password.  When I was on before we were in the process of doing a bungalow extension.  Well the extension still isn’t done, but we hope to change that shortly. In the meantime since we lost our two dogs, we have been doing some travelling, finding ourselves in Australia when Covid became an issue and making it back from there 2 days before Qantas temporarily stopped flying overseas, and 2 days after lockdown started here, after a number of flight cancellation and changes. A stressful time.

Planning permission for the bungalow, lapses in about 3 months, so I need to get a build over agreement to Thames water ASAP.  Below is the existing and proposed drainage designs.

Finances are tight so the aim is to do it as economical as possible. 

The now internal manhole will be removed. I have moved the connection to the sewer forward as this seems easier given the island sink.  Is this necessary or can it curl around backwards, with the rodding eye to the left of the building?


The new store room, previously the bathroom has holes through the wall for the sink and bath outlets and through the floor for the toilet. Can these be reused? The door to the store room can be changed.

I show the utility room will need multiple waste water outlets (I have shown 2 only  )  joining to a stack, the same stack for all the new bathroom outlets . Is this possible? I did come across a fitting which allowed 5 inputs at the same level for a stack ( sorry can’t find it again, so don’t know it’s name).
 The rodding for this stack would have to be from an opening in the stack.  Is this wise and is there an alternative.?   I am very concerned about this as in a previous house we had an outside stack totally blocked at first floor level. Rodding from the loo and from the drain did nothing, so the plumber had to loosen off the mid joint of the  stack, to remove the contents, it was not pleasant to say the least!

The other manhole to the rhs, will need to have one of inputs removed, unless we use it for a new upstairs bathroom.  All the pipes are 4”, new stacks  as far as I understand are 110mm, will this be a problem? Can we easily block off this input if not reused?

Sorry a bit long winded.  Any help very gratefully received as I find understanding what is good and permitted very  confusing, Jenny.


existing drainage.PDF proposed drainage.PDF

Edited by Jml
Adding pdfs

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There are rules in Part H on what can join a stack at the same level. Typically you can't have a 110mm pipe from a WC join a stack opposite a smaller pipe from a basin or similar.


I believe the thing you are referring to is a "Soil manifold" intended to prevent "cross flow" from one branch to another. Thereby allowing multiple branches to connect at same height.

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That’s it. The example I saw had wc with 4 other waste water attachments. I assume this gets around the part H requirement.

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Just to update.  Have spent long hours looking through lots of posts.  Have to say there is now a wealth of valuable information avaliable from all the posts.  Decided as advised by many, that having an internal stack with only rodding point at its base not a good idea. So will create a new IC Outside the new extension and from there to the mains sewer, which is to be built over.

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