Rear Extension - Drainage Layout

20 Mar 2015
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United Kingdom
I am in the process of planning an extension i am trying to resolve the issue of the new drainage requirements. I live in a terraced house where the sewer runs parallel along the rear of the house taking foul connection from the WC Soil Vent Pipe and a foul water (kitchen/wc) from a gully.

As part of the build over agreement i will have to remove the inspection chamber and ensure the foundations bridge the sewer pipe adequately. We wont be able to reroute the sewer pipe so i am thinking that we will need to do the following:
1. Replace the inspection chamber with a piped through 45 degree joint from the SVP
2. Provide a rodding point and foul water connection on the opposite 45 degree angle
3. Internally we will have a substack in the new wc that will need to take multiple feeds from wc and sinks

We have a suspended timber floor so falls should not be an issue.

My key areas of concern are the position of the internal sub stack & AAV. Is it best to take the sub stack and run it to the SVP pipe before it meets the sewer connection or would you suggest another connection to the main sewer. Effectively i am trying to get my head around how to reduce the amounts of bends and rodding points required?

I have attached a couple of images to explain. Any help would be really appreciated.


  • Drainage 1.pdf
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  • Roof Detail 1.pdf
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A connection from a high use (and high risk) SVP would normally need a manhole connection and not a branch.

Get this sorted out before having the rest of the design work done if the extension viability depends on it.

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