Do I need a double vented soil pipe run.

19 Nov 2008
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United Kingdom
I'm installing an ensuite bathroom in our loft room and in the process of planning the soil pipe run.

I plan to run the soil pipe out of the house, then drop vertically for about 2m then run across the house (under a window) for about 3m to the existing soil pipe. The existing soil pipe is a bog standard vertical 110mm pipe with a single branch connecting to the existing toilet. I plan to swap the single branch for a double branch.

My question is, will I have to vent the first vertical drop. i.e. will I have to use a 110mm T-piece (single branch) on exit from the building, at the top of the 2m drop and then extend upwards.

The soil pipe run inside the building will be about 2.5m and will have connected to it shower and basin waste. I'm concerned that the suction generated by the waste going down the first vertical drop will empty all the traps in the bathroom.

In the picture below would a) be OK or would I have to do b)

Thanks for any comments,

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I would do b) as it would not involve much extra work either and ensure all works well.
Definitely b; but be prepared to fit an AAV on the waste runs as you may still draw the traps when to loo is flushed. At the moment, air can only enter through the vertical stack & you really need to be pulling air in from the othside of the w/c as the loo is flushed. If you’re subject to BC inspection (which it should be!), they may insist on it. Also watch the maximum permissible unvented branch length(s) of the waste runs, the BCI will pick you up on that as well. Separate waste connections into the vented stack are always best.

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