Route of soil pipe for new downstairs loo

27 Nov 2013
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United Kingdom
Hi everyone, I wonder if any of you helpful lot would be able to help me with my downstairs bathroom project:

I'm planning on installing a loo, as well as a sink and shower. The floor is suspended timber and I can get underneath into a crawl space below which extends throughout the whole house. I am planning on routing the soil pipe through this space on brackets attached to the floor joists (with spacers to ensure the gradient is OK). The pipe will run for an approximately 7 metres under the house before going through the cavity wall below outside ground level and eventually joining a manhole about 2 metres further on. The shower and sink wastes will have to join the pipe near the start of the run.

My question is about the initial route of the soil pipe - should I:
1. Route it horizontally out of the wall, form a vented soil stack outside and then route it back into the floor void under ground level, or
2. Route vertically downwards into an elbow immediately under the floor, and straight into my 7 metre run, or
3. Route as in option 2, but add an air admittance valve inside the bathroom.

Obviously it would be simpler to go for option 2, but would that cause problems with negative pressures after flushing? I don't want my shower trap to gurgle or be sucked dry every time the loo gets flushed.

I know about getting building control sign off and will do so.

Many thanks for any thoughts on this!

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Option 3, don't put any more bends in it than you have to, and this arrangement will work perfectly well.

Provided you already have a vented stack elsewhere on the property, then an AAV would suffice internally. Technically you can connect directly to the drain if the distance between the invert of the drain and the crown of the WC pan trap is <1.3m, (which I am pretty sure it would be in your case), but an AAV is belt and braces and will avoid any possible pulling of the shower and/or basin wastes.

(Sinks are for kitchens, we have basins in bathrooms. ;) )

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