Adding soil stub stack to existing branch external to internal detail

[Dannss]

Morning all, as always can never find an exact existing thread / discussion where I don't feel like I'm making one too many assumptions.

Can I get a quick sense check on the below, always wanting to work 1 step ahead, eventually planning to add an en-suite to my existing loft conversion. Got the perfect opportunity to set myself up for it now whilst re-jigging the bathroom below and adding shower niches (ie will have void)

Assume all soil 110mm and falls and gradients correct.

Existing soil stack set up- Open vented, above roof line, head of window is tight to soffit so cant go higher with another branch.

What I've got in my head is the following, all yellow is internal, AAV in a nicely vented space in the eaves with plenty of space. shifting WC to right is putting it back where it originally was when house built.

I've attempted a 3D version below for clarity;

If anyone has any thoughts on the following it would be much appreciated, couldn't find anything exact in doc H.

1. Adding a stub stack with AAV onto the branch... any issues with this? Keep distance of 200mm between AAV and loft conversion wc inlet? (no issues achieving this) The vertical section of yellow stack will be around 2.5/3m.
2. (different issues but...) I Believe the wastes into the horizonal branch isn't the best of ideas, but at least they are all separate. (currently have issue with siphoning on the inherited waste design)
3. The internal yellow additional 90/92.5 bend... use a big old swept/slow bend on this part?
4. Any ideal / necessary rodding points to add in?
5. Yerrrr there's a lot of bends there, trying to think of a nicer way around this without raising the external branch as this would make the waste connections tricky. It might be possible by rotating the bend that penetrates the external wall.

In short.. anything fundamentally wrong with taking a AAV stub stack off a branch to open vented soil stack? Can only see reference to bends to be avoided in wet section and kept as large radius as possible.

Things in my mind are -

1. 3 bends??! That'll clog before you know it
2. Waste from en-suite is going to pick up some serious velocity before hitting the branch - new game of count the splats
3. Some mystery issue with AAV + open vented at similar elevations

All input appreciated

 

[Deleted member 120897]

Afaik, adding to an existing vented soil pipe doesn’t need any extra venting.

 

[Chuck]

Have you considered keeping the new sub-stack outside? if you want to vent it to mitigate the siphoning issue you could have an external AAV so you can terminate near the window. Alternatively, a dog-leg would get you up past the soffit. An external sub-stack would remove the nasty triple-90, the internal AAV and the space wasted inside. You could also have rodding access to the horizontal section if you want it.

Drains are not really my thing so my thoughts are 'ideas to consider' rather than firm recommendations.

 

[Dannss]

Afaik, adding to an existing vented soil pipe doesn’t need any extra venting.

Should have added in, naturally there will be a shower waste and a basin waste on the attic en suite

Have you considered keeping the new sub-stack outside? if you want to vent it to mitigate the siphoning issue you could have an external AAV so you can terminate near the window. Alternatively, a dog-leg would get you up past the soffit. An external sub-stack would remove the nasty triple-90, the internal AAV and the space wasted inside. You could also have rodding access to the horizontal section if you want it.

Drains are not really my thing so my thoughts are 'ideas to consider' rather than firm recommendations.

Input appreciated, unfortunately, to the best of what I can picture, without lowering the ceiling in the current bathroom/boxing in, I wont be able to take the stub stack outside and then back in at a higher point. on the wall. unless I could get thru the soffit with a 45 degree up into the eaves of the loft conversion. Quite keen if possible to avoid any additional external pipework where possible or having the soil exit the tiled section of roof.

The bathroom ceiling internally is some 300mm higher than the header of the window.

No issue with the wasted space in the attic, its in the eaves anyway and there is plenty of space free.

Looking at seeing if I might be able to ditch one of the bends and bring the branch higher up (ie turn straight into room), whilst still trying to keep 200mm separation on the new wc connection from the right of the stack, and also whilst getting the wastes to enter the top of the branch.. but it looks pretty tight / a non starter.