External Air Admittance and Waste Pipe Design

[primetime]

Hi All,

I have an odd situation where my loft waste pipe for the toilet runs close to the roof as it exits the wall. I need to do this so the toilet isn't too high in the loft. Because of this I can't get a branch over the eave in order to run the pipe above the roof line. I have sketched up a run using 90's but then breaking into the horizontal section and adding an air admittance valve.

Can anyone advise if the proposed sketch below is suitable?

Not to scale, all minimum falls assumed, admittance valve noted but not sketched in.

Thank you.

 

[denso13]

Can you not run the pipe up further from the branch and have an open vent instead of the AAV?

 

[dilalio]

Also best not to drop that basin waste into the branch like that!
You should really boss it into a vertical stack.

 

[Steelmasons]

Also best not to drop that basin waste into the branch like that!
You should really boss it into a vertical stack.

Oh dear.....guess I've been doing it wrong all these years........could you educate me?

 

[dilalio]

I obviously just did

 

[Steelmasons]

I obviously just did

Would you mind explaining the reason why???..
..possibly self/induced syphonage????

 

[dilalio]

I obviously just did

Would you mind explaining the reason why???..
..possibly self/induced syphonage????

In this setup, basin trap could be pulled and also, depending on reducer/boss used, the basin waste can drop down into the 4" branch and create a nice toilet roll and sh#tcatcher! Just what happened at my sisters house last Christmas when we all went round for din-dins... Guess what my job was that day

 

[Steelmasons]

In this setup, basin trap could be pulled and also, depending on reducer/boss used, the basin waste can drop down into the 4" branch and create a nice toilet roll and sh#tcatcher!

Not too sure how basin trap could pull unless of course the flushing of WC caused a solid plug within the 4" "float" ..
....are the boss pipes you use void of pipe insertion stops???...jeez I'll have to remember to mark the pipe for insert depth next time I use a boss pipe.........come on now...
...I've always been a bit unsure of "plumbers" who live in potters bar..

 

[dilalio]

...I was taught that way by the wise old chap I started plumbing with... I'm not saying I "haven't" done it as above, or that you 'shouldn't'... my opinion is that it's 'best' to go into vertical.

Yes, I use stops... when I get fittings from my local but that's not always possible and have to get what's available from nearest place to job sometimes... also, attending existing installs can reveal also sorts of fudgery

I live in Potters Bar, but not from it. Care to explain why this is an issue for you?

Where are throwing your stones from then?

 

[primetime]

Thanks for all the replies guys. Am please no-one has taken excepting to putting a branch in the horizontal. I could extend the vertical above the roof line but will have to negotiate the guttering with more bends. I quite like the clean look with the air admittance valve. I could extend the new vertical stack and run the basin waste horizontally.

Picture below of the dormer in question.

Sorry to hear about your geographical differences!

 

[dilalio]

That picture would have saved you a lot of work in Sketchup

I would put a vent/stack up directly above the existing stack at 1st floor and bend it backwards towards wall of dormer and upwards to roofline and terminate as normal with vent cap.
Branch off to wc outlet along the wall with clips. Keep it off the 1st floor roof as it looks neater and stops leaves etc being trapped behind. Then run basin waste parallel to soil branch and boss it into vertical vent pipe above soil branch - use a boss connector rather than a strap on.

 

[dilalio]

OP: As Gwiz mentioned, the basin into the horizontal soil pipe 'could' "pull" itself when being emptied.
Not a dead cert but possible.

 

[primetime]

Thanks for that. Unfortunately what I didn’t make abundantly clear in the cartoon is that the horizontal WC pipe needs to run tight the tiles because of the floor height inside the loft. If I raise the pan I will need a step or plinth to get onto it.

With the horizontal WC pipe along the tiles I won’t get a branch under it as you suggest, that’s why I went with all 90° bends and then a branch along the way. Although not a usual setup is it acceptable?

All input appreciated thus far.

 

[dilalio]

Can you post a photo taken from above? From the balcony along to the wc outlet would be useful!

 

[denso13]

It's ok but I'd still extend the branch pipework to an open vent above the gutter. The AAV should be above the spill level of the basin so you'll be a good bit up the wall anyway.