Drain design puzzle

[dware]

I'm hoping someone can help me, I'm coming to the end of an extension project and I'm having trouble figuring out how to squeeze the waste pipes of a new en-suite into the drain. I hope my explanation isn't too long...

The drain has been extended from the existing drain by the builder who completed the basic building works and I've been doing everything since then. And I'm now finally at the point of adding a ground floor (this is a bungalow) en-suite into one of the existing rooms.

After studying Part H of building regs (this is all under building control) I can see that following all the rules and gaining the right drop on the shower is going to be tricky as the drain is now pretty close to the surface after being routed round/under the extension. And if I have to join the shower into the base of a new stack 450mm above the invert I'll have to raise the shower on a 250-300mm platform. Which isn't too nice (I'd have to add a step too).

So, I had a bit of a brain wave last night, I suddenly thought if I branched the drain to both feed the stack (for the toilet + kitchen waste) and a new bottle gully, do I suddenly lose the 450mm rule for the shower (the toilet is still just high enough)? This picture might help! :

http://media.diynot.com/119000_118176_28620_26512846_thumb.jpg

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• dware
• 30 Nov 2010
• 2

(the drain on the right, just up to where the stack would be, is already laid, the rest will be new. There will also be a hand basin, not marked, which could either join the toilet waste or go to the gully, either would be fine)

Am I missing something? That solution seems too easy, although I think I worry too much!

This would be great as I'd only need to raise the shower slightly, and it would also solve another problem I spotted which was the raised shower and toilet having to join the stack at about the same height, which got tricky with the joints.

My final question is, does this have to be a vented stack or can it be a stub? Everything is ground level and within limits, and it does join into a drain with an existing vented stack. However, that's around 10m away, does that cause problems? I'd rather not have to vent it as I have a velux above this (within 3m) so I think I'd need to have the stack 900mm above that!

Thanks for persevering until the end!
David.

 

[John506]

I'm not too sure on some of your questions, but all I do know is the building control officer (if he visits at this stage?) isn't going to shoot you down for not having the correct fall on your waste pipework

 

[Hugh Jaleak]

Could the proposed stack be moved along to the new W.C position? If there's an existing vented stack then I think you'd be ok with a stub stack and AAV, box this into a corner, then tee in shower waste using a manifold at or just below floor level. Move the bottle gulley (to your stack position in the diagram) for the kitchen sink waste.

Teeing a stack into a run from above can lead to problems if solids collect upstream of the Tee, personally i'd put the W.C at the head of the run.

 

[dware]

Thanks John, Hugh.

John, the falls should be OK, it's that 450mm minimum on a stack that is the problem for me. But thanks for the reassurance, building control scares me, but it does mean I'm learning new things!

Hugh, this is all external, and I'm hoping to avoid an internal stack and AAV (there's not much space in the en-suite). And I can't use an AAV outside I believe.

I could easily swap the stack and the gulley around, putting the gully downstream of the stack. But as I understand it, my shower has to join any stack 450mm above the invert, and this would mean I'd have to raise the shower by a fair amount to do that. However, I can't see any rules for how high anything going into a gully has to be above the invert, in which case I would be able to lower the shower by enough to reduce the step.

I'm guessing the 450mm minimum on a stack is for backwash maybe? And that wouldn't be a problem on a gulley?

Does that make sense?

David.

 

[Hugh Jaleak]

External AAv's are available, but there is a price premium....

The 450mm rule applies to stacks on multiple storey buildings, as an upper floor W.C. is flushed, the falling water and waste can fill the stack, causing a pressure build up as it hits the bend at the foot of the stack. By avoiding connections within 450mm of the base of the stack, this prevents any traps being 'blown' out by the pressure. The problem wont occur with a gulley as this is directly connected to the drain, not the stack.

As you are on the ground floor this rule shouldnt apply. The B.C.O. may allow an external stub stack with either an AAV or rodding access for the W.C, then simply boss the shower waste connection into this, however I would check before finalising anything.

 

[dumbrill]

I think the answer to this lies in paragraph 1.30:

"1.30 Stub stacks – A stub stack may be used
if it connects into a ventilated discharge stack or
into a ventilated drain not subject to surcharging
and no connected water closet has a floor level
more than 1.3m and no other branch into the
stub stack has a centreline more than 2m to the
centre line above the invert of the connection or
drain (see Diagram 5)."

I struggled with then 450mm until I realised that it didn't apply to stub stacks.

I have a similar situation and I'm not clear whether the stub stack needs an AAV. Also can the stub stack be capped off at a level lower than the highest water level in the event of a blockage, ie. top of the basin. Also does using HepVos negate the need for an AAV?

 

[dware]

Excellent Hugh, so it sounds like I have more options than I thought (that's the problem with not knowing where the written rules can be bent/broken).

I'm actually quite taken on the gulley approach if it's possible as it would mean if anything did go wrong with the drain, any blockage wouldn't result in the unmentionable coming up out of the shower. Is that a valid concern?

Because of the shallowness of the drain it'll be easier to keep the stack down a couple of metres along from where the toilet exits the wall, I'm now thinking of the following:

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• dware
• 30 Nov 2010
• 1

Am I right in thinking I could use a 'back inlet bottle gully' in-line after the stack (plan view A), or should a normal bottle gulley be tee'd into the drain (plan view B)? I would have though the back inlet gully could cause an obstruction to the waste from the toilet.

As you say, I'll follow this up with the BCO, but it really helps me to have some idea in my head to start with - so thanks again for your help.

Thanks,
David

 

[Hugh Jaleak]

An AAV must be fitted above the spillover level of the highest appliance, so above the basin. Not sure about the HepVo's whilst they will open under the pressure of the discharge, I dont think a negative pressure in the drain would be sufficient to allow them to open to admit air to the drain.