mini access chamber minimum invert depth question

4 Apr 2013
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United Kingdom

while waiting for my building regs application for a number of jobs.

we need to install a new soil stack to serve new upstairs toilet and groundfloor toilet.
looking at installing a mini access chamber into existing clay pipe work as per photo attached.

issue for us is that the original clay pipe drain invert is only 420mm to ground level and once mini access chamber is installed the invert for the new soil stack pipe work at the chamber end would only be approx 310mm therefore at the rest bend end it would be close to surface and probably some of the rest bend would be above ground.

not sure if this would comply with building regs - as our application has only just gone in but wanted to gather some advice / suggestions as to if this would be ok or if we need to re think the position of any new access chamber further down the line.

obviously the current position is most suitable for accessing the existing drain.

flow of drain is from left to right in photo.
the left hand side of drain also supports a single gully outlet as single sewer/drain serves both foul water and rain water.

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In addition to your already shallow fall problems, a lot of mini access chamber designs are going against you.

The through channel is the lowest point of the chamber and the inlets are often higher, thus exacerbating your problem.

A solution maybe is to connect a 'y' junction onto the main line, in place of a chamber and have a rodding eye between the 'y' junction and the stack.

In addition to this, you may have to install a knuckle bend instead of a swept rest bend because of the lack of falls. You could counter this problem by fitting a soil pipe access/rodding fitting at the bottom of the stack.
thanks for your reply and suggestion.

I'm guessing as long as there is some rodding access then I'm ok however we do it.

as per your suggestion I guess I could have a rodding eye both behind the y section for access to the through flow and then one between y section and stack.
I understand that its best practice to have rodding access but wasn't sure what the building regs requirments were that you have to have an access chamber or just rodding access.

ah yes, the measurements I gave take into account that mini chamber inlet height being higher than the through channel so there is just enough fall for the new pvc pipe leading to the new stack.

its really a question of - am I ok to have a portion of the rest bend above ground and that the new pvc pipe is only 150mm below the ground level.
(just that most underground pipe doesn't do well in sunlight above ground)

it is a very old house and there are no other access points in the drain and its some 26m long before it gets to the pavement lol. although i know it certainly has a decent fall as it drops some metres further down.
I agree with Nose - you can get a Y junction with a screwed access for underground - I got one for a song because they are " handed" and Drain Center had ordered the wrong hand for someone - was right for me :mrgreen: - Then I put a Clark Drain small manhole cover over it . that long radius rest bend is for a Single Stack system to stop air compressing @ the bottom and affecting trap seals - you`ve got 2 toilets on the 1 stack - see if you can get a long radius bend for above ground - the rest part is not relevant to your install . Or work out something to stop the UV getting to the underground pipe . It certainly fades it , but does it damage it :confused: ask a manufacturer ?
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hi guys

yeah the Y junction definitely seems the way to go, thanks. will look at getting a 45deg equal junction in the main through drain.
Plus this means I would easily get a bend at the stack end which would be underground and not sticking up assuming I'm reading it right what you say about having two toilets on stack only needs a radius bend and not a long radius - or do I have that wrong

as for the rodding eye:

If I put the rodding eye on the branch between Y junction and stack will that give rodding access to the main through drain as well or just the new pipe from the stack to the Y junction.

and in that case would i need to put in an additional Y junction just behind the first one on the main through drain to give future rodding access to the main drain(left to right). I know it seems obvious, but I guess the only thing with this is a small extra cost of a junction and a rodding point but if its not needed then i would leave it with just the one.

thanks again
Maybe worth looking at doing the underground section in clay drainage. Hepworth do a 'Single Oblique Access junction', these have a level invert on the branch, so will allow a bit of extra cover on the drain. Raising piece to suit required depth and cover to match.$FILE/Clay_Drainage.pdf (Page 17)

Secondly the clay long radius bends are, (if memory serves me correctly) not so tall as the plastic equivalent. Avoids sunlight issue (UV in sunlight does fade the plastic, and makes it become very brittle), and due to shallow depth of drain, clay drain may be less prone to getting damaged. Supersleve to plastic collar on the L/R bend and take stack from there in plastic.
Providing that the pipe is covered by some form of hard permanent surface such as slabs or concrete, BC do not seem bothered about minimum invert depths.

I've just had a long run just below the surface (10cm below soil level) approved with no trouble, I just mentioned to the BC inspector that we'd be installing a patio over the top.

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