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New Bathroom Soil Pipe Design Critique - Backdrop shaft & Internal Stack

Discussion in 'Plumbing and Central Heating' started by Undaunt, 19 Oct 2021.

  1. Undaunt

    Undaunt

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    We are about to install a bathroom in a new location at the front of a Victorian terrace (to regain rear bedroom) & as such we will be running a new soil pipe.

    The plan is to chase it in to the wall slightly in the hallway downstairs, 'hiding' it within the lobby door lining design, using acoustic soil pipe to reduce chance of noise problems. Next door neighbour is on board with it, building control say there is no reason not to (although unusual), and structural engineer is happy. I've considered and mocked up many alternative routings, but getting it across the hallway would mean too many un-roddable bends.

    The main issue is getting it past the first floor joist which I would rather not cut and trimmer joist for strength reasons, hence I have designed in two 45* bends as you will see from the video animation of my sketchup model :



    The second issue will be the fact that the outside inspection chamber will be at too low a level & therefore would have a gradient of much greater than 1in40 - 1in80. I have therefore mocked up a backdrop shaft. There is a risk this will end up too close to the to-be-constructed new wall of the extension (with mini-pile foundation) - the only way to reduce this risk would be to have the backdrop shaft running parallel to the back wall of the house, but this will introduce more 'bends' - although admittedly at easily roddable areas (but I'd prefer to have a design least prone to clogging up in the first place!)

    Potentially 5 inlets are needed in the chamber before the backdrop shaft - the main bathroom (plus loft bathroom), the understairs toilet/washing machine, the kitchen sink/dishwasher and one or two rainwater inlets from guttering (it's a combined sewage/rainwater victorian sewer)

    Anyway, how does this look? Any thoughts/suggestions?

    One big question is which fixed point to we work from when installing this... from the bathroom, dropping down, getting the rest bend situated then digging the correct gradient in the subfloor towards the inspection chamber then setting the height of the new access chamber/inspection chamber then fitting the backdrop shaft between the two chambers? Or the other direction? Any thoughts from those with more experience would be massively appreciated since I feel like its one of those things you could paint yourself into a corner with and no matter which way I think it through, I can't come up with a definitively obvious answer!

    Here's a model of the house (current and proposed rough future design) for reference :


    Edit : A Mascar bowl has previously been suggested but I get a sense they aren't used much or liked. Is this purely cost, or are they bad for blockages?

    P.S. I also hope to bring the bath trap above the floorboard level to avoid cutting the original floorboards. Please ignore the pipes that don't quite meet - it is difficult to model in sketchup & is just to give a rough visualisation!
     
  2. Exedon

    Exedon

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    Couple of questions
    Do you intend to have air admittance on new stack or vent through roof?
    The deeper of two chambers does it already exist or do you need to create it?
     
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  4. Undaunt

    Undaunt

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    1. Air admittance stack preferably, if necessary - we could do vent through roof, but due to trying to get as close to a retrofit passivhaus standard as possible, the AAV would be superior from a thermal perspective.
    2. The deeper chamber was installed a few years back as a 'temporary' setup as you can see in this thread https://www.diynot.com/diy/threads/how-to-connect-to-6-old-clay-collar-solved.493923/page-4 (we currently have an overground soil pipe sort of bodged in whilst the place is still a building site - it never has any problems remarkably!) The hole is still open, so whilst we would have to un-stick it from the cement underneath (dry mix so hopefully not too difficult) it could be adjusted somewhat/rotated/replaced with mascar if needed.
     
    Last edited: 19 Oct 2021
  5. Hugh Jaleak

    Hugh Jaleak

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    Not had a lot of experience with the Marscar bowl system, but always thought it an interesting concept, seems the general consensus is they are a PITA to clear should they block. Lot of people seem to be complaining, but none saying why! Given the number of pipes you need to common into the one outlet, and the setup you have, the Marscar Bowl looks like a sensible option, and would avod the need for 2 chambers. Keep the outlet pipework on as shallow an angle as possible, and that should negate any issues with rodding if the worst happens.

    As for digging the trench, usual practice is to start at the bottom and work upstream, just make sure the pipework under the floor is suitably protected from any possible future damage.

    Vent the stack, drains need to breathe, and Regs require the Highest point of the drainage system is vented to atmosphere. AAV's can cause issues if there is no ventilation anywhere else on the system.
     
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