Soil Pipe / Macerator in understairs toilet

Discussion in 'Plumbers' Talk' started by dumbdodo, Mar 20, 2017.

  1. dumbdodo

    dumbdodo New Member


    I plan on installing a small bathroom in the space under the stairs and was hoping some advice.

    Is it possible to use a traditional system soil pipe in the space available below floor boards or would i need a macerator (seen in image below for layout)

    Understairs v2.jpg

    For info:
    depth between joist and ground (below floorboards and outside) = 400mm
    total length of pipe using straights = 5.4m (2.7m toilet to dining room + 2.7m dining room to soil stack)
    number of 90 degree turns = 2

    If a soil pipe is acceptable what is the minimum distance above the ground the soil pipe can enter stack

    Your help will be much appreciated as i have been unable to figure it out.


  2. b4xtr

    b4xtr Active Member

    You have enough depth to give the fall required for soil pipe.
    I did more or less what you are suggesting in a previous house.
    To connect into the soil stack, i dug down a bit outside and put a branch in just below ground level, but did replace the old cast soil stack with plastic at the same time.
    Supported the new pipe run under the floor floor by hanging it from the joists.
    I was younger and fitter then but it was reasonably easy, provided you don't mind crawling about in small spaces.
    Can't see why you won't be able to do it but i expect a few of the more knowledgeable members will be along to give a few more pointers
    It was well worth the effort though, good luck with it
    KIAB likes this.
  3. sospan

    sospan Well-Known Member

    Getting through the exterior wall could be interesting but on the face of it, it should be doable. Fortunately all the lengths are shorter than 3m so you shouldn't have joints under the floor space.

    In the event of a blockage, it also shouldn't be too difficult to rod either.

    The only unsightly bit is going to be the horizontal run in the yard.

    Main advice would be to try a pressure test before you seal everything up, the last thing you need is a leak at the 90 under the dining room floor
  4. CGN

    CGN Well-Known Member

    You need very little fall for a soil pipe so you have more than enough by the looks of things. Just finishing off an under stairs toilet at mo, but concrete floor so had to kango a trench across the lounge.
    KIAB likes this.
  5. KIAB

    KIAB Well-Known Member

    Why can't you go diagonal from toilet through hall & kitchen to outside stack, would shorten the run, & maybe only one bend at toilet end.
  6. sospan

    sospan Well-Known Member

    Was thinking similar,

    Two walls to go through and getting the line of the pipe correct against the benefits of removing a 90 and replacing it with a 45 as it gets towards the current pipe

    I guess a lot depends on the direction of the floor joists and accessibility in the kitchen

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