When I had a dishwasher installed, the plumber refused to install the waste water pipe horizontally, saying that this would get it to clog all the time. He left a roughly 1:10 slope on it. I thought it made sense.

Now I find that the drain pipes that my sink and bath are connected to (and which clog up all the time) are lying on the floor completely horizontally, for a length of at least 1.5 meters. They are 4cm diameter PVC pipes.

Is this a bad installation, and do you think this is why it keeps clogging up? Are there legal requirements in the UK for the drain pipes to be sloped, so I could get my landlord to fix these?

  • Can't comment on the UK rules , but FYI fixture branches from the main drain the a fixture here are supposed to have 2.5% fall in most circumstances. And 1 Inch??? That's really small drain for a bath and sink should be at least 50- 65mm (2 1/4"???)
    – UNECS
    Commented May 6, 2012 at 21:24
  • 1
    @UNECS my bad... went in with a tape measure, the actual diameter is 40mm. This is the single pipe that drains the bath/shower, two sinks, a dishwasher and a washing machine. Still a bit thin?...
    – RomanSt
    Commented May 7, 2012 at 0:15
  • Yes I would say so even at one in ten but at only 1.5 long you could get away with it... I would recommend 50mm min if you had to and of course it will need a slope especially with a kitchen sink and W/M ( grease,food scraps and hair). Having to small a pipe with a large load on it will suck your water seals out of your traps also.
    – UNECS
    Commented May 7, 2012 at 0:19
  • 4cm is about right for a single fixture's drain. For stacks I would recommend a bit more, like 2.5-3", and the main sanitary line should be 3-4".
    – KeithS
    Commented May 7, 2012 at 19:22

1 Answer 1


I am not in the UK but my city uses the International Plumbing Code.

Everything needs to have the required slope. The required slope depends on the size of the pipe.

IPC table 704.1 shows that a pipe with an ID (interior diameter) of 2.5" or less needs 1/4" per foot. While a pipe that is 3" to 6" only needs 1/8" per foot.

  • Found 10cm deep blockage at a 90° bend - was pretty disgusting. It seems that 90° bends are not recommended by plumbers? The blockage would explain why... Also, there's no air vent anywhere at all - I guess that would explain the constant glugging. Overall, must be a really terrible installation here!
    – RomanSt
    Commented May 7, 2012 at 10:28
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    True, standard 90s are often avoided in drain lines for this very reason. Instead you can either use a "long sweep" 90 or two 45s, this arrangement will flow better than a standard 90 (and not clog as frequently).
    – auujay
    Commented May 7, 2012 at 14:24
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    As for the vent, there MUST be a vent stack within 5 feet of a 2.5" line. The vent does two things; first it provides a ready escape for sewer gases, and second it provides for the free flow of air "behind" the water which speeds draining. Without a vent, or with a vent improperly placed, some water will enter the pipe, creating a vacuum which must be broken by air bubbling up into the stack from downstream, similar to upending a bottle of water (glug-glug-glug).
    – KeithS
    Commented May 7, 2012 at 19:29

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