soil-pipe gradient 110mm vs 82mm

23 May 2010
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Tyne and Wear
United Kingdom

Connecting a new and repositioned loo to an existing stack which is inside the house and I really need to put the soil-pipe branch beneath the floorboards. Bog standard (no pun intended!!) family bathroom. Planning on a 300mm ish drop from the loo pan into the pipe, through a 90 then along to the top of the stack, joined with another 90. The horizontal distance is 1800mm and the void beneath the boards is 180mm deep.

110mm pipe would give a gradient of approx 1:30, 82mm pipe a gradient of approx 1:25.

Appreciate any advice upon, and answers to, to following:

1. Are my gardient calcs about right? (I've allowed a bit of a faff for the connections - but I may have missed something)

2. Would these gradients be acceptable?

3. I feel a bit more comfy with the increased gradient offered by the 82mm option; would that be the best option or is 82mm not good for a soil branch of just under 2m?

Many thanks indeed for any and all pointers,
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Thanks for that :) Read through the regs and now content that 82 or 110 would meet them (the loo has an 80mm circular outlet and is preset to 6ltr flush) - what I'm hoping for now are the views of professionals as to the pros/cons of 82 vs 110 in this situation - it may be that I'm worrying needlessly and that 110mm at a gradient of 1:30 will be fine and offer distinct advantages over the 82 given the meagre flush capacity.

(Additionally I intend to add a Durgo about 2 ft upstream of the loo if that info makes any difference)

Much appreciate any thoughts
Personally I wouldn’t risk 82mm. How are you going to access the drain section between the two 90 degree bends under the floor in the event of a blockage? (see clause 1.34)

Is there another open stack on the foul drain to allow positive ventilation? You can only fit a Durgo valve if there is (see clause 1.33).
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Hi Richard,

Yes there is another open vented stack, at the back of the house (where there's another loo, which is at the upstream end of the house's soil piping into the sewer). There are 4 houses connected to this particular sewer and all the others have open vent stacks too.

As far as a blockage is concerned, the front panel of the toilet "unit" piece of furniture will be removable, the AAV will sit atop a small stub inside the unit, I was planning on a FloPlast Push-fit AAV. For a blockage, I was planning to be able to remove the AAV and rod down the small stub, round a swept 90 and along the 1.8-2.0 m of pipe run to the top of the stack.

I'm getting the vibe that as I can achieve a gradient of 33mm per m with the 110mm pipe, I should use that and sleep peacefully knowing that the gradient is sufficient. It probably seems screamingly obvious but I suppose I'm after the reassurance :)


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