Calculations behind 2 storey limit on reducing size of stack ventilation pipes?

[Shevek]

Does anyone know the calculations behind the limit on reducing a stack ventilation pipe to 75 mm and to one and two storey buildings?

We have an en suite to our 2nd floor loft conversion and I want to reduce the soil vent pipe down to 75 mm and run it to and out through the centre of the flat roof.

Problem is I can't do this according to Approved Doc Part H1 1.32:

Sizes of stack ventilation pipes – stack ventilation pipes (the dry part above the highest branch) may be reduced in size in one and two storey houses, but should be not less than 75mm.

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They need a cut off point somewhere, but is there any BS or something I use to argue that a 75 mm reduction at 3rd floor will be fine in our situation?

 

[Mike Jackson]

You can argue until you're blue in the face but you need to comply with the building regs.

 

[AWheating]

i sure you would be fine with 82mm soil pipework tbh, the other question is would anybody notice anyway? ask your building inspector if he has a problem with it?

 

[chris watkins]

BS en 12056-2 sanitary pipework, layout & calculations.
Not very helpful I am afraid min size for a vent on a system II (primary ventilated soil sack is DN 80) what ever the height & can not find anything to counter the >2 stories bit but not been through the whole lot.
What about using an air admittance valve instead of the vent (I don't like em myself but) or could you not vent the main part of the stack outside before it goes up into the loft? Was it /is it internal or external at the moment?

 

[Shevek]

It's not the regs I'm particularly worried about. We're a self-build so I just want to get it right and not cause any syphoning. Don't want to use an AAV either.

Thanks Chris for the DN 80 minimum size. That'll do us.