Soil Stack

[grantown]

Hi I am new here, so please be gentle.
I have to replace a cast iron soil stack on the outside of my property, I am thinking of moving it inside (it would look better) however it then dawned on me why do they use such a large pipe (5”) above the toilet connection/fitting. I was wondering if the dry pipe work (the leg up onto the roof) couldn’t be installed in a smaller size (i.e. 2”) as this would be much easier to fit and disguise in the house.
I can’t see any reason why it needs to be so large just to break the vacuum I would of thought a 2” one would do the same.
Does anyone know any reason why I couldn’t do this?
Best wishes to all

 

[seco services]

you can reduce it but only to 3"

 

[muggles]

As seco says, minimum permitted dry pipework diameter is 3", however if you can actually find anywhere to buy this you'll almost certainly find it's the same price or more expensive than 4" as it's hardly ever used (certainly last time I needed some it worked out more expensive).

Incidentally your present stack will probably be 4" (you measure the internal diameter for imperial measurements, and the external diameter for metric).

Hope this helps

 

[grantown]

Thanks for your help Serco & Muggles, one final thing, can you confirm the soil stack sizes are 4” bore pipe and the vent could be reduced to 3” bore or is it measured outside diameter??
Sorry for the stupid questions, I was wanting to cut down the box sizes as much as possible (i.e. have the smaller pipe sizes as possible).
Thanks again for your help

 

[muggles]

Yes that is correct, 4" bore for stacks, may be reduced to 3" bore for venting purposes. 4" is actually 110mm on new soil pipe (4" internal bore, 110mm external diameter) but if you go to the merchants and ask for 3" and 4" they'll know what you're on about. You'll probably need to order the 3" in specially as few places carry it ex stock, and as already mentioned be prepared to pay through the nose for it.