Soil stack vent for 2 connections to foul drains

My house has 2 connections into the main foul drainage within 1m of each other and the idea is that 5 toilets will connect to both of these following an extension. I fell out with the builder that did it like this so I'm not sure what the exact plan was. I know that usually you would have one soil stack that sticks out of the roof or you have an air admittance valve but when you have 2 connections do you need 2 soil stacks or do you connect these 2 stacks together within the property and have 1 soil stack? One connection will be used for 2 ground floor toilets and the other will have 2 first floor and one loft toilet going into it.

 

Ground floor WC's can be directly connected to the drain, but given you have 2, I'd use a stub stack, and cap that with an AAV. Upper floors into a Vented stack. No need to join the 2 stacks.

 

Thanks for your suggestion Hugh. In that scenario, would the pipe that has toilets on it on the upper levels have a soil stack that goes through the roof or a high AAV and the other pipe with toilets on the ground floor have it's own AAV which would be a bit above the ground floor toilet height?

 

Yes, one stack ideally needs to be vented to allow any positive pressure in the system to escape, if you cap them both with an AAV then you may find you have issues going forward and will end up fitting a vent anyway.

Upstairs WC's put into vented stack, ground floor into stub stack, AAV needs to be above spillover level of the highest appliance, so if you're having a basin in the WC, bear that in mind when doing your calcs.