Curious why our new self build house has a stack going up through roof (all 3 are internal stacks) and two other stacks have dergo's on them and terminate in bathrooms/ensuite etc?? Why not  dergo's on all 3.
I Have to do a drain test soon- does this mean climbing onto roof with a drain blank and sealing it prior to a testing for leaks?? Can I not simply cut it and put a dergo onto it or does one stack always have to vent to outside air???

Think it's the Building regs. I think the furthest stack from the main sewer (eg the head end) has to be an open ventilated stack. Stacks between that one and the mains sewer can have AAV fitted.

Bag the drain, flush WC until water backs up to WC rim.  Allow to settle for a few mins, should then hold level if drain is sound.  Remove bag and quickly get out of the way of the rush of water.

Needs to be done on lowest stack connection, ie cloakroom WC if that is the lowest or bathroom / en-suite if that is the lowest etc.