Help please, Air admittance valve, placement, soil stack moving

[Mr Chibs]

Wondering if you drainage experts could advise me please.

I changed a flat roof to pitched, and along with a reworked bathroom, the soil stack needs to move.

Where the soil pipe comes through the bathroom wall, my idea was to use an air admittance valve and box the whole soil stack into the room below. Reading up can't get a definative as to wether this is OK to do.

My next brain wave was to add a breather pipe (in green) outside and just take the was straight down, (blue pipe) from the back of the loo into the floor and out... as the image.
Would the green pipe/stack mean I could get away with not using a AAV?

Before someone asks, there isn't really room to take through the roof, as it's a vaulted ceiling in the single storey extension, and after avoiding rafters, it will be too far away from the wall and the boxing in will look bad.

 

[Lower]

The air admittance valve will need to be above the highest drain point that is feeding into the soil stack to avoid any risk of overflow if the soil stack was to ever block and back up.

If you don't have an air admittance point in the same stack as the toilet you run the risk of pulling the water seal out of any other drain that feeds into the same stack.

 

[Mr Chibs]

Thanks, I have a shower and basin that was going to feed into the same stack, so if I worked these separately into a secondary stack, would I not need to worry about the AAV?

 

[muggles]

This is notifiable to Building Control, so contact them to see what they'll be happy with

 

[Mr Chibs]

When they looked at the roof, the only comment was... 'you'll have fun with the stack'.
That's why I'm here.

 

[Lower]

Can't you run the waste from the toilet sideways and down into a new soil stack in a similar position to the proposed green one?

 

[Mr Chibs]

I wish, but sadly I don't think so, as the isn't room to get the soil pipe in between the wall and first roof rafter, plus there is a steel beam sat on top of the extension wall.