Connect to old cast iron soil stack

[Me!]

I have an old and now unused cast iron soil pipe on what was once the outside of the back wall of the house, but a more recent extension built on the back of the house means that this soil pipe is now internal in the extension.
I would now like to add a WC and sink into this extended part of the house and connecting to this old soil pipe seems to be an easy way of getting the drainage I need without having to dig up the nicely laid brick floor in that room.
But, how do I do that? I'm not aware of any way that I can connect a new 110mm plastic pipe to the side of the existing cast iron, is there any way of doing this that people know of please?
Or, I could cut the old cast iron pipe at ground level and remove all the redundant part above, then insert one of these type of connectors and then use pvc pipe to connect to my new WC. But, by the time I have inserted that adaptor and then an 87.5 degree bend as well to take the pipe where I need it, its height above the floor will be too great by the time I get to the WC. A solution would be an 87.5 degree bend that connects directly into the cast iron that I cut off at floor level rather than using the adaptor that I linked to above, but I can't find any such bend, does anyone know if they exist and where to find them?
Or, any other suggestions on how I can resolve this will be much appreciated please.

 

[Me!]

Ah! I've just found one
https://www.ebay.co.uk/itm/90-Elbow-Cast-Iron-to-Plastic-Drain-Connector-110mm-Soil-Pipe-to-4-Cast-Iron-B/253698121628?hash=item3b11961f9c:g:4p8AAOSwBNtbKOzd

 

[malcolm]

Will you also vent the top of the pipe? I gather that is necessary for upstairs toilets.

 

[Westholme]

I assume from the mention of a brick floor that the new WC and basin will be on the ground floor.

You could certainly use what you've found to connect a WC only, but how do you propose to connect the basin waste? You could take it outside to a gully, however if you want all the connections internally you may need to have a stub stack with a branch pipe leading to the WC, not a bend, and the basin waste connecting to the stub stack higher up.

See the Building Regs part H or
http://www.polypipe.com/sites/default/files/Above_Below_Ground_DTIG_v5_Low_Res.pdf pages 8 & 9

 

[Me!]

Yes, I do intend to vent the new pipework with an air admittance valve.

My plan is for the pipe to turn 90 degrees with the new connector as soon as it exits above the newly cut cast iron pipe at (ground) floor level and to then run approximately 2 metres horizontally(ish) including going behind the new WC and connected to it with a branch, and to then turn 90 degrees and head vertically up to the ceiling height of the ground floor room where it will be capped with a durgo valve. Part way up that new vertical bit will be where I connect in the waste from the new sink.

There is also another stack that is fully vented to the atmosphere as another part of the system.

 

[skier-hughes]

Good luck with the cutting. I tried it and the pipe just cracked and shattered.

 

[Flyfisher]

I'm not aware of any way that I can connect a new 110mm plastic pipe to the side of the existing cast iron, is there any way of doing this that people know of please?

Not sure what you mean by trying to connect a new 110mm plastic pipe to THE SIDE of an existing cast iron pipe. It's possible to connect smaller pipes to the side of larger ones using an appropriate strap-on boss, but I've never seen anything similar for a similarly sized pipe. Sounds like you'd have to cut off the cast iron pipe completely and then add a Y-branch if you really want to keep all the pipework at 110mm.

If you want to keep everything in cast iron then this place seems to have all the bits you'd need: https://www.drainagesuperstore.co.uk/product/cast-iron-soil-pipe-875dg-branch-traditional-express.html

Or use the adapter you've already found and go with plastic from there.

As skier mentioned above, cutting the cast iron pipe will require some care as they can be very brittle.