Any lateral thoughts on how to connect up a new shower drain?

[Beau]

We are slowly renovating my late parents farmhouse. We are looking to put in a small bathroom where currently we just have a toilet. There is a cast iron downpipe behind the lead flashing on the slate hanging, and the blue line marks the height a shower drain should come out through the wall. I am loathed to take off all the slates, so I’m hoping there is a discreet alternative I haven't thought of. The room below is nice and I would rather take lots of slates off than drop a waste pipe through it. The wall has EWI behind the slates, and I think the buildup is 3x2 battens with 60mm PIR set between them, then batten and slates.

 

If I do have to remove lots of slates, should I just bite the bullet and replace the cast downpipe with plastic while I'm at it?

 

Thanks

[Nickfromwales]

You’ll be biting that bullet, but you already secretly knew that .

 

No real reason to change the stack unless it’s in a bad way, and you can cut into these with a decent Starrett hole saw without issue. 
 

Just keep the chuck tight as you don’t want to finish drilling the hole and then watch the hole saw fall into the stack, and then listen to it go ‘clunk’ as it hits the bottom clay bend and shatters it. 
 

Ask me how I know…..

[Nickfromwales]

Thanks for the sympathy guys… 

[Beau]

2 hours ago, Nickfromwales said:

Thanks for the sympathy guys… 

I can imagine how gutting it was but still going to laugh at your misfortune.

 

So accepting I need to take a whole pile of slates off would you look to go into the stack behind the slates or drop down to near ground level behind the lead and break in there? Less hight for dropping hole saws and easier access for what I imagine is a tricky join. If its a boss with a strap I'm going to be digging holes in the wall to get behind the pipe as it looks like it was rendered after the pipe was fitted so partially set into the wall. 

 

The advice is much appreciated even if I dont like whats involved haha

[Nickfromwales]

I’d tee in as high up as you can, and actively avoid dropping down with a waste pipe, as you’ll get slurps and traps being sucked dry, so will need air admittance valves if you do that.