Soil pipe in the way of flat roof joists, help

[Kido44]

I am having a wall removed, due to a kitchen extension, and a flat roof fitted on the extension.
The soil pipe from the toilet runs in the way of where the steel will be going and the flat roof joists.
How can I get the soil pipe down through the steel, and the joists, without the fall being to great.
I will be getting a plumber out but I'm worried this is going to cause a major problem.

I have provided a pic, and I'd appreciate any help.

Thank you

http://www.diynot.com/network/Kido44/albums/

 

[Nige F]

If I was your plumber - I`d rip out all that iron stack and re run it in plastic around the new steels and joists when they are in . Only problem you`d have is if that`s your Only WC . No such thing as too much fall - I know about venting stacks and Hep V O waste valves , and a load of stuff that has no bloody relevance to modern plumbing

 

[Kido44]

If I was your plumber - I`d rip out all that iron stack and re run it in plastic around the new steels and joists when they are in . Only problem you`d have is if that`s your Only WC . No such thing as too much fall - I know about venting stacks and Hep V O waste valves , and a load of stuff that has no bloody relevance to modern plumbing

Oh ok so we can drop it down between the steel/joists and then angle it under ceiling level?

Just box it in?

Roughly how much will it cost to replace?