Loft Conversion - Soil Pipe through steels

I am currently mid-way through a loft conversion in Brentford, Middlesex and first fix plumbing is just about to begin. I have been told by my contractors and architects that I have to have a Saniflo because they can not cut a big enough hole through the steel beams for a traditional soil pipe. Another friend of mine is doing a loft conversion and his builders have said they can do it with no problem. My question therefore is, what is the maximum sized whole you can drill through a 150x150 steel and what is the smallest soil pipe that could be used? I really want to not have a Saniflo but am really being pushed down that route.

Don't know for sure, but I would be very surprised is you can cut a 4" hole through a 6" steel.

But late in the project to be thinking about this though?

I agree it is very late in the day. This is what happens when builders suddenly tell you as they are first fixing...

You should NOT drill ANY hole through a steel beam without approval from the structural engineer who has signed off the beam design. Your architect should have designed the support and pipe run so you can have a conventional waste. How is the pipe run shown on the drawings which you have submitted to building control for approval as you have to build in accordance with those drawings?

If you can get full access to the steel you can cut as big a hole as you require PROVIDED you weld in a reinforcing bush to replace the structural strength lost. You need to get a structural engineer to determine the bushing required. Principal strength of a steel beam comes from the top and bottom flanges but cutting out the web (other than a relatively small hole on the neutral axis) requires some alternative reinforcement.