Mounting timber joists on new steel beam

[nwnw]

Hi folks,

I'm working on the design for a new extension (all in Sketchup).  Loads of queries to come and I'll probably put my drawings up here soon for scrutiny.  My biggest headscratcher just now is how to support the existing roof structure when I remove a 6.5m length of structural stone wall.

I've seen various ways to do this with joist hangers and sandwiching timber on the steel, but I'm wondering why I can just mount direct as shown below.

I've coloured for clarity...Blue = existing roof trusses and joists.  Red = new extension flat roof joists and firrings.  Pink = Steel

 

 

Existing timbers I'm sure will not be perfectly aligned so my thinking would be to leave a nominal gap and shim to the steel, or to profile the face and underside of the joists a tiny bit to fit.

Thoughts?? Pitfalls??