Pantera2075

Hi all, Noob here, and i'm hoping to get some advice on the roof of my new garage if you would be so kind? Garage is 9m wide by 6.5 deep being built under pd but due to the footprint building control are involved. We have dug into a slight incline so we are looking at the 2.5m pd plus about 600mm gained by the excavation. Currently the raft base is part poured and shuttered for the final pour which should be complete in the next few days It will be brick inner and outer skin with a cavity as I can get extra cheap reject bricks 2x garage doors and a pedestrian door in the long side which is also the gable. It will be a very shallow pitch side to side around 12 degrees (Can't remember exactly but its something like that). Now, i'm a mechanical engineer but not a builder so I might ask some really stupid questions in an attempt to square it with my mechanical mind. With the price of building materials these days i'm looking to make it the most cost effective - not necessarily the cheapest but that will be a major factor. Roof will very likely be steel sheet, dark grey but of as yet undecided design. Roof will be effective 1m high above the 2.1m eaves. I want to leave the roof inside as open as possible so I dont want multiple traditional trusses. So, what are my options for the structure of the roof? I can see that an 8" zed purlin will do 23feet in a single span which is long enough to do the 6.5m. Is it ok to screw steel purlins to a wooden wall plate? I'm assuming current steel prices mean its best to stay away from universal beams if possible? What do I do at the gable ends where the purlins meet? Would these need to be steel? Would it be an idea to put in steel uprights within the walls to bolt the superstructure of the roof to? Is it an option just to have a truss, either steel or wood on the top of the inner leaves of brick?Could it jut be a wall plate angled up the inner leaf?? Obviously tied down to the walls. Would it need a substantial wooden ridge board to hold the ends apart and transfer load to the gables? Is there a more widely used more sensible design for this? Thanks in advance Phil