Seating overhang area

[new24]

Hi all. I'm doing some outside porcelain tiles or even granite slabs at the side of my house. Gable side of a dormer bungalow. Looking to build something similar to the first picture.

What I have in mind is 8mm thick100mm box section with some plates welded to rake a ub or even an rsj. Something similar to the second pic. The thing that I'm concerned about is thst I'm wanting a 6m span and wondering will I have any issues. It will only be taking the load of 9x2s spaced at 600mm centers along with some osb and epdm covering.

I'll either Bury the box section in the concrete or a plate welded and bolted into a 750mm sq foundation pad 400mm deep.

The over hang area I'm wanting is 6x3m . I know structurally it will be better to have another box section in the center but this will interrupt my nice views so hoping for just two box sections on each end. The ub or rsj will be returned I to the house and bolted onto a block on flat chimney and the other side it'll be bolted into a gable. There will be a 9x2 joist runner bolted into the gable with chemical resin or expanding bolts. Joist hangers for the 9x2s and so on.

Would this be achievable with a ub or even a 150mm rsj or would I need a very large rsj.

 

[DAZB]

I would say you would be better talking to a Structural Engineer as it is the kind of job that needs proper calculations drawing up and, as capable as some of us are on here, maybe best not relying on the informed (or not) ideas on an internet forum. It would also then cover you were you to sell the property in the future.