kaffy,
I designed a wooden shed for my back yard. Like my shed, your enclosure is small enough that building codes (probably) do not apply to it.
I worked out that snow loads were not a concern for me. I analysed for me (210lb) walking about on the roof. Perhaps I should have analysed for me, a friend, and a case of beer, on the roof. My walls were 2[×]4 framing with something called channelply, screwed on the outside. This is typical building construction, so I anticipated no problems other than graffiti. I covered my walls in paint, rather than stain and varnish.
In your case, your thin steel walls will resonate like crazy if anything hits them, like rain or hail. Sheet metal away from support structure like angle bends, is very flexible and not very strong. It will not be a good place to lean tools like rakes and shovels. You will have quite a bit of design to do.
Can you examine an existing steel enclosure? Generally, if people do things a certain way, there is a good reason.
Can you just go out and buy something? Home Depot sells garden sheds that almost certainly will look way better than a sheet metal enclosure. I know very little about Pre-Engineered Metal Buildings (PEMBs), but maybe you should investigate. Making a home designed metal enclosure look good could be challemging.
I used SolidWorks to design my
Backyard Shed. Either SolidWorks' weldment feature did not yet exist, or I did not understand it. If I had it to do over, I would have used it for the wood framing. It would have been helpful if SolidWorks had had a feature that popped up and told me how circular saws worked.
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JHG