I didn’t attach the OSB to the bottom of the base plate at first as I didn’t want it to be badly weathered.

However, I realied that I needed to attach the boards to the underside of the base plate before I got too far with the rest of the building otherwise. It’s OSB3 so it lasts pretty well in the rain anyway. I used bottle jacks to lift it up, and then glued and nailed them on. This was pretty tricky.