RBM usually goes on the external walls only as standard. Unless you have specifically asked for sarking under the roof.
Ok, my roof on my plans just happens to say sheeting iron and RBM. Only reason I asked and noticed it for a start was because friends walked through our build and first thing they said being builders was why have they not put any insulation directly under roof. It goes there to stop moisture from condensation dripping down into your roof thats why you have it and other things. I was like I don't know don't think they do it. So looked at my plans and it does actually say it on them so yeah I have no idea, do your elevations say sheet metal roof with RBM at all like mine?
By the way your place is looking great. I meet sparky tomorrow, met with plumbers today they did couple extras for us outdoor shower webt in and extra tap. The sparky has a whole house of extras to do he doesn't know that yet though but will tomorrow morning