I've always had good advice from this new group, so I'm hoping you'll be
able to guide me in the right direction again.

I'm planning the cable runs for my kitchen rewire (yes, Part P Building
Control notice already submitted) and am trying to establish how to run
the cable, in a permitted manner, along the line of the ceiling where
the pitched roof joins the wall. The angle of the pitch is roughly 45
degrees.

My OSG states, in Section 7.3.2, that the cable does not require earthed
metal conduit if "installed [...] horizontally within 150mm of the top
of the wall or partition". I'm assuming from that that even if the
cable remains within the 150mm channel below the angled ceiling it will
still require earthed metal conduit, because it is not horizontal.

I may also have to negotiate an RSJ which has been boxed with plaster
board. Do the 150mm channels also extend to cover the area around the
side and bottom of the boxing? Effectively the underside of the boxing
now makes up part of the ceiling, albeit 30cm lower than the main
ceiling and a touch higher than the lowest point of the pitched roof.

I know I've not explained that very well, so If that makes no sense I
may have to draw a diagram.

Thanks for your help (again).