On Tuesday, June 15, 2021 at 10:37:22 AM UTC+1, Chris Bacon wrote:
The intent is to sheath the steelwork of the roof with OSB or similar,
and go on that with 34/1000 box profile.

Roof pic: https://i.ibb.co/Krf1z2J/IMG-20210615-102128-1.jpg

It might be possible to screw straight through the box profile and OSB
into the steel purlins with TEK-3 screws, but there's scope for missing
the target, which would be unfortunate. The purlins are equal angle
40x40x3mm mild steel, about 1000mm between the trusses, purlin apacing
each side about 1150mm.

I could secure OSB onto the steelwork, and mark out the purlins'
positions on that I suppose. I could secure the OSB onto the
purlins/trusses and fasten the box profile to the OSB only. Hm.

Any suggestions as to how to approach the job?

I think I would attach some battens to the angle something like 2€� X 2€� square then you have something more to aim at when sheathing with OSB. What I would advise against is any drilling of the vertical part of the angle iron as that is where the strength is.

Richard