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?

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?

Could you not mark up the purlin position and pre drill the holes. Beside which when you lay up a sheet the positions of the purlin will be visible at one end making it easy to eyeball the screw position

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