Steel
Do you mean purins or rafters ?

I mean purlins. The counter-battens run up the slope at right angle to
the purlins, but are
only 50mm.

Steel must be boxed in. You need to go back to the engineer and ask
him to recaculate. The extra weight may effect him, and he can advise
what size you need. Or simply go back and ask if the roof can be done
thinner.

The ceiling construction will box them in.
What engineer ? This is a DIY group ! I'm doing full plans of course.

I have looked at the multi-beam purlins from kingspan. They are light
for their strength.
The load tables take some figuring out though.

Simon.