If his joists are running parallel to the purlins he had a good reason
to use a structural engineer.

(He can sue the pants off the fool if he lives long enough.)

It won't cost you much more than the time you put into it and a few
sheets of board and some insulation.

Edit to an earlier post:

If you put the plasterboard on top of the newly positioned joists, you
will not need to bother with the battons.

Take the time to put a light in the roof space. Space the joists
according to the board you use. Ply would be better than plasterboard as
you can then utilise the attic.

If you put the ply under the purlins as suggested you might want to put
lights in it, so put the wire behind them as you go and drill through
for the feeds.




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