Why do Hardie and BGC specifiy battens for cement sheet?

From BGC:

Durasheet™ must not be fixed directly to the bottom
chord of roof trusses. Timber battens or metal furrings
should be installed.

From Hardie:

2. Do not fix (Villaboard) sheets to the bottom chord of trusses. Batten these out
first with timber battens or steel ceiling battens.

Can't see a reason for needing battens, I have pine gable roof trusses (600mm) on my carport ceiling which would make perfect mounting points.

Anyone shed (no pun intended) some light on this? :~


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