Outdoor Deck Ceiling - what type of plasterboard?

I want to add a ceiling to my outdoor deck using plasterboard. The deck is covered with a ColourBond steel roof.

Despite being an outdoor deck, I am contemplating using standard plasterboard - because of the weight. A square meter weighs approx. 4kg and I have 20m2 to cover, so this would add about 80kg of weight to my roof.

The (probably) more appropriate water resistant types of plasterboard weigh twice that, so it would add 160kg to my roof....

I am sure the roof structure could handle it but I am not comfortable with adding this sort of weight, considering the roof is 100% watertight AND I plan to paint the plasterboard ceiling with an outdor paint anyway....

Any thoughts on this? Thanks!


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