Veranda ceiling - what material to use?

[Joff-Turbo-Nova]

I am in the midst of building the wife a garden office, I want to box in the area above the veranda and create a ceiling, ideally which can be painted.

Given that one side will be exposed to the elements what material would you recommend that I use - I was considering marine ply which seems to be quite robust against moisture and the like.

Pic below of the area in question....

Cheers

Joff

PS everything is straight and level, its the camera lens pulling things in a bit!

 

[tomfe]

As it's sheltered from rain normal ply will do, if you are worried use marine ply like you said or cement board.