I am installing a 200sq foot deck using reclaimed ironwood (aka ulin) from Borneo. I believe this is a similar wood to ipe, perhaps even the same species. The deck will be built on the (flat) roof of my house, which is in Hong Kong. The surface is tiled and slopes towards a drain in a corner, so there is no pooling. The deck cannot be more than about 6" above the tile, however, so clearance is an issue. I have a couple of questions:

1. The first is what material to use for the sleepers? The climate here is hot (in the summer) and humid. I also live about a quarter mile from the sea. I also don't trust the quality of local PT timber, so I would prefer to avoid it. That leaves various sleeper options, in particular:

a. Composite
b. Recycled (HDPE) Plastic
c. Aluminium
d. Steel

Composite seems the cheapest option, but I am unsure if it is suitable. Any opinions on this? Any other ideas about ventilation? I suppose I could install a fan if it would help.

2. If I go for one of the metal options, what kind of screws should I use? I am concerned about catalytic corrosion due to reaction between the screw and the framing. Also, do I need to worry about differences in expansion rates between the framing and the wood in high heat or humid conditions? Is this likely to result in sheared screws, or otherwise unseat screws due to constant movement vis-a-vis the metal/wood?

Thanks
Colin