LeanTo Roof Plywood Thickness & External/Internal?

[ILikeDIYS]

LeanTo roof - approximately 3 meters out from house with 570cm spaces between rafters (will add noggins too).

Buying plywood from Wickes. Options are:

9mm thick, non load bearing (internal or external if treated).
12mm thick, non load bearing (internal or external if treated).
12mm thick, structural load bearing internal or external.

Not sure which one to go for. I'm thinking 12mm is overkill and might add too much stress on the framework?

I will be walking over it but only for house paint touch ups. I weigh 58kgs. I don't think that justifies needing structural plywood.

It will be covered by thick felt (not the shed type but the thicker version that's usually torched/glued on).

Thanks.

 

[ILikeDIYS]

For perspective here's a 12mm sheet I have lying around placed onto roof:

 

[foxhole]

Too wide spacing for 12mm ply, needs 18mm for a proper job.

 

[ILikeDIYS]

Had another idea...

Onduline sheets

This stuff good for a lean to? It's less than a tenner at bunnings per panel!

 

[big-all]

thinner material like corrugated plastic need timbers at 90% to those timbers going side to side every 450-600mm

 

[foxhole]

Had another idea...

Onduline sheets

This stuff good for a lean to? It's less than a tenner at bunnings per panel!

The black bitumen stuff gets water logged easily and sags or can even flatten under weight of water.