The roof covering really has little to do with the structure nowadays unless you are doing a flat roof with gravel on top as a ballast.
The new to residential material in the last 20 years is EPDM roofing. Check it out.
EPDM Rubber Roofing - Flat Roof Solutions
If the roof needs more structural support, do it from the top if that is possible, you will already have the roof exposed, and removing and replacing any plywood at this time would be benneficial for the new roof material.
Another option for the structure is SIP. Structural insulated panels. They are able to be installed over the existing roof sometimes.
SIP Construction Systems Technical Details - Structural Insulated Panel Association
Enjoy the homework.