Growing trees in a rooftop garden would be challenging. You need to evaluate the growing location and size. If you want a large tree, think about the logistics of moving it to a rooftop. Will the tree be protected in any way to be out of the wind and sun? Are you located in an urban location, close to high buildings? Container trees also need a lot of care to overwinter.

The issues with container trees on rooftops are wind and intense sun. Thus, a protected location is preferred. Evergreens and ornamental grasses are better suited to harsh environments than deciduous trees. Some ideas would be Dwarf Alberta Spruce, or upright junipers such as 'Skyrocket' juniper. Upright grasses, such as 'Karl Foerster' feather reed grass or pampas grass would be good choices.

You did not specify if you want to keep these plants year after year. In that case, overwintering will be needed. This would involve moving them to an area protected from wind, cold, and sun. Other ways to protect them are to wrap the plant in burlap and insulate the pot to protect the roots. You may want to try overwintering them, but if unsuccessful, treat them as an annual plant and plan to replant new ones the next year.

I hope this helps you make the right decision for your given situation.