Marine-grade aluminum or fiberglass can be used for a rooftop application since both materials are light-weight and much lighter than concrete or steel.
Fiberglass and aluminum are both suitable materials to use when overall weight is a concern with rooftop planters.
One factor to consider is the material used to fill the planter. You want to fill the planter with light-weight material. Adding internal shelves and using non-biodegradable materials in the base of planters is a way to decrease weight.
Please click here to learn more about different options for planter materials and here to learn more about options for filling a planter.