When you are building a roof deck out of plywood, you need to make sure that it is thick enough to support the weight of people standing on it.
The minimum thickness that you should use for this project is 15/32 inches. You could just try to get ½-inch thick plywood as it will be a little thicker and more durable.
You should also make sure that you are dealing with plywood that is rated Exposure 1. This type of wood can be exposed to the elements and used in construction projects without any issues.