When determining what kind of soil to use for the garden there are two different strategies each with their own set of pros and cons.
The first option is using a less dense fluffy type soil to keep the overall weight down.
The biggest side effect of using this fluffy soil is that over time it will condense seem like it is disappearing.
Also you might have to worry that such soil could not support any substantially sized plants.
The next option is using a dense soil that contains more rocks.
Often times this soil will be richer containing more nutrients for the plants.
This type of soil also would not have the shrinking affect that the fluffier soil would have over time.
The drawback of using the denser more rich soil is that it can be many multiples the weight of the fluffier soil.
So it can be a give and take.
Sometimes it would be best to use a thin layer of a denser soil or a much thicker layer of a lighter soil.
It is important for many reasons that the soil used is native to the area.
If the soil is imported often times the habitat will not be suitable for different wildlife.
The same goes for the vegetation planted.
Birds and insects would stay away from non native plants making a waste of the resources put into the rooftop garden.
Some great native plants to use on rooftop gardens in western Washington would be evergreen shrubs like Snowbrush or Pacific Rhododendrons.
Depending on personal preference and rooftop specifics there are endless options of different native plants to choose from between trees, shrubs, vines, herbs, and ferns.
Once these are planted they should keep coming back year after year with no assistance of a gardener.
The point of a rooftop garden is creating a self sustaining environment that requires no other resources once it is built.
This means that watering systems as well as care taking beyond a light weeding a few times a year are not needed.