Just because a plant may do well in the same area that you want to create a green roof does not mean it will thrive in a green roof setting. The growing media is artificial and often not even remotely similar to the soil around it. Soil depth, wind exposure, and sunlight in the roof environment are also things to consider. If you do spy a plant in your area that is growing in rocky low nutrient soil you may have a candidate for that roof garden of yours.
Please don't dig up the plant if you find it in a park or nature preserve as this can have a negative chain reaction in local ecology.