CNN: If your neighbor mentions their green roof you might think they have a moss problem. Maybe they are simply referring to the color. But you’re unlikely to think that they have just had a mini ecosystem installed. Simply put, green roofs are gardens on your roof. They come in all shapes and sizes and range from a simple layer of turf to bite-sized hanging gardens of Babylon. But green roofs are not just aesthetic. They have important environmental benefits: they absorb storm …