With roof-top gardens it is really hard to maintain a lawn area because it needs constant care and attention and constant watering.
Grass lawns do look great in any garden whether it be a normal garden or a roof top one but they bring a garden down if they are unkempt or not looking their best. This means that roof top gardens more than any other need constant attention.
Alternatively you could brighten up your roof-top garden using imitation grass, this would allow you to choose your exact colour, to have a beautiful lawn all year round and to have a space you can use all the time to enjoy your roof space.
Grass on a roof garden is not practical so to get the garden look without the constant work artificial grass is the ideal solution.