When you have a flat roofed building, something to strongly consider is having a skylight. You can have a skylight in a normal shaped roof, however, having a flat roof can give you an advantage of space! A flat roof will normally mean you have a lot of space, you could even consider making most of the roof glass, opening up your room and letting the light in.
You have two main considerations, plastic or glass?
If you are having a large window and it is a way of naturally lighting your room, we would strongly suggest to go for the more expensive option being glass. Glass made for skylights is being made to have more durable qualities, it can also be laminated or tempered to increase its strength.
Unlike with plastic which can discolour over time or even warp. Plastic also allows ultraviolet rays in from the sun which can fade your furniture and can also damage your skin. (To stop this it must be coated in a special film, however this does not stop it from warping and basically being less durable.)