Flat roofs can be very good for installing PV modules on. Access for maintenance and repair is also easy. Mounting structures are either ‘free-standing’ (held in place by ballasts) or fixed to the roof (if the roof cannot bear the additional weight of ballasts).
PV modules on a flat ‘green’ roof: mounting stucture held in place by brick ballasts.
Larger system on flat roof, mounting structure fixed to building structure.
Is is easy to overestimate how many PV modules can fit on a flat roof. Structures on a roof can cause shading, the rows of modules need have enough distance between them in order that they not shade each other, and the orienation of the roof may not ideal.