Fitting Rooflight?

Bungalow. Are fanlights fitted ON the rafters?

I have a truss roof and no trusses can be cut without problems, which I am aware of. Tiles: Interlocking Bold rolled sand faced [a pain fills gutters] because the centres are around 17in, part of the rafters will show but will be half boxed.
When we moved in the local loft window shop wouldn't touch it because of the truss rafters.