Wickes Skylight ?

I have a Fink style struss roof and no part of trusses can be cut [Local Loft Shop wouldn't do the job because of the truss rafters] and want to install a Wickes Skylight from Wickes and only supply to order.

Q: Do roof windows sit ON the rafters or lower down in between on additional trimming? The tiles are double roll top

Many thanks in anticipation of replies.