Skylights can go in flat or pitched roofs. Usually, the less pitch there is, a 'conversion kit' is used. This kit is to help deal with water and snow. With a pitched roof, obviously, the water runs off very easily. But with slightly pitched, or flat, roofs, you want to raise the window ABOVE the roofline in order to prevent water from rising up and into the house.
The Dealers will talk to you about this. Just be sure to bring up the 'problem' w/them. And before you go in to talk w/them, take a moment to measure the RISE of the roof (to the middle of the house), and the run over to the roofline. (Rise and Run is what the Dealer will probably want to best answer your questions/concerns.) Below are just 2 popular mfgrs.