Deck mounted skylights are attached directly to the roof sheathing and attaches to the drywall that sits below. These are typically installed in slightly sloped roof pitch situations so the water will run around the skylight and not into the building. For a deck mounted skylight, the minimum pitch is 3:12 / 14 degrees or greater.
Curb mounted skylights sit on a 2x4 or 2x6 curb or raised frame when the roof pitch is lower or flat. The curb itself will channel water away from the skylight keeping the space below dry. The roof pitch for a curb mounted skylight is is 0:12 / 0 degrees to 20:12 / 60 degrees.