Locate Ceiling Rafters, Take Measurements
This project takes place in a ranch-style home with a simple sloping roof. The complexity of the roof, the amount of attic space and other factors help determine the following steps.
Locate the ceiling rafters with a stud finder and mark the inside of their dimensions to indicate where the skylight will go.
Measure from the peak of the ceiling to the point where the wall and ceiling meet (the plate) to obtain an overall rafter length. You want to position the skylight hole in a place between the rafters that will maximize the dispersal of light while optimizing the ease of installation. Generally speaking, placing the top of the skylight a few feet down from the roof peak is ideal, but every roof is slightly different, so adjust as you need to.