I’ve got a backward technique for installing deckmounted skylights in existing roofs. As shown in the drawing below, I install the skylight before I do any framing. I cut the sheathing out of the opening, leaving the rafters intact. I’m able to locate the skylight exactly where I want it based on either exterior or interior conditions. Putting the framing modifications off until later buys me some time with the weather— always a consideration here in Portland, Ore. Once I’ve got the skylight woven into the roof with new shingles, I can proceed with rafter work rain or shine. Because the skylight sits well away from the rafters, the skylight is not at risk of an errant reciprocating-saw blade, and I can be precise about the framing.
Sam Yoder, Portland, OR