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
Edited and Illustrated by Charles Miller
From Fine Homebuilding #226
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I would have never thought about putting the skylight in before framing, but I live in Colorado where it hardly rains versus Portland, so you have to do what makes sense. If it rains a lot, the hole in the roof is big deal, so minimizing it is crucial. Another advantage I could see of your approach is the light coming in through the skylight while you finished the job.
I'm curious how the roof sheathing is fastened to the after-installation framing? If the roofing is finished around the skylight then there's no opportunity to drive fasteners from the top. I suppose a few beads of adhesive would be one way to connect the framing and sheathing.