I'm a big fan of natural light. We installed a simple skylight in the laundry room a couple of years ago, and it was fairly straightforward, so I'm going to put one in our upstairs bedroom. This one will be openable, which will provide ventilation and light, and of course a place to stick a periscope up and pretend my house is a submarine.
The roof rafters are 24" so a common 2' x 4' skylight will fit right in with no change to the structural framing. I bought a Velux VCM 2246 (manual opening, curb mount, 22" x 46" nominal size) and a ECL flashing kit at Lowe's. They stock them locally, they're made in the USA, and we had a good experience installing the first one. I marked out some locations with tape so we could visualize them from inside, and then nailed in a header and footer to form a frame.
|This is what happens when I'm left alone with a recip saw|
These skylights are sized based on expected framing spacings and thickness. Ideally, your rafters are 24" on center and 1-1/2" thick, so that there is 22-1/2" between then, and of course everything is square, level, and plumb... Few of those parameters are ever true, so you have to watch the important ones. In the end, all the skylight cares about is that it's sitting on top of a framed curb that is square and has outside dimensions of 25-1/2" x 49-1/2". That's easy, since you'll be building the curb yourself, and it sits on top of your roof sheathing so it's independent of the house framing, but you have to make sure that it will align reasonably with the rafters or it will get ugly when you go to trim the inside.
Now, the part you really care about is whether or not it leaks. If the roof is very flat, or very steep, things are tricky, but my roof is 12:7.5 (32˚) which is in the normal range. We just followed the instructions on the Velux flashing kit and have had no problems. They supply everything you need but nails, shingles to patch your roof, and a bit of tar paper underlayment.
I use a long (~12") 1/8" drill bit to locate through structures. I used it to poke through at each corner from the inside, and then I stuck a bamboo skewer through each hole to easily find it from up above. I drove a screw into each corner hole up on the roof deck, used those to snap chalk lines, and started cutting with a circular saw. I have an old beater I use for stuff like this. It's a low-powered saw, so I can hold the guard back and use it freehand with little risk of it grabbing and coming at me. I set it shallow and use it to score through the roofing material, then remove that and make another deeper cut through the wood strandboard sheathing. This house also has a layer of 1x8 skip sheathing under the osb, so I cut those with a sawzall from inside. You could do the whole thing with a sawzall, but I think this way is cleaner and a circular saw has a much easier time with the asphalt roofing.
|Scoring the roofing with a circular saw|
Then I went around with the circular saw again, and set really shallow, just barely cut through the roofing 4-1/2" out from the hole on the sides and bottom, and 8-1/2" at the top. These dimensions are in accordance with Velux's directions. I used a knife to slice whatever the saw didn't get and stripped it all down to the wood. Then I attached the curb, which was just four pieces of 2x4 lumber screwed together at the corners. I figured toenailing it to the deck wasn't going to cut it for an operable skylight that will have some dynamic wind loading, so I used 6" Timberlok screws, countersinking each one into the top of the curb and driving it all the way into the rafters. I predrilled pilot holes with my long 1/8" bit to reduce the chance of splitting the framing, and drove them home with my little Milwaukie impact driver to clamp everything together very solidly.
|Long Timberlok screws fasten down the curb|
|Ready for flashing|
|Finished view from the inside|