I built the walls of my 12' x 12' shed before deciding what type of roof I wanted. Now I'm left with the problem of how to secure the roof to the lower walls. If I build a jack wall as shown below (image probably from this video), how would I secure the bottom plate of the jack wall to the top plate of the lower wall? I don't want the joint between the walls to be the weak link against wind.
You have to notch your roof rafters (birdsmouth) at the bottom and at the top so that the rafters can sit on the plates. You'll then nail from each side to secure it to wall plates. Familyhandyman has a diagram on what this is.
For the jack studs, you'll need a top and bottom plate for it so you can nail/screw the bottom plate to the top plate of the existing wall.
Once the siding, roof and rafters are put on it'll be secure. I live in a hurricane prone area and I built my shed similarly and it has so far withstood a few hurricanes.