The roof is a simple frame with clear plastic or glass in it. We will make the frame out of cedar or pressure treated 2 x 2's just like the walls.
The roof panels will mount flat on the roof as opposed to fitting in between the studs like the walls. Measure from the outside edge of the greenhouse to the center of the first or second rafter for the first panel. This will be the same on both ends of the greenhouse. Measure between the center of each remaining rafter for the middle panels. As you can see here I spanned two rafters on this side with a center piece in-line with the rafter.
Cut your 2 X 2's to these dimensions to form the panels. On each of the 2 X 2's, dado a groove 1/4" wide (or whatever width your glass or plastic is) and 1/4" to 1/2" deep. Position the groove approximately 1/4" from the edge.
Next cut the plastic or glass (glazing) to fit inside the groove. I like to leave a little slop for expansion and contraction but not too much or else your glazing will fall out.
Lay out all the pieces, insert the glass and using deck screws, screw it all together.
Mounting the panels is simply done by screwing with deck screws into the rafters. I mounted one panel with hinges and a temperature actuated Univent. This will let the heat out or keep it in depending on the outside conditions. Univents can be found at most greenhouse stores.