My probably over-engineered method to attach solar panel to the roof
Here are a few pictures of the way I attached a 100 W solar panel to the roof of my x213. In brief, I used aluminum flats mounted to the roof with two through bolts, to which the solar panels are attached with the regular z-brackets.
Once the flats were attached to the roof with three fasteners each, I covered everything with dicor and let it gas out for two weeks. Then, I covered the flats with a strip of 4 inch eternabond tape, and covered the bolt protrusions with a small dab of dicor. This provides double insurance against leaks. Note that I didn't come up with this myself, I watched Tom Silva from This Old House attach a snow guard to a flat roof, using a similar method.
Now, the second advantage is that the solar panel is removable, I can take it off any time I like without having to reseal the roof.
For fastening the flats, I used the self-tapping screws from the z-brackets into the rafters. However, at the back of the panel, there were no rafters. I felt that the screws didn't really have the bite in the decking that I would have liked, so I used well nuts for attachment. Now, it feels like I can pull the decking off with the panel, before the panel would come off the roof.