Mena, you may want to investigate installing rails for mounting, especially if modules are adjacent. It's more expensive but a lot more elegant in design and use. There was a thread from someone that used rails in the last couple of years.
There's lots of way to skin the mounting cat. I used "U" channel and pinned reinforced module flanges to the "U" channel. The "U" channel allowed me to span three rafters, offered a lot of surface to seal/adhere to the roof, and since I located the mounting holes towards the top of the "U", it allows for more clearance. It also worked out that the tilt struts fit inside the "U" channel when stored.
