The installation instructions for my panels specifically require a ventilation gap underneath. If you drive around and look at the big multi-kilowatt installations on home roofs, you'll find that every installation of crystalline silicon panels has a substantial spacing from the roof. As others have pointed out,some thin-film panels are designed for installation in contact with the mounting surface.
Some details and photos of my installation are at http://www.damouth.org/RVStuff/SolarInstall.shtml.
If I'm boondocking in the same place for a few days, I tilt my panels up to maximize output. In winter, this can increase output by 40%. But on a windy day, the tilted panels are pushed pretty hard by the wind. So I made sure the attachment to the roof was very secure - see details on the website.
