Roof mounted solar systems are most always less expensive compared to ground mount for several reasons.  The roof already exists as foundation to attach the mounting hardware and modules, also the distance from the modules to the meter is usually shorter.  This means less wire, less conduit, no dirt trenching, no concrete, no building ground racking, equals less cost and less installation time.  When the solar panels are mounted on the roof, the wire is run either through an attic or crawlspace, or sometimes across the roof or exterior wall inside of a conduit depending on the home.  The module mounting hardware is screwed into the roof trusts to ensure that the whole system is sturdy.  One important thing to keep in mind before a rooftop solar installation is the condition of the shingles and roof itself.  You will want to make sure they both are in good condition or even new.  While it is technically possible to re-shingle with a solar system mounted, it would be difficult and no one wants that, trust me.  While there may be a specific part of the roof you would prefer the modules be on, the important part is that they're mounted on a side that receives the most sun exposure for most of the day in any season.