The installation of solar water heating(SWH) collectors is an investment in one's energy security. Proper installation protects that investment.
A roof-mounted array has the economic benefit of autilizing an existing structure your home or garage's a roof rather than having to build a separate structure to support the collector array, which can be costly(see Ground-Mounted SWH Systems" sidebar).
Even so, a roof-mounted SWH system should not affect the building's structural integrity. Examine the roof structure and be certain that the rack can be attached to the roof framing-not the sheathing! If the roof already looks like it is having a difficult time doing its job i.e., it is sagging or bounces when you walk on it you may need to reinforce it If you plan to use a tilt rack(see "Tilt or Parallel?" sidebar), be sure to understand collector loads on the roof. It may be necessary to consult a professional for advice or have the system engineered.