Perhaps if we can narrow the scope a bit  it will help you get  more than trivial answers.!

What type of panels,  hot water panels or photovoltaic?,

If electric grid synchronous or isolated ?

how much sunny roof - in northern hemisphere south facing - roof have you got?

and how much power or heat do you need?

What Latitude  - sets angle of panels. and maybe other environmental factors ?

local building styles, suitable roof material,  safe weights etc ?

Easy access or not to both the roof and underside ?

Any limitations from local weather - hail snow, sandstorms, hurricanes ?

local wildlife or plants that may be affected or may attack the panels? 
Mike

That's a really broad question:-

• How to fix them to the roof so they don't blow away in a storm?
• How to wire them safely (you can't turn solar panels off while you're wiring them up)?
• What isolators do you need?
• What inverter do you need - and is it type approved in your country?
• Do you need to meter the generation?
• Do you need permission from the electricity supplier.?

If they're PVs the wiring regs requirements take up and entire section of the wirings regs (712) - in addition to all the general requirements of course.

   - Andy.