Is my roof strong enough

The solar panel weights about 80kg - this is really not much weight for a roof. There should be no reason to strengthen a house roof prior to mounting the solar panel. However if you are fitting a panel to an old roof, you should check the timbers are not rotten or sagging. If in doubt ask a roofing contractor to check.

On Commercial building, the roof should be checked as putting 40 or 50 panels on the roof does add a lot of weight but again it is usually much less than the roof is already designed for,
 
Also roof are designed to take a compression load. ie the weight of snow on the roof. If you are mounting solar collectors that stand up from the roof, there is a possibility that the wind loading trying to blow the panels off the roof could affect the roof. This effect is much more pronounced with flat plate solar panels. The Evacuated Tubes allow wind to blow between the tubes and the wind loading is usually much less than with flat plate collectors.