In order to determine safety class of PV module two tests are done according to IEC 61730-2 standard:
1. Spread of flame test
2. Burning brand test
In spread of flame test a glass flame is applied to module for 10 mins. The flame should not spread more than 1,82 m for class A.
In burning brand test a burning object (300x300 mm dimensions for class A) is put on PV module and fast air stream is blown to it for 1,5 h. The test sample should stay firm so that the burning object would not fall through it and the test object should not create any glowing/burning particles by itself.
Both rooftop materials and solar panels have separate fire class ratings from A to C. From a safety perspective, the goal is that installing PV modules on the roof does not decrease the overall fire class rating of the roof + solar panels.
If your client has a class A roof, putting class C panels on it would decrease the overall fire rating significantly. Do they want to reduce the fire safety of their roof just to get some cheaper panels?