Flat roofs are often the most susceptible to hail damage. In fact most types of flat roof do not even come with a hail rating. This either means that the material has not been tested, or that it wont hold up long under hail storms. This does not mean that there aren’t flat systems that hold up admirably.
To show the differences in roofing materials, I am going to compare three common flat roof systems. One is rubber roofing. This stuff is usually around a half inch or more thick and instead of being adhered to the roof, it is mechanically fastened with screws. This requires a further layer of insulation to complete the roof system. My Denver roofing company found hundreds of large diameter hail impacts because of the soft rubber material in one example. However, there were no leaks on this building.
PVC roofs do not hold up as well from hailstorms because they are so brittle. You actually want a slightly flexible material if that material is not very strong. In once case, our roofing contractors in Denver found impacts that looked like shattered glass. Hail can go right through thin PVC. This immediately causes leaks and water damage. So make sure you use thick, impact resistant PVC in hail regions.
Rubberized asphalt is a construction system that performs well under hail conditions. Rubber is mixed in with the asphalt to increase the life and elasticity of asphalt. Asphalt’s main weakness is that it dries and cracks over time, so this attempts to combat this. It will not completely protect underlying fiber glass roofing, mainly because of how thin it is. However they generally do protect from hail pretty well. My roofing contractors in Denverrecommends gravel surfacing to be added to the bitumen or rubber for maximum weather protection. Gravel is very strong compared to other built up roofs. A rock will always be harder than ice. The rocks absorb the energy from the hail instead of having the fragile roofing material do the job.
Generally a well built gravel built up roof never has any problems with hail damage. Other strong roofs include metal roofs, which may dent but rarely need to be replaced. Coated acrylic foam roofs are actually some of the toughest roofing materials out there. High density foam will absorb and dissipate pretty much any hail stone. To really get a strong roof, you could substitute river rocks for gravel on any of the built up roofs. This results in a much heavier but basically impervious roof.