Hail can cause damage to the underlying insulation of a flat roof causing lessened R-value. The damage is not usually noticeable when you just look at the membrane surface of the roof, but it will cause the insulation to “shattered” from the force of  the hail strike.

If your flat roof is modified bitumen or torch down it can cause fracture of the mat and loss of granules diminishing the life of the material and causing potential leaks.