Hail guides
Hail size and damage thresholds: when which material suffers
Estimated hail size helps you judge what damage a storm can cause. Vehicles, roofs, solar modules and glass have different thresholds – together with location, time and confidence level, this gives a picture of the event.
Vehicles: dents from about 20 mm
Car body panels show first dents from about 20 mm (0.8 in); larger hail also marks roofs, hoods and trunk lids. For vehicles, an alert from class 2 (20 mm) fits. Dent repair businesses looking for large damage areas often choose class 3 (30 mm).
Roofs: tiles break from about 30 mm
Clay tiles and slate usually withstand small hail and tend to break from about 30 mm (1.2 in). In Switzerland, building parts are tested in hail resistance classes: HW 3 means a part withstands a 30 mm ice ball. Roof windows, skylights and gutters suffer earlier.
Solar modules: tested with 25 mm
Type testing under IEC 61215 fires 25 mm (1 in) ice balls at the module. Larger hail goes beyond this test, and micro-cracks in the cells are invisible from the ground. For PV systems, an alert from class 2 (20 mm) is useful.
Glass, plastics and shading: from about 20 mm
Thin glass, polycarbonate (skylights, conservatories) and the aluminium slats of shutters and external blinds can be damaged from about 20 mm (0.8 in). These values are guides; material, age, wind and impact angle shift the threshold.
Size is an estimate
Radar-estimated size is not a measurement at every point. Individual hailstones and the size reaching the ground can differ, and hail does not fall evenly across a track. In Germany there is no size in millimetres, only “hail possible” or “large hail possible”. Always review confidence level, location and track together.