Hail in the Garden: Why Polycarbonate Protects Roofing Structures
23.07.2026
Severe storms regularly cause massive hail damage across Europe – and the trend is rising. What makes garden roofing structures resilient, and how can you identify durable materials?
Just on July 13 and 14, 2026, a severe hailstorm struck northern Germany: according to police and weather reports, tennis ball-sized hailstones fell, shattering car windows and damaging roofs. This event fits into a broader European trend: according to a recent study by Munich Re and the European Severe Storms Laboratory (ESSL), published in Nature Geoscience, the frequency of particularly severe hailstorms is increasing faster in Europe than anywhere else in the world. Northeastern Spain, southwestern France, and northern Italy could be among the regions most affected by these changes going forward. Following heat and the associated UV radiation, hail is therefore another weather extreme putting plastics in the garden to a new test. For patio roofs, greenhouses, and carports, this raises a central question: which material can withstand such stress?
Unlike glass or acrylic glass (PMMA), polycarbonate (PC) deforms plastically on impact instead of shattering. This is made possible by its molecular structure: long, highly entangled chains with stable carbonate bonds absorb energy instead of breaking brittly.
The difference can be measured: in the standardized notched impact test (Charpy), polycarbonate achieves values of 65 to 75 kilojoules per square meter – the sample often doesn't break at all. Acrylic glass (PMMA), by contrast, reaches only around 2 kilojoules per square meter. Polycarbonate also remains robust in cold temperatures, while PMMA loses impact resistance significantly.
Plastics in the garden that are particularly vulnerable to hail damage include patio roofs and conservatories with multiwall sheets, greenhouses and cold frames, carports and bicycle shelters, as well as privacy screens and facade elements outdoors.
To allow objective comparison of building products, a standardized testing system exists: the Hail Register maintained by the Association of Cantonal Fire Insurers (VKF) in Switzerland. Components are tested at accredited institutes in Switzerland, Austria, and Germany by firing ice balls of defined size and velocity at them, and are classified into five hail resistance classes:
- HW 1 to HW 5 – the number corresponds to the maximum hailstone diameter in centimeters that a component can withstand without damage
- Recommended minimum for residential buildings: HW 3 (hailstones up to 3 cm in diameter)
Several manufacturers now offer multiwall sheets with documented hail resistance. POLYVANTIS provides a multi-year guarantee on hail and weather resistance for its impact-modified multiwall sheets. Exolon Group, which emerged from Covestro's former plastic sheet business, has recently expanded its range with a new, more sustainable generation of sheets for roofing structures and canopies.
Further suppliers of hail-resistant plastic solutions can be found in the K exhibitor database.
Anyone planning roofing structures or greenhouses should therefore not rely on the material name alone, but should specifically ask for a hail resistance class documented in the Hail Register.
Did You Know?
- A hailstone 3 centimeters in diameter hits at around 86 km/h according to the official VKF test standard – the basis for hail resistance class HW 3.
- In the standardized notched impact test (ISO 179), polycarbonate reaches 65 to 75 kilojoules per square meter, while acrylic glass (PMMA) reaches only around 2 kilojoules per square meter.
- The frequency of particularly severe hailstorms (hailstones larger than 5 cm) is increasing faster in Europe than anywhere else in the world.