Determination of hail resistance

The damage caused by recent hailstorms has made headlines, and unfortunately, these events are occurring more frequently. But is there a way to quantify the damage caused by hailstorms?
The answer lies in the Torro Scale, which measures the intensity of a hailstorm and helps assess the extent of the damage caused.
If the damage cannot be quantified, the intensity is instead related to the size of the hailstone rather than the potential damage it could have caused (TABLES 1 and 2).



In Italy, from a regulatory perspective in the construction sector, reference is made to the following standards:

• UNI EN 13583:2012​                                                                    Flexible waterproofing membranes - Bituminous, plastic, and rubber membranes for roof waterproofing - Determination of hail resistance      
• UNI 10890:2000                                                                           Roofing complementary elements - Plastic domes and continuous skylights - Determination of hail resistance and acceptance limits

In both standards, the test method is based on determining the impact velocity of a sphere with known characteristics, which causes the breakage of the tested element.
The reference sphere is made of polyamide-based plastic, with a mass of (38.5 ± 0.5) g and a diameter of (40 ± 0.5) mm, featuring a smooth and defect-free surface.


Both standards are quite outdated and do not take into account climate change, which is unfortunately showing its worst effects in the increased impact energy of hailstorms. It is no longer uncommon to observe hailstones the size of a tennis ball, with impact speeds exceeding 100 km/h.
In terms of Torro Scale intensity, this corresponds to at least a two-class increase, resulting in a significant jump in the level of damage.

    



 

H5

Damaged roofs, broken tiles, torn-off windows, shattered glass panels, visibly damaged car bodies, and similar damage to the fuselage of light aircraft. Fatal injuries to small animals. Severe damage to tree trunks and wooden structures.

4-7

A more up-to-date regulatory document addressing hail resistance, specifically applied to photovoltaic panels, is the following:

• IEC 61215-2:2021 "Terrestrial Photovoltaic (PV) Modules - Design Qualification and Type Approval - Part 2: Test Procedures"

The document examines various tests, including Chapter 4.17 "Hail Test", which focuses on assessing the ability of photovoltaic modules to withstand hail impact. This is done using a detailed and realistic performance table.

Table 2 - Ice-ball masses and test velocities
Diameter mm	Mass g	Test velocity m/s	Diameter mm	Mass g	Test velocity m/s
25 35 45	7,53 20,7 43,9	23,0 27,2 30,7	55 65 75	80,2 132,0 203,0	33,9 36,7 39,5

The method considers spherical ice projectiles, produced in a freezer, with diameters ranging from 25 mm to 75 mm and impact speeds between 83 km/h and 142 km/h. This accounts for high-energy impacts, similar to the catastrophic hailstorms that occurred this summer in northern Italy.

As the launch device, the standard specifies the use of a compressed air cannon, similar to the one available at Istituto Giordano's laboratory.

 

Launch cannon and 65 mm ice sphere

The Security and Safety laboratory at Istituto Giordano is capable of offering a rigorous and realistic testing methodology inspired by the IEC 61215-2 standard, extending its application to the construction sector as well.

 

CONTACTS
Andrea Bruschi
Tel. 0541 322.388
E-mail: a.bruschi@giordano.it