Understanding Very Severe Hail (VSH) Ratings – Part 1

Historically, Factory Mutual’s (FM) Severe Hail (SH) designation has been the most stringent hail approval in roofing. The test consists of a 1-¾ inch steel ball dropped almost 18 feet, with virtually any rigid insulation board with a single-ply membrane resulting in an approval. In 2017, FM introduced the Very Severe Hail (VSH) approval, consisting of a 2-inch diameter ice ball shot out of a pressurized air cannon. The ice ball impacts the membrane and substrate at approximately 110 miles per hour, resulting in four times the energy impact as SH testing.

Hail Rating	Impact Medium	Method	Impact Energy
SH Severe Hail	1 3/4" - Diameter Steel Ball	17' 9 1/2" Height Gravity Drop	14 ft-lb (19 J)
VSH Very Severe Hail	2" - Diameter Ice Ball	160 Feet/Second Shot Velocity	58 ft-lb (79 J)

VSH testing is completed on new and heat-aged membrane samples. The pass/fail criteria require a membrane and substrate inspection after impact. Observing any of the following items results in a failure:

• Cuts or punctures in the membrane
• Adhesion loss between the membrane and substrate
• Cracks or damage to the top or bottom of the substrate

Due to the impact force of the ice ball, only a few membranes and substrates can obtain VSH approval ratings. Stay tuned to our blog for Part 2 of Understanding Very Severe Hail, where we will dive into VSH approved systems.

To learn more about Very Severe Hail (VSH), contact Brandon Reynolds.