Understanding Very Severe Hail (VSH) Ratings – Part 2

With new Very Severe Hail (VSH) requirements in place, it is important to understand the single-ply membranes and cover boards that can be utilized in approved systems. VSH requirements now include parts of 14 states as designated by Factory Mutual. VSH requirements can also appear in specifications outside of these 14 states.



The cover board is an essential part of a VSH singly-ply system. Common rooftop cover boards such as gypsum and HD polyiso are not approved, whereas cover boards with very high compressive strengths like plywood and OSB are approved. Outside of wood products, DuraStorm VSH is the only other approved product sourced from post-industrial and post-consumer waste streams. It has a compressive strength of nearly 4,000psi. You can learn more about DuraStorm VSH by clicking here.

Fleece-backed and bareback membranes can achieve VSH approvals; however, cover board attachment is critical. With fleece-backed membranes, the cover board can be mechanically attached using plates and fasteners or adhesively attached using a two-part polyurethane adhesive. With bareback membranes, the cover board can only be attached using a two-part polyurethane adhesive. See the chart below for clarification.

VSH Approved Membrane	80-mil VersiWeld TPO	115- and 135-mil VersiFleece TPO
Membrane Attachment	Roller or Spray Applied Bonding Adhesive	Full Spray/Splatter/4" Beads
VSH Approved Cover Board	DuraFaceR, DuraStorm VSH	DuraFaceR, DuraStorm VSH
Cover Board Attachment	Flexible DASH	Flexible DASH or Plates and Fasteners

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