Hail Resistance of Metal Roofs

Hail is a major cause of roof damage in many regions. Metal roofing is generally difficult to puncture, which is why it is often chosen in hail-prone regions, but its behaviour depends on metal type, thickness, profile and what lies beneath it.

Testing

The most widely cited test in North America is UL 2218, which drops steel balls of increasing size onto a roof covering. Results are classed from 1 to 4, with Class 4 the highest. In the Class 4 test, 2 in (51 mm) steel balls are dropped from about 20 ft (6 m), and the covering passes only if it shows no cracking or penetration. Many metal panels over a solid deck, as well as stone-coated steel products, are listed as Class 4.

Dents versus leaks

Class 4 ratings address whether the covering is broken, not whether it dents. Large hailstones can leave dimples in metal, especially in softer metals such as aluminium and copper or in thin panels. Dents are usually cosmetic and do not cause leaks. Very severe storms can still damage seams, flashings and fasteners, and long storms with large stones on low-slope roofs are the most damaging.

Reducing visible damage

• Thicker, more rigid metal, which resists denting better than thin sheet.
• Profiles with ribs, striations or deep textures that stiffen the panel and disguise small dents; smooth, flat panels show dents most.
• Solid, well-supported decks.
• Stone-coated or textured finishes.

Insurance

Some insurers offer premium reductions for impact-rated roofs, while some policies or "cosmetic damage" waivers exclude dents that do not affect function. Owners in hail regions should read policy terms carefully, since waivers can lower premiums but limit claims.