​Impact resistant shingles are designed with a thicker gauge base mat, and composed of more flexible materials than standard shingles. Their durability rating is established by administering the UL 2218 impact test. Shingles are granted a rating of Class 1, 2, 3, or 4 based on the degree of impact they can withstand without cracking. The highest rating is Class 4, offered by most major roofing manufacturers. To evaluate according to Class 4 criteria, a 2-inch steel ball is dropped from a height of 20 feet directly onto the shingle being tested. If the mat of the shingle does not crack on impact, the shingle receives Class 4 impact rating. To achieve Class 3 rating, the shingle must withstand a 1 ¾-inch steel ball dropped from 20 feet.