The IIHS’s roof strength test is different than the front and side crash tests. The IIHS uses a machine that pushes a metal plate against the roof of the vehicle. The test stops when the roof is crushed by five inches. The force applied to the roof is steadily increasing and is known as the strength-to-weight ratio.
The rating for this test comes from the strength to weight ratio when the roof is crushed five inches. Cars that achieve a “Good” rating feature a strength-to-weight ratio of four, which means that the roof can withstand four times the weight of the vehicle before the roof is crushed five inches.An “Acceptable” rating requires a strength-to-weight ratio of 3.25. A “Marginal” rating is earned with 2.5, while anything below that is considered “Poor.”