Flexibility. A roof will apply substantial stopping force to a fast moving object over a very short distance until the roof breaks. (And bit more as the broke

Flexibility. A roof will apply substantial stopping force to a fast moving object over a very short distance until the roof breaks. (And bit more as the broken piece of roof accelerates.). A pillow or mattress will deform a lot without breaking, so the smaller stopping force will be applied over a much larger distance. (Energy change = force times distance)