Traditional shingles do not create a watertight seal. But if the roof is adequately pitched or sloped the roof does not need to be completely watertight. Gravity works so that water and melting snow runs off the roof. But if the roof is flat, or has very little slope, water from melting snow and rain can pool on the roof. In these situations, rubber roofs work better as they are watertight.
This home had a rubber roof. Unfortunately a wind storm lifted the rubber roof. The weight of the rubber roof destroyed the chimney and once the rubber was lifted, the rain that was a part of this storm poured into the house and caused quite a bit of water damage throughout the home.