Another contribiting factor that can cause a collapse is drainage. Roofs will only hold the weight that they were designed to hold. That includes dead weights or/and snowloads. If a roof isn't designed to shed water over the edge then it requires an internal drain system. Some roofs is designed to direct the water flow over the edge by way of scuppers & sometimes these scuppers are install in parapet wall. If scuppers or drains become clogged then the weight of a flooded roof can easily cause a collapse.

Another cause can be by water entering through what's called the "weather envolope". This would be the roof, wall & roof insulation & ventilation, brick or wood or other types of siding, windows, flashings. Water will cause structural damage if left alone long enough.

Barry