This video shows what happens when a building's roof drains are clogged. The water builds up and starts to cascade out through the overflow scuppers. This means the water on the roof can be between 3"- 6" deep. A gallon of water weighs 8.34 lbs and buildings are not meant to hold this kind of weight. A simple rain storm can collapse a building. This is why all roof systems must be inspected and cleared of debris especially those surrounded by trees. A roof inspection is a small price to pay to avoid catastrophe.