The biggest problem with leaving collapsed plastic water tank roofs sitting for too long is that the collapsed roofs can become water entrapment especially when they collapse.
Such roofs start filling with rain water leading to extreme weight sitting on top of the collapsed roof leading to the tank side wall folding into the water tank.
Collapsed roofs over time if not repaired, lead to mosquito infestations and become a funnel that funnels leaf matter into the tank and fouls the water. This also attract animals to drop into the water through the split roof where they die and rot.
If the water tank roof is left long term without repair, the roof nominally will become a larger repair or eventually cannot be repaired at all.
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